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Bob McDavitt's ideas for sailing weather around the South pacific

16 September 2018

Bob Blog

WEATHERGRAM

YOTREPS

Compiled Sun 16 Sep 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

Equinox is next Sunday:

Next Sunday, 23 Oct at 01:54 UTC marks the vernal equinox. At this point in time the overhead sun is directly over the equator, shifting from the northern hemisphere to the southern hemisphere.

 

If you check sunrise /sunset times for your place for Sun 23 Sep, you will find that these compute to something like 12hr 7mins of sunlight. This is because sunrise and sunset are defined with respect to the top limb of the sun, rather than the centre of the sun, and that adds a few minutes of sunlight to the day. You’ll find (if you look) that the day that has closest to exactly 12 hours of daylight, called the equilux, is around three days earlier than the September equinox.

 

Somehow, I was thinking that New Zealand switched to Daylight time on the fourth Sunday of September--- but no, the change occurs on the last Sunday of September, and so that’s 30 September

 

The change to daylight saving occurs on different dates for the nations around the South Pacific so I think it’s a good idea for you to have a summary for reference

 

Date Place Daylight saving time zone

30 Sep New Zealand UTC+13 (NZDT)

30 Sep Chatham Islands UTC+13h45mins (CHADT)

7 Oct Lord Howe Is. UTC+11 (AEDT)

7 Oct New South Wales, Tasmania UTC+11 (AEDT)

4 Nov Fiji UTC+13 (Fiji Summer Time FJST)

No changes in Tonga or Norfolk Island or Queensland or New Caledonia.

 

As yachts start their end-of-spring migration, with the approaching South Pacific Cyclone Season, it is also useful to have a summary of the various national holidays, as these limit available departure dates.

French Polynesia 1 Nov All Saints Day, 11 Nov Armistice day (100th year).

Cook Islands: 26 Oct Gospel day

Niue: Oct 19 to 22 Constitution day celebrations, Oct 22 is also Gospel day.

Tonga Sep 17 HRH Crown Prince Birthday, Nov 5 Constitution Day

Fiji Oct 10 Fiji Day, Nov 7 Diwali, Nov 20 Muhammad Birthday

New Caledonia Sep 24 National Day, 1 Nov All Saints Day, 11 Nov Armistice Day (100th year).

New Zealand Oct 22 Labour day.

Australia: OCT 1 Labour Day (ACT, NSW &SA), Queen’s Birthday (QLD)

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TROPICS

MANGKHUT travelled over the northern parts of the Philippines, with a death toll of at least 24 so far. It is now skirting around Hong Kong. FLORENCE weakened as it made landfall over Northern Carolina and has a death toll of 13 so far.

It is purely a coincidence that the Atlantic systems all have female names at present: The males: Gordon and Isaac have faded away

Also the Southern Hemisphere cyclone season is showing signs of a possible early start with the formation of Tropical depression ONE in the South Indian Ocean.

Map of current storms is seen at tropic.ssec.wisc.edu/

Looks like HELENE may end up over the UK this week.

Looking at the weekly rain maps we can see the tracks of last weeks’ cyclones.

There has been an intense area of rain last week over New Britain to northeast of Papua New Guinea.

See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay in the region from PNG to just south of the Solomons Islands to north of Fiji this week.

Trough over Vanuatu on Monday UTC travelling to south of Fiji on Tuesday UTC and fading over south-of-Tonga on Wednesday UTC.

Convergence zone over Tahiti on Tues UTC and Tuamotu Islands to Gambier Islands on Wednesday UTC. Then a Low may form south of Tahiti by end of the week travelling off to the SE and deepening, stealing the wind for Tahiti

 

Subtropical ridge (STR)

HIGH over 1030hPa to eats of NZ travelling east along 40S. Squash zone of enhanced winds on its northern side between 20 and 25S reaching peak on Tuesday UTC.

Next HIGH should move from Australian mainland into Tasman Sea on Mon/Tuesday UTC, and then fade over central NZ.

 

Around Tasman Sea, NZ.

Low1 crossing the South Island on Monday UTC Low2 is expected to be crossing the area between Fiji and NZ on Monday/Tuesday 17/18 Sep, followed by Low3 on Fri/sat 21/22 Sep. Southerly winds follow these lows.

For those sailing towards New Zealand from Fiji/Tonga/Noumea, there may be a narrow gap between these lows, so that the best-looking voyage this week maybe with a TUESDAY departure, but this does involve zig-zagging around some southerly headwinds.

Traffic going from New Caledonia to Australia have useful south to southeast winds for starters but may encounter a front and southerly winds near Australia from Thursday UTC.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

09 September 2018

Bob Blog 9 Sep

WEATHERGRAM

YOTREPS

Compiled Sun 09 Sep 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

The “Olde Man” Southerly

When a low crosses the North Island and the deepens as it moves away, the resulting southerly gale thru Cook Strait lasts for around three days. Such an occurrence is nicknamed by many Wellingtonians as an “Olde Man Southerly”. Its steadiness feels belligerent, lingering longer than desired. At this time of the year the arriving air hovers just below 10 C, but when the wind chill is taken into account is feels less than 4C—colder than the inside of a fridge. This wind has been described as “blowing straight through you” for you feels it’s cold in your bones .

 

We had an Olde Man last week and I was in Wellington to experience it. The weather map (Noon Wednesday local, courtesy of MetService) shows the low responsible.

 

Because this low is close to Auckland, I have heard some smart people claiming that the reason Wellington gets such cold southerly outbreaks can be summed up in two words: “Auckland  sucks”. The map illustrates a classic “eggbeater” with a clockwise spinning wind around the low to the east of the North Island, and a counter-clockwise spinning wind around the High to east of the South Island.

Of course another contributing factor to this Cook Strait gale is the layout of the land, with mountain chains over the South Island and the North Island, Cook strait is the right sized gap to make a “river of wind”.

 

This can be seen on windy.com and also by clicking on Wellington airport a graph of recent data may be seen showing temperature, wind and pressure profile during this “Olde man”

 

This river of wind crosses the isobars toward s low pressure.

1) The switch from Northerly to Southerly is relatively quick, with no real “calm” in between. Also the barometer starts rising hours before the northerly winds fade. image

2) It takes around 12 hours for a SW wind to swing to a southerly and rise to a gale, along with a period of rapid rising pressure.

3) AFTER the Southerly gale starts , the pressure continues to rise, not so fast, but a steady rise from 1015 to 1025hPa over 2 days.

4) Once the pressure peaks and steadies, the wind relaxes just a little. It continues to stay strong for another day or more.

5) Temperature remains much the same throughout the event with no day/night signal. Showers continue until the air dries out and dewpoint drops to below 5C.

 

TROPICS

It is now “busy time” for the Atlantic Ocean. FLORENCE is likely to make landfall in North Carolina, and OLIVIA may visit Hawaii but is expected to weaken in the process. MANGKHUT is heading to the northern Philippines /Southern Taiwan area.

The weekly rain maps show the tracks of last weeks’ cyclones with JEBI across Japan and NORMAN just missing Hawaii. Note hoe the rain associated with the low east of NZ just touched the east coast of the North Island. See www.facebook.com/MetService/videos/891981477639175/

There has been very intense rain associated with a low just south of Tahiti – I’m not sure why this low was so wet.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay in the region from PNG to just south of the Solomons Islands to north of Fiji this week.

Low to south of Tahiti is expected to go south and fade next few days.

Passing trough to south of Fiji on Monday UTC and then over southern Tonga on Tuesday UTC and Southern Cooks on Thursday UTC, consisting of erratic winds and possibly a few showers.

 

Subtropical ridge (STR)

HIGH less than 1020hpa is expected to form near 25S South of Tonga on Monday UTC and travel east along 20 to 25S to be south of French Polynesia by Wednesday UTC.

HIGH over 1025hPa over New South Wales on Monday UTC spreading east along 30 to 35S reaching NZ around Thursday UTC and then spreading further east along 35S. There may be a zone of enhanced SE wind so the north side of this High, mainly near 20S.

 

Around Tasman Sea, NZ.

Low is expected to cross northern NZ on Monday and southerly winds following this feature are likely to affect area between NZ and 20S on Tuesday --upsetting sailing between topics and NZ, but OK going t’other way. After that, with high pressures over NZ and trade winds to north of NZ, some good-looking sailing voyages from the tropics to NZ are likely.

Mainly easterly winds in North Tasman sea, so OK to sail New Caledonia to Australia, but not the other way. Traffic going from Australia to NZ may need to get to at least 35S to find a way eastward.

 

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

02 September 2018

Bob Blog 2 Aep 2018

WEATHERGRAM

YOTREPS

Compiled Sun 02 Sep 2018

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

REVIEW OF AUGUST  2018

Sea Surface temperature anomalies as at end of August  may be seen at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

The eastern equatorial Pacific around Galapagos is the focal region for ENSO and is now on a steady warming trend. Temperatures around Australia to New Zealand are becoming below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America and the Kuroshio current off Japan still stand out as warmer than normal.  These may help steer tropical features away to the northeast.

Warm anomalies continue off west side of Mexico to Hawaii indicating a busy cyclone season for next few months.

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To see how the annual weather cycle and the seasons are working out, check the average isobar maps from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

Average isobars for past 30 days and their anomaly:

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust, as are the North Atlantic and Northeast Pacific HIGHS. The August lows in the Southern Ocean have been favouring three positions: Tasman Sea, SW of South America, and SE of South America.

Lower than normal pressures between Europe and Philippines show a more active monsoon. 

Zooming into the NZ area, the 1010hP (between dark and light blue) isobar has shifted from Christchurch to Gisborne.  And the 1015 has retreated off Tonga.  This explains the increase in westerly winds onto western NZ, and can be taken as an early start to SPRING weather patterns. Not much change around Australia .

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The last 30 days of rainfall, and its anomaly, are seen at TRMM at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

The rain map shows extra convergence in the ITCZ from Bangladesh to Philippines , and across  the North Pacific Ocean, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere a large dry region stretches from Australia to Vanuatu/Fiji.. It seems that the SPCZ has shifted northwards and estwards (an El Nino trait).

In summary, it seems that, although the atmospheric and oceanic parameters are “neutral” at present, as seen in my last week’s blog, there are also some signs of an El NINO weather pattern.

 

TROPICS

We have reached the time of the year when cyclones cluster, as is happening this week in the northeast Pacific Ocean. 

 For Map of current storms see tropic.ssec.wisc.edu/

There is MIRIAM, NORMAN , and OLIVIA in NE Pacific, FLORENCE in north Atlantic, and JEBI in NW Pacific.

NORMAN is expected to sideswipe Hawaii later this week.

And JEBI is expected to visit Japan this week

 

Looking at the weekly rain maps we can see that the Asian monsoon is active over south China and the Philippines/Indonesia area, and the ITCZ is active across the northeast  Pacific and around Panama. South Pacific Convergence zone is weak and further east than normal.

 See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG to the Tuvalu/Tokelau area this week.

Passing trough over southern Vanuatu on Monday and Tuesday is expected to extend SE on Wednesday into a trough southwest of Fiji, the on Thursday turn into a low south of Fiji that then should go SSE to east of NZ.

Trough is expected to form over Tahiti area by local Tuesday, the convergence zone associated with this feature is expected to visit Tuamotu Archipelago to Gambier group around local Wednesday. 

 

Subtropical ridge (STR)

Large High 1035+ at 35S to south of Tahiti should fade and travel east by end of the week.  The squash zone of enhanced easterly winds between it and the Low over Tahiti should peak around mid-week with strong wind and large swells affecting Southern Cooks.  Avoid.

HIGH travelling east across Tasmania on Monday should skirt around south end of NZ on Tuesday and then travel east along 45S to east of NZ on Wednesday.  Squash zone between this high and Lows to east of the North Island should bring a classic “olde man southerly” to central NZ from Monday night until Thursday.

 

Around Tasman Sea, NZ.

High pressures in central Tasman sea this week should ensure good voyages from New Caledonia to Queensland. Traffic going from Australia to NZ may need to get to at least 37S to find a way eastward.

Low is expected to be crossing North island don Monday followed by three days of strong SE winds.  Conditions should be OK again around Friday or Saturday for departing Northland, As for sailing to NZ from tropics, nope, too much southerly this week.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

26 August 2018

Bob Blog

WEATHERGRAM

 YOTREPS

 Compiled Sun 26 AUGUST 2018

Bob McDavitt’s ideas for sailing around the South Pacific.

 Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

The state of the ENSO = neutral

 El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather: the La Nina, caused by cooler than normal seas along the equatorial eastern pacific. shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

The Ocean:

 NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

 

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017,then a warm period until July 2017, and then a cool period until May 2018.  Ove the last three months we have been having another warm period. 

 

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM another half a degree during the rest of this year, and then relax slowly next year.  The most likely period for this parameter to reach +1 and be called an EL NINO is NDJ (November December January).  And that is the early part of our South pacific cyclone season.

 CPC/IRI predictions are at  iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

 

The Atmosphere:

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number ofisobars between them on the weather map. When the SOI is more than plus one(standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

 

For the past year the SOI has been mostly around plus 0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. However, there have been several brief periods of negative SOI over the past six months and we are having another of these at present, indicating that there is a tendency towards an El Nino.

See www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

(Note that in this graph on the vertical axis 10= 1standard deviation)

 Latest SST anomaly map shows the remains of a large blue pool of cooler water across the central equatorial Pacific. Also, there are warmer yellow waters appearing around the Galapagos.  The  Australia/Tasman Sea is getting colder than normal at the coldest time of the year, and so is the Humboldt current off western South America.

Sea surface temperatures across the Pacific on 23 Aug is at  www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

 

In Summary: We are neutral but watching as parameters trend towards an el Nino. There is a good chance of a weak EL NINO for the turn of the year.  This should weaken the trade winds, allowing the South pacific Convergence zone and subtropical ridge to shift to the north a little--- this may disrupt cyclone formation to the east of the norm, somewhat lessening the likelihood near Australia and increasing it in the Fiji /Samoa area.  Still too early to be sure of this, so just keep it on the backburner for now, and don’t make any decisions based on that idea just yet.

  

TROPICS

TC LANE brought wind and rain to Hawaii, much as forecast. There are a couple of tropical depressions that seem to be vaguely moving westwards or north-westwards.

 After a week of heavy rain over Japan and Korea, the North Pacific is relaxing now,  and the Atlantic continues to remains quiet.

 Looking at the weekly rain maps we can see that the Asian monsoon is active between India and the Philippines, and the ITCZ is active across the Pacific and around Panama. South Pacific Convergence zone is weakening.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

 The SPCZ is expected to stretch from PNG and Solomon Islands to Samoa and from there to the southeast.  

 Broad trough to east of NZ is expected to travel eastwards and associated passing trough should reach Society Island around local Tuesday and Tuamotu Archipelago around local Wednesday.

 There is a jetstream over New Caledonia tonight and this is expected to combine with a trough deepening east of Australia on local Monday to form a trough over Vanuatu on local Tuesday, reaching Fiji on local  Thursday and southern Tonga on local Friday.

Subtropical ridge (STR)

 HIGH crossing NZ tonight is expected to is expected to move east along 35S to 40S this week and intensify to over1040hPa by end of the week. A squash zone of enhanced trade winds on the north side of this high is expected to peak along 12 to 20S between Tahiti and Tonga from local from local Tuesday to Saturday.  Avoid.

 This squash zone detracts from a comfortable voyage from Tahiti to Tonga this week., unless  you skirt around it via Suwarrow and avoid its peak on local Thursday.

 From Thursday to end of the week, Another HIGH is expected to travel east from Australia to South of Fiji along 30 to 20S.  Small squash zone is expected in Coral Sea on the northwest side of this High.

  

Around Tasman Sea, NZ to tropics.

Passing High with light winds on Monday.    Low is expected to deepen in Tasman Sea bringing strong northerly winds to northern NZ on Tuesday, then a front on Wednesday, and then the LOW over central NZ on Thursday and Friday.   It’s a stay at home week for NZ yachts, but there are good voyages from Australia to New Caledonia.   If sailing to NZ avoid the Wednesday front.

 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

 If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

19 August 2018

Bob Blog

WEATHERGRAM

YOTREPS

Compiled Sun 19 AUGUST 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

Why is the Sky Blue?

After my recent blog on twilight colours , I have been asked to offer my explanation of the blue sky.

Sunlight is made up of a spectrum of colour – a bundle of waves of electromagnetic energy all travelling at the same speed but each vibrating at different frequencies. When talking radio stations we speak about its frequency (e.g. 95FM) but when talking about light we speak about its wavelength. Red light has a wavelength of around 650nm and at the other end is violet light with a wavelength almost half that at 400nm. Red Orange Yellow Green Blue Indigo Violet.

 

A beam of light travels in a straight line until something gets in the way and does one of four things

1) Reflects all of it on a straight path (like a mirror)

2) Bends it (like a prism)

3) Scatters it (read more below)

4) Absorbs some of it and reflects/scatters off the rest (we see this scattered/reflected light and call it that the COLOUR of the object).

 

Back in 1859, John Tyndall described how wavelengths in a light beam get scattered when it shines thru a colloid (a mixture in which the floating particles do not settle out) e.g. cloud/fog, smoke, milk, ink, paint – the particles in a colloid are 1 to 1000nm or o.ooo1 to 1 micron, This scattering is called the TYNDALL EFFECT. Shorter wavelengths get scattered sideways and only the longer wavelengths get transmitted forwards.

 

LORD RAYLEIGH, or John William Strutt, the third Baron Rayleigh (1842-1919)was born in Essex and educated in Trinity College, Cambridge. In 1871, he described how light is scattered by the molecules of air, and this is called RAYLEIGH SCATTERING

As sunlight travels thru the atmosphere it encounters air molecules and reacts with their electrons so that some light gets scattered. The shorter wavelengths are deflected at a greater angle than the longer wavelengths. Lord Rayleigh described how this works when the scattering is done by molecules showing that “the amount of scattering is inversely proportional to the fourth power of the wavelength” This means that the shorter wavelengths are deflected away into the sky and the longer wavelengths get transmitted forwards.

 Or: Blue scatters Best (ten times more than red).

This scattered blue light fills the sky giving us skylight.

See www.tandfonline.com/doi/abs/10.1080/14786447108640452

 

OK I hear you: so if the shorter wavelengths are scattered best, then why isn’t the sky VIOLET or INDGO? It’s true that Indigo and violet are scattered even better than blue, but the sun emits more energy as blue light than indigo or violet. That’s only part of the answer for we can see the violet in a rainbow. The rest of the answer lies in the way our vision works.

Humans have evolved with three types of colour-receptors in our retina. They respond most strongly to light at the red, green and blue wavelengths. When they are stimulated in different proportions our visual system constructs the colours we see.  To see the response curves for the three types of cone in the human eye visit math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

When we look up at the sky, the blue cones are stimulated strongly and the red and green cones are stimulated less so, and almost equally. If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the indigo and violet wavelengths stimulate the red cones as much as the blue, producing a purple tinge. The net combined effect is what we call the “sky blue” colour.

It may not be a coincidence that our vision is adjusted to see the sky as a pure hue. We have evolved to fit in with our environment; and the ability to put natural colours such as sky-blue or grass-green into the background and focus quickly on anything that does not have natural colouring is a useful survival trait.

 

 

TROPICS

Map of current storms at tropic.ssec.wisc.edu/ shows TC LANE is SE of Hawaii. It is expected to weaken and go west along 15 to 16N, but may bring some windy rain to western parts of Hawaii in a few days. See www.tropicaltidbits.com/storminfo/14E_gefs_latest.png for an update.

TC SOULIK is expected to skirt around the south end of Japan and visit Korea this week,and CIMARON may travel NW towards Osaka Bay in Japan.  The Atlantic continues to remain somewhat quiet, for now.

 

Looking at the weekly rain maps we can see that the Asian monsoon is active between India and the Philippines, and the ITCZ is active across the Pacific and over Panama. South Pacific Convergence zone has a wet week over Solomon Islands. Kerala, the Southwestern state of India , stands out with recent flooding.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Fiji and around the Tokelau/Samoa area.

A passing trough at present over southern Cooks is expected to deepen into a LOW that should continue deepening to below 989hPa by mid-week as it travels ESE/SE to o 35S 125W, Associated trough is expected to cross Tahiti on local Sunday and Tuamotu Islands on local Tuesday.

A passing trough is expected to visit New Caledonia /Vanuatu early this week and stall over Fiji/Tonga during mid-week then move onto Southern cooks late in the week

 

Subtropical ridge (STR)

HIGH is expected to move along 25 to 35S from south of Fiji/Tonga early this week to south of French Polynesia by mid-week, followed by another High later in the week,

Next HIGH from Australia is finally expected to move into the central Tasman Sea late in the week and visit NZ early next week.

 

Around Tasman Sea, NZ to tropics.

Low is expected to travel across southern NZ on local Monday.

There may be a brief ridge over northern NZ on local Tuesday, and then another Low from central Tasman Sea is likely to cross the North island on local Wed/Thu/Fri. Avoid these lows.

 

Tahiti to Tonga

Wait until local Monday or Tuesday for the winds to return to be from SE after a passing trough.

There is expected to be a small enhancement of the trade winds late in the week near 20S as a High to the south travels east.

 

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

12 August 2018

Bob Blog 12 Aug

WEATHERGRAM

YOTREPS

Compiled Sun 12 AUGUST 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

The South Pacific Convergence Zone explained:

Now that many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, as if it guards the eastern entrance to the South Pacific like a protective dragon, some have asked what is it, why is it there, how does it differ from the ITCZ and what makes it tick.

Weather is a mix of pattern and chaos, and meteorological teaches concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell.

Because the sun is most directly overhead at the equator, that’s where the warmest seas are, and this causes rising air. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds. The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ

But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W (it is quasi stationary, just like the High between California and Hawaii, and has a gyre that is collecting a rubbish heap just as badly (see Henderson island). blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-in-paradise/

There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. The easterly winds travel well to west of the dateline around 10 to 15S, and the South/SE winds come and go according to the migratory high and are usually found around 15 to 25S. The convergence zone between these easterly and Southeasterly winds is called the South pacific Convergence Zone, or SPCZ.

It is typically located from the Solomon Islands southeastwards to the Southern Cooks, but sometimes may have large gaps or be very quiet.

It is affected by many things: the PDO which takes many years to switch, by the El Nino/La Nina which last a year or so, and by the strong annual cycle which repeats each year, and by the MJO which comes for a week or so every six weeks or so.  Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone

 

TROPICS

There is a regular procession of tropical features off the western Mexico coast.

The Atlantic remains somewhat quiet. YAGI is travelling NW to China, and LEEPI to south Japan, and HECTOR is heading for south of Hawaii, with another feature following.

Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific. Not much change in the past week.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Fiji and spreading onto northern Tonga/Niue.

A weak passing trough is likely over Society Island and Tahiti on Monday/Tuesday UTC.

A passing trough, associated with a Low travelling east along 30S, should reach Niue on Mon UTC, and fade between Southern cooks and Tahiti on Wed/Thu UTC.

 

Subtropical ridge (STR)

HIGH is expected to move off Australia into northern Tasman Sea on Tue/Wed UTC and then travel east along 25S

 

Around Tasman/NZ

Front moving onto NZ on Mon expected to deepen into a Low over central NZ on Tuesday and then move off to the east, followed by W/SW winds until Friday.

 

Tahiti to Tonga

The best-looking date for departure is Wednesday local. This voyage is likely to encounter a weak passing trough near160W on Saturday and then should be able to go onto Tonga . >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (offers link to FOLLOW)

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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05 August 2018

Bob Blog 5 Aug 2018

WEATHERGRAM

YOTREPS

Compiled Sun 05 AUGUST  2018

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

REVIEW OF JULY 2018

Sea Surface temperature anomalies as at beginning of August  may be seen www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

The eastern equatorial Pacific around Galapagos is the focal region for ENSO and is now on a slowly warming trend. Temperatures around Australia and into the Tasman Sea are becoming below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America and the Kuroshio current off Japan still stands out as warmer than normal.  These may help steer tropical features to the northeast.

Warm anomalies continue off west side of Mexico indicating a busy cyclone season for next few months.

 

To see how the annual weather cycle and the seasons are working out, take  a quick look at the average isobar maps from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html showing average isobars for past 30 days and their anomaly for JULY.

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust. The North Atlantic and Northeast Pacific HIGHS are looking stronger than this time last month. The July lows in the Southern Ocean have been favouring three positions: south of Australia, SW of South America, and SE of South America.

Lower than normal pressures between Europe and Philippines show a more active monsoon. 

Zooming into the NZ area, the 1015hP (between blue and white) isobar has stayed put in the tropics and to east of NZ.  There has been a radical change south of Australia and in the Tasman with a DROP in pressure allowing the 1015 to shift from Invercargill to Auckland.  This explains the increase in westerly winds onto western NZ, and can be taken as an early start to SPRING weather patterns.

 

The last 30 days of rainfall, and its anomaly, are seen at TRMM at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

The rain map shows extra convergence in the ITCZ from Bangladesh to Philippines , and across  the North Pacific Ocean, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere a large dry region stretches from Australia to northern NZ, and the South pacific Convergence zone has been weak around Vanuatu and : Fiji.  It seems that the SPCZ has shifted northwards (an El Nino trait).

 

In summary, it seems that, although the atmospheric and oceanic parameters are “neutral” at present, as seen in my last week’s blog, there are early signs of an El NINO weather pattern.

 

TROPICS

There is a regular procession of tropical features off the western Mexico coast. 

The Atlantic remains somewhat quiet, and SHANSHAN is heading for Japan. As seen at tropic.ssec.wisc.edu/

 

Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific and to central America. 

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Vanuatu to the Tuvalu/Tokelau then Southern Cooks area.  

A passing trough is likely over Southern Cooks on Mon 6 Aug UTC (squally) and Wed 8 Aug (not so squally), These troughs should weaken east of the Cooks and not affect Society Group (apart from some northerly winds).

Another passing trough is expected to develop around Fiji/Tonga around 11 Aug UTC This may turn into a  Low near 22Z next week, Avoid.

 

Subtropical ridge (STR)

Weak HIGH is expected to appear in north Tasman Sea from Mon to Wed 6-8Aug, and another should cross northern Tasman Sea /North Island from Thu 08 to sat 11 Aug UTC.

A polar HIGH is expected travel east along 50 to 45S this week, east on NZ, there is likely to be a squash zone of gale easterly winds east of NZ between around 30 and 40S. Avoid.

 

Around Tasman Sea, NZ to tropics.

Passing troughs on Monday and Wednesday. Best date to depart NZ for the tropics this week is Thursday (local). 

 

Tahiti to Tonga

With the passing troughs, perhaps the best on offer this week is a Friday local departure to Palmerston or Aitutaki. Then again, if you don’t mind encountering a trough over the open sea, can depart any day, but get some waypoints to reduce the squalls. .

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

29 July 2018

Bob Blog 29 July 2018

WEATHERGRAM

YOTREPS

Compiled Sun 29 July 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

The state of the ENSO = neutral, but leaning towards an El Nino.

 

The Atmosphere:

The tropics influence our seasonal weather, swinging from El Nino to La Nina, and occasionally hovering in-between (called neutral). The La Nina is caused by cooler than normal seas along the equatorial eastern pacific and it shifts the subtropical ridge away from the equator. The El Nino, with its warmer than normal seas in the target region, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

 

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number of isobars between them on the weather map. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

 

For the past year the SOI has been mostly around plus 0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. However, over the past two months, the SOI has settled into a sub-zero state, but has not been strong enough to be called an El Nino, and so is deemed to be NEUTRAL.

Neutral conditions are seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

 

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, then a warm period in mid-2017, and then a cool period until May this year. In the past two months NINO 3.4 has been around plus 0.5C, in NEUTRAL territory.

Weak La Nina is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

Waters beneath the surface are slightly warmer than normal, as seen at www.cpc.ncep.noaa.gov/

 

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM over the next few months and then remain steady a few months and then ease back in 2019. SO there may be an El Nino event, and it should peak between October and February.  CPC/IRI predictions are at  iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

And the latest SST anomaly map shows warm yellow waters across the central Pacific.

See www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

 

In summary, it seems that there may be an incoming El Nino, and it should peak late in the year. This is likely to cause the subtropical ridge between New Zealand and the Tropics to shift to the north, and thus to encourage the cold west to southwest winds to be more noticeable than normal around New Zealand and the Tasman Sea during the coming spring.

 

TROPICS

After a busy July in the NW Pacific, conditions are returning to normal for the start of August.  As seen at tropic.ssec.wisc.edu/

 

Rain eased over India and Japan last week, but the monsoon is active from Pakistan to SE Asia and over the Philippines

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Vanuatu to the Tuvalu/Tokelau area.

A passing trough is likely over Southern Cooks on Tue 31 July reaching Austral Islands on Thursday 2 August UTC.

A passing trough is expected to cross New Caledonia on Mon UTC and travel east along 20 to 25S reaching Tonga on Thursday UTC and then fade.

Trough crossing NZ on Thursday is expected to develop a subtropical LOW near 25S 180 around Friday 3 Aug UTC that may travel slowly along 25S and deepen to BELOW north of 1000hpa on Sun/Mom 5/6 August UTC and then further east. Associated trough should reach Cooks by Sun 5Aug followed by W/SW winds. Avoid.

 

Subtropical ridge (STR)

Weak HIGH is expected to travel east across northern NZ on Monday and then off to east.

Next HIGH is expected to travel across southern Tasman sea on Thu and build to east of South Island from Sat 4 August.

The squash zone between this HIGH and the LOW near 25S should produce an easterly gale with 5m+ swells near 27 to 30S next week. Avoid.

 

Around Tasman Sea, NZ to tropics.

Ridge over NZ on Monday and trough on Tuesday NW flow on Wednesday and a trough on Thursday. Then an easterly to NE flow for late in the week with Low to northeast and HIGH to southeast. A week to stay put.

 

Tahiti to Tonga

May be able to so some Island hopping north of 20S until Wed or Thu, then should stay put. Incoming trough reaching Cook Island, including Suwarrow, by Sun 2 Aug UTC followed by W/SW winds.

 

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

22 July 2018

Bob Blog 22 July

Bob Blog

WEATHERGRAM

YOTREPS

 

Compiled Sun 22 July 2018

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

After my notes on lightning last week, one of my readers has asked me to try and have ago at explaining the old folk-lore expression “Red sky at night, sailors delight; red sky in the morning, sailors warning”

(land dwellers change the sailor into a shepherd).

 

Reading weather signs is one thing that humans have been doing ever since we have been human enough to detect pattern amongst the chaos. And this sign is a good one, often proving true. In Matthew 16.2, Jesus told the Pharisees that “When evening comes and the sky is red, you say the weather will be fair, and in the morning when the sky is red and overcast, you say today will be stormy—you know how to interpret the appearance of the sky, but not the signs of the times!

 

“A Red sky at night is a delight.” Explanation, it’s as easy as 1,2,3: 1. The sun sets in the west, and, for many parts of the planet, 2 that’s where tomorrow’s weather is usually coming from. Usually the winds at cloud level above us are from northwest/west/or southwest, so weather features and clouds ride this steering field from west to east. If the western horizon is cloud-free, then there is a good chance that our tomorrow’s weather may be cloud-free.

3. Sunlight shining slant-wise loses all but its red colouring.

Air near the ground gets dirty: it accumulates particles of dust and salt and smoke that are small enough to float around (aerosols). Sun shining on these materials undergoes Mie scattering, whereby red light is mostly scattered forwards and blue light is mostly scattered laterally or backwards. As the sun sets, it beams through an increasingly longer path than at noon. It takes on a golden hue, or, if there is a lot of dust about, then the clouds above are side-lit in a rosy red. Colours can be seen to go through the spectrum and some colours are enhanced according to the aerosols of the day. Once the sun is set, red light from the aerosol layer mixes with the blue light scattered above it to form purple light.

 

“A Red Sky in the morning is a warning” Explanation: Sun rises over the eastern horizon. If that’s where the clear skies are, incoming light is rendered reddish by passing through cloud-free but dirty air. If there are clouds overhead, this usually means we are on the back end of a departing area of dry weather, and thus, as is the nature with weather, on the forward side of the next lot of rain approaching from the west. Dawn horizontally side-lights the overhead clouds with a reddish glow, enhancing our senses of pending gloom.

 

TROPICS

 

Tonight, we still have a “zone of interest” between Mexico and Hawaii.

It is a very busy time in the NW Pacific. This is in part to do with a MJO or Madden Julian Oscillation travelling into the Pacific

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with a weak stretch across the Wallis and Futuna area to south of Samoa.

A passing trough over the Southern Cooks is expected to turn into a low near 30S 160W by 23July UTC that should move off to the Southeast.

Another passing trough is expected visit Vanuatu on Mon 23 UTC, Fiji /Tonga on Tues UTC, Niue on Wed UTC and Southern Cooks on Thu 26 July UTC.

And yet another passing trough is expected to affect Eastern Coral Sea/New Caledonia area on Sat/Sun 28/29 UTC.

 

Subtropical ridge (STR)

HIGH is expected to linger in the northern Tasman Sea from Monday to Thursday and then travel off to the east along 30S.

Too many passing troughs for another HIGH this week. Me-thinks this is a sign of an incoming El Nino, that’s if I’m reading the sign of the times correctly.

 

Around Tasman Sea, NZ to tropics.

Developing low over central NZ on Monday. Then a Low is expected to form near the Kermadecs on Tuesday (as associated trough crosses Tonga). This low should deepen to below 980hpa and travel south along 170W on wed/Thu UTC and then go off to the east along 45S, bringing a SW to westerly flow to NZ, OK for departure to the tropics.

Near Australian coast a new low is expected to deepen off Sydney from Sat 28 July

 

Tahiti to Tonga

Winds around the Society islands look OK for departure any day this week. Try and stay north of the passing troughs.

 

Galapagos to Marquess

Useful southerly winds over Galapagos for going west at first. Go to 2S 106W and then 6S 128W then direct.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

15 July 2018

Bob Blog 15 July 2018

WEATHERGRAM

YOTREPS

Compiled Sun 15 July 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

Kayaker Scott Donaldson made successful land fall in new Plymouth early last week after departing from Coffs harbour in early May.  See www.stuff.co.nz/national/105168174/Kiwi-Scott-Donaldsonreaches-Taranaki-the-first-person-to-kayak-the-Tasman-solo

 

I have been helping Scott with twice daily txt messages looking at currents, wind and swell. Scott is an adventurer motivated to raise money to help those with live with Asthma (such as he and his son)

If this success inspires anything in you then please show it by funding his donation page:

givealittle.co.nz/cause/tasman-kayak--- Do this soon as the page closes by end of July.

Surely such an adventure is worth a $10 donation?

================

 

WINDY.COM have added a new layer, called THUNDERSTORMS, which comes from the global ICON model from DWD as a combination of rain and lightning density. More resolution than the CAPE parameter.

See community.windy.com/topic/6269/windy-is-the-first-weather-service-in-the-world-to-globally-forecast-thunderstorms.

Technical details are at confluence.ecmwf.int//display/FCST/45r1+new+parameters%3A+lightning+flash+density

Lightning is static electricity. Any object that spikes upwards from an otherwise flat horizon is asking to be struck by lightning. When updrafts in a shower cloud exceed around 25 km/hr (13 knots), air and water molecules are rubbed together, creating static electricity (making lots of free electrons). In the updraft, rain, ice, and snow are jumbled, and the lighter particles are lifted to the cloud top, whilst freed electron tend to attach to the heavier particles at base of the cloud. Like charges repel, and so this reservoir of electrons in the base of the cloud causes the free electrons in the ground beneath the cloud to go elsewhere. This forms an area of positive charge in the ground under the cloud. When the cloud gets blown around, this positively-charged area follows the cloud like a shadow. When the reservoir of charge in the cloud gets large enough, an explosive discharge occurs.

 

First, groups of electrons leap outwards and downwards in all directions towards the positive area near the ground. This happens in jumps, called leaders, each taking around a nanosecond, and at the end of each jump the pause may break the leader into two. When one of these leaders is within about 100 metres of the ground a group of positively-charged particles streams up from the earth to meet the leader. These are called streamers and are best made from a spike or tower (or tree). When the streamer meets the leader, the stored energy (100 million volts) is released suddenly enough so that the air in the stream is ionized. It turns to plasma, 20,000C+, or 10 times hotter than the surface of the sun for a fraction of a second. This ionization works its way up the streamer (so the light starts at the bottom and ends at the top), and this is called the return stroke. On its way up all the places where the streamer has divided get lit, hence the spread-out pattern of some lightning photos. After the first strike, the main pathway between the ground and the cloud reservoir of charge can go through another ionization episode (or strike). This is the flicker seen in a lightning strike.

 

A normal lightning path or bolt may be many kilometres long and is only as thick as a thumb. The speed of sound is 1 kilometre in about 3 seconds or 1 mile in about 5 second, and you can use the time difference between the flash and the bang to measure the distance between you and the strike. Lightning may be seen when it is 60 kilometres away, but thunder is only audible for maybe 10 to 20 kilometres. In the 19th century, thunder was thought to be caused by the implosive collapse of the partial vacuum of the lightning bolt. Nowadays, the consensus is that it is the shock wave of the sudden thermal expansion as the bolt forms plasma (air breaking the sound barrier). The rolling sound after the initial clap is sound arriving later from parts of the lightning bolt that are further away. Over land, rolling thunder also echoes off surrounding terrain.

 

80% of lightning discharges occur from top to bottom of a cloud, or between two nearby clouds (C2C or cloud to cloud). The ones that reach the ground are called C2G or cloud to ground.

 

I handle weather forecasts for around 400 yachts each year, and in the past year two have lost their navigation instruments due to lightning damage (both in tropical waters).  This seems to be a higher rate than the normal quote of 1 in a 1000 per year.

 

A yacht mast “spikes out from a flat surface” and thus tends to act like a lightning rod. Normally it is connected to the keel reasonably well by its rigging. Since lightning is static electricity it tends to flow on the outside of a surface, and so a metal mast and rigging should take most of the charge to the keel and surrounding seawater. Some extrapolate from that to say there is a cone of protection underneath the mast, but some say that’s a myth (see www.lightningsafety.com/nlsi_pls/cone-of-protection-myth.html). Installing a proper lightning rod and connecting this via copper plate to the keel may just divert some of the lightning charge so that it blows thru the copper plate, holing the keel. Opinions differ on this point.

 

When you see a lightning squall coming, the standard advice is to get the sails down and the engine going on auto helm, and standby in the companion way or below deck and cross your fingers. I’d like to add that you protect your electronic equipment: Disconnect your antenna, and gather any mobile navigation equipment such as GPS, satellite phone, tablet, laptop, etc., and toss them into the galley oven (or a microwave if you have one) as an approximation to a Faraday cage.

 

TROPICS

Tonight, we still have BERYL in the North Atlantic (now out of the tropics), and a “zone of interest” between Mexico and Hawaii.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Malaysia and Indonesia. This is part of a MJO event, a trend to increased convection spreading east into the NW Pacific. This is likely to trigger tropical cyclone activity around Micronesia over the next week or two.

- see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with a weak stretch across the Wallis and Futuna area.

A passing trough is expected to visit Tonga on Monday UTC and then travel east to visit Rarotonga on Thursday UTC and linger there until Sunday 22 July UTC.

 

Subtropical ridge (STR)

HIGH near 40S and east of NZ near 180 tonight is expected to slowly travel east along 40S this week. Associated squash zone of enhance d trade winds is expected to peak near 20 to 25S over Gambier Islands on wed to Fri 18 to 20 July UTC.

Next HIGH is expected to travel east along 25S across the north Tasman Sea on from Mon to Thu and then linger over northern NZ on Friday and Saturday UTC.

 

Around Tasman Sea, NZ to tropics.

Trough moving off NZ on Monday followed by a westerly flow. A weak trough is expected on Wednesday followed by a SW flow on Thursday.  Light winds with a passing ridge on Friday and then another trough is expected from the Tasman Sea on Saturday/Sunday.

 

Australia to tropics

A slow-moving ridge over the northern Tasman Sea this week.

 

Tahiti to Tonga

Winds around the Society island may reach a peak on local Sunday/Monday 15/16 July as a squash zone (associated with a passing HIGH) travels east.  For the remainder of the week there should be an OK east to ESE flow from Tahiti to Tonga with a minor squash zone along 25S.

South of 18S, A passing trough should reach Rarotonga area from Thursday/Friday UTC.

 

Galapagos to Marquess

SE to E to NE winds, mostly less than 20 knots. To use available currents, go to 2S 93W and then 6S 125W and then direct.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

08 July 2018

Bob Blog 8 July

WEATHERGRAM

YOTREPS

Compiled Sun 08 July 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

Acceleration zones

Terrain effects near mountainous islands interrupts the wind flow and divides it into rivers of wind and puddles of calm. Today’s EC (9km resolution) model as seen on windy.com shows this in the trade winds over Fiji, with an onshore seabreeze developing around Momi Bay. The EC model data is running with only a few real world observations but its portrayal closely matches the observed data at Nadi Airport.

When I was a forecaster in Fiji back in the 70s I discovered that whenever the pressure difference across Fiji gets around 2 hPa or higher (isobars drawn 2hPa apart, the winds in the accelerated zones such as Vatu-i-Ra passage, Bligh water, and the SW end of Viti Levu may be funnelled to over 25 knots. Today the EC model shows orange-coloured speeds in these acceleration zones, for 21-averaged knots or more. These zones are strongest over the sea, and it would be good to be able to position some wind stations on nearby land.

The Fiji MetService has several local rules they use for compiling the Fiji Marine forecast. This evening’s edition responds to that timeless “isobar rule”:

Marine Weather Bulletin

Issued from the National Weather Forecasting Centre Nadi

at 7:30pm on Sunday the 8th of July 2018

A STRONG WIND WARNING REMAINS IN FORCE FOR ALL FIJI WATERS.

Situation:

A high pressure system to the south of Fiji continues to direct

strong east to southeast wind flow over the Fiji waters.

 

Forecast to midnight tomorrow for Fiji waters:

East to southeast winds 20 to 25 knots. Rough seas. Moderate southerly swells.

 

Further outlook: Easterly winds 20 to 25 knots. Rough seas. Moderate southerly swells.

===

Interestingly, the GFS model, with its 13km resolution, and .25x.25 degree grib distribution does not capture these terrain effects:

 

TROPICS

Tonight, we have Storms THREE and BERYL in the North Atlantic and MARIA in the NW Pacific.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Japan (severe flooding there in the past week). Also, the rain across the South Pacific has become more intense. It remains very intense around central America.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to weaken during the coming week. It is expected to hover from Solomons to Samoa, and the zone over the Solomon Islands is expected to drift slowly south this week.

The trough which visited Samoa over the weekend is expected to travel SE across the Tahiti area on local Monday and fade over the Tuamotu islands on local Tuesday.

The next trough from the west is expected to reach the New Caledonia area on local Friday, Vanuatu on Saturday, and Fiji/Tonga/Samoa early next week.

 

Subtropical ridge (STR)

HIGH is travelling east along 30S to east of NZ this week. It should have a squash zone of enhanced trade winds on its Northern side, reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

Next HIGH is expected to travel across South Tasman Sea on Tue/wed UTC, northern NZ on Thu UTC and then to east of NZ along 35S from Fri UTC, fading near 170W by Sunday UTC.

 

Around Tasman Sea, NZ to tropics.

Large trough is crossing NZ on Monday and Tuesday with winds backing to SW then southerly and bursts of squally showers.

Too unsettled to depart on mon/Tue. OK to get away of Wed, but will need to allow for encounter with northerly winds ahead of the next trough

A Trough is expected to move off Australia and onto the eastern seaboard on local Thursday, and then deepen into a low between Lord Howe and New Caledonia on Friday. It is further is forecast to go southeast towards northern NZ over the next three-four days, turning in to a multi-centred system with centres below 1000hPa.

 

Australia to tropics

Stay put until after that low forms and moves away. Should be OK to go from around Sun 15 July.

 

Tahiti to Tonga

It may be better to stay put next few days – due to squash zone of enhanced trade winds reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

A departure on Thu/Fri UTC may be able to get as far as Palmerston Island before the next trough arrives from the west, maybe Niue.

 

Galapagos to Marquess

SE to ESE winds, mostly less than 20 knots. To use available currents, go to 2S 95W and then 6S 125W and then direct.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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01 July 2018

Bob Blog 1 July 2018

WEATHERGRAM

YOTREPS

Compiled Sun 01 July 2018

 

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

JUNE REVIEW

Sea Surface temperature anomalies as at end of June may be seen at www.ospo.noaa.gov/data/sst/anomaly/2018/anomnight.6.28.2018.gif

As has been the case for a few years now, the warm anomalies outweigh the cool, so there is a net discontinuity in our oceans. The eastern equatorial Pacific is the focal region for ENSO and is now on a slowly warming trend. Temperatures around Australia and into the Tasman Sea are about to below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America still stands out as warmer than normal.

Warm anomalies continue in the north Pacific indicate a busy cyclone season for next few months.

To see how the annual weather cycle and the seasons are working out, here is a quick look at the average isobar maps for past 30 days from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

 

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust. The North Atlantic and Northeast Pacific HIGHS are looking less strong than this time last month. The June Lows in the Tasman Sea have been less than the May Lows.

 

Polar westerly winds are stronger around Antarctica from NZ to South America but weaker than normal from South America to South Africa and around Australia to New Zealand. It’s complicated.

 

Zooming into the NZ area, the 1015hP (between blue and white) isobar has stayed put in the tropics and travelled east across NZ. It’s subtle, but observable. This indicates a change from a westerly flow to a (cooler) SW flow:

The last 30 days of rainfall, and its anomaly, as seen at TRMM at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

shows extra convergence in the ITCZ from Bangladesh to Philippines , across s the North Pacific Ocean, around the Mexican west Coast and across the equatorial Atlantic, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere there are large dry regions: Around Madagascar/ Western Indian Ocean, Vanuatu to Southern Cooks (the normal position of the South pacific Convergence zone), and the Amazon.

 

The Indian Monsoon 2018 arrived faster and earlier than normal, as seen at www.imd.gov.in/pages/monsoon_main.php

 

 

TROPICS

Tonight, we have a TC off the west of Mexico (FABIO) and in the China Sea (PRAPIROON). Also, the remains of EMILIA are travelling to the NW, leading the way for FABIO.

 

If we compare the past week’s rain with the previous week we can see that the intensity of the rain in the South Pacific Convergence Zone and around central America is intensifying.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from northern Coral Sea /Solomon Islands to northern Vanuatu, to Samoa to Southern Cooks. A small low should form on the SE end of the SPCZ, to SE of southern Cooks, on Tuesday 3 July UTC and deepen as it gets further SE.

And a small but rather intense passing trough/tropical low os expected to form over Wallis/Futuna are on Thu/Fri UTC and travel over Vava’u are on Fri UTC and Niue/Southern Cooks area on Sat UTC. Avoid.

 

Subtropical ridge (STR)

New HIGH is expected to spread into the Tasman sea on Mon/Tue and then travel NE across northern NZ on Wed/Thu and further NE next weekend.

 

Around Tasman Sea, NZ to tropics.

Looks Ok for a voyage for NZ to tropics any day this week.

Small low is expected to form off Queensland coat on Monday and fade in situ by Thursday. Strong disturbed westerly flow is expected to spread east from Tasmania/southern New Wales from Wednesday, reaching South Island of NZ by Friday.

TROUGH that crossed NZ on Sunday is expected to deepen into a LOW over Chatham Islands on Monday UTC that is expected to go NE on Tuesday/Wednesday UTC and then east/southeast. Avoid this Low.

 

Australia to tropics

Not this week, too much head wind. But an opportunity may be appearing around Sat 7 July.

 

Tahiti to Tonga

A dose of strong SE winds is expected to affect northern parts of this area on Monday and Tuesday UTC. And on Fri/Sat UTC a small but intense low/passing trough is expected to affect the Vava’u to Niue area. Next week may be better.

 

Galapagos to Marquess

SE to E winds, mostly less than 20 knots. To use available currents, go to 2S 95W and then 5S 120W and then direct.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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