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

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.

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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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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.

 

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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

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

 

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

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

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. .

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

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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