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

25 November 2018

BobBlog 25 Nov

WEATHERGRAM

YOTREPS

Compiled Sun 25 Nov 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.

 

A friendly KIWI welcome to all the visiting yachts who have recently arrived in NZ.

When you are sailing around our cruising grounds you may wish to check out our coastl weather:

Where to get NZ weather

MetService has 18 Coastal areas and 11 Recreational areas and produces marine forecasts for these compiled by a dedicated team of specialized marine forecasters. These forecasts are updated four times daily and carry an outlook covering the following three days.  They are available at www.metservice.com,

or for your mobile at m.metsrvcie.com.

or download the MetService Marine Ap: see more at

about.metservice.com/our-company/ways-to-get-the-weather/weather-on-your-mobile/smartphone-apps/

or via MetPhone at

about.metservice.com/our-company/ways-to-get-the-weather/metphone/

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If you have access to email when at sea the you can download the text of a coastal or recreational marine forecast by sending a email to query@saildocs.com, no subject needed, with message

SEND http://m.metservice.com/marine/coastal/NAME

Or SEND http://m.metservice.com/marine/rec/NAME

where NAME is the name of your desired region (e.g: brett or bay-of-plenty or lake-taupo)

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Marine Radio

Taupo Maritime runs ZLM radio HF/SSB/SW stations on 2207,4146,6224, 13356 and 16531 kHz in English

Maps are sent via Radio Fax on the following frequencies 3247.4, 5807, 9459, 13550.5, and 16340.1

Schedules are on the MetService website

NOWCASTING: New Zealand Coastguard Weather information (Nowcasting) is available via VHF for 15 places around the North Island and 5 places around the South Island.

See the map at

www.coastguard.nz/media/418138/20161019_final_coastguard-nowcasting-weather-information-channel-map.pdf

or download their app: see more at 

www.coastguard.nz/boating-safely/coastguard-app-the-boaties-best-mate/

 

Trip planning

A lot can be said about the usefulness of windy.com for checking if it may be good to go sailing over next few days. 

During the past week one of my regular readers shared with me his blog on trip planning

See twoatsea.com/that-time-of-the-year-weather-watch-2018/

Weather enthusiasts are always seeking new web sites for delving into the weather, and the best I have come across I heartily recommend is weather.geek.nz/

In particular, when planning for the week ahead, check out weather.geek.nz/nz_model_tiles.php

The weather geek site contain many weather treasures, have fun browsing.

 

El Nino:

 The latest sea surface temperature map shows an anomaly of warmer than normal seas along the equator in the eastern equatorial Pacific.   This anomaly is getting stronger and when it becomes strong enough the event is called an EL NINO.

 

Now, as we move into December we can be reminded of the origins of the naming of this tropical anomaly as El Nino.   The full title is “El Nino di Navidad” or the Christ Child of the nativity.  It got this name because in Peru, once every few years, rain fell over the usually very dry interior. These rains arrive along with warmers seas along the Peru coast around Christmas time and so the event was named after the festival they were celebrating (Spanish is the leading language in Peru).   In moderation the rains are a blessing, and the title seems fitting.   But in the past few decades an El Nino brings flooding and landslides to Peru, It also makes the anchovies harder to catch, as these fish stay in the cooler waters, and in an El Nino the top of the ocean becomes coated with warmer seas from equatorial regions, so the anchovies go so deep they are out of reach.

 

This incoming El Nino is already showing itself in Peru:

www.freshplaza.com/article/9044580/effects-of-el-nino-landslides-in-peru-tornado-in-argentina/

 

NIWA have stated that this summer’s incoming El Nino is likely to behave differently from normal.  The warmer seas along the equatorial Pacific that make up an El Nino normally occur near South America, but this year they seem to be occurring more towards the central Pacific.  This is known as a central-based El Niño, or El Niño Modoki. Modoki is a Japanese word that means "same, but different".

See www.scoop.co.nz/stories/SC1811/S00066/update-on-el-nino.htm

The unsettled weather we have been having during November is typical of an incoming El Nino. We can use this as an indicator that the coming months may bring the Bay of Islands more days with southwest winds than normal, but there will still be plenty days with sea breezes or with northeast winds.  Maybe around a third each way.

 

THE TROPICS

Latest cyclone activity and TCFP tropical Cyclone Formation Potential are seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

MAN-YI is heading towards Japan and is expected to peel off to the east of Japan.

 USAGI  is expected to make landfall near Ho-Chi-Minh city.

Looking at the weekly rain maps, last week’s shows the tracks of MAN-YI and USAGI, and also an intense buildup of convection along the equator near 140E, just north of PNG.

This is weakening at present, but the forecast is for a robust MJO event to move into the Pacific over the net two weeks, and that may indeed increase convections along the South Pacific Convergence zone.

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

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to strengthen between Solomon Islands and north of Fiji this week.  There may be a tropical low forming near Vanuatu next week in early December.  The convergence zone between Samoa and French Polynesia this week is expected to weaken away.

 

Subtropical ridge (STR)

The STR has been shifted to around 20 to 35S and is very weak.  One small High is expected to travel eastwards from New Caledonia to south of Tonga from Tuesday to Friday this week.

Tropics to New Zealand

A series of trough and lows are continuing to cross NZ, with major lows over the North Island on Tuesday 27 and Sat 1 Dec this week.  There should be an interlude of relative calm on local Thursday. Also, there should be more settled weather for arriving in NZ from Mon 3 Dec.

 

Between Tropics and Australia.

Low from interior of Australia is expected to deepen as it moves into the Tasman Sea by around Thursday and then travel to northern NZ by Saturday. This offers opportunities for sailing from Australia to New Caledonia, but closes opportunities for sailing t’other way this week.

From Tahiti to Tonga

Looks OK with easterly winds, something light.  However, there is expected to be a weak trough traveling east to south of 20S reaching Tongan area around wed UTC and then Southern Cooks area around Sat 1 Dec, with light variable 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

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

18 November 2018

Bob Blog 18 Nov

WEATHERGRAM

YOTREPS

Compiled Sun 18 Nov 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.

 

SAM and the Polar Vortex

The Southern Annular Mode (SAM) is a parameter that measures the westerly winds that circle Antarctica. When it is positive the belt of strong westerly winds contracts towards Antarctica—so there a higher pressures and weaker winds over southern Australia/ New Zealand. When the belt moves north towards NZ, SAM becomes negative, and storms from the Southern Ocean can reach NZ.

 

One way of thinking about SAM is that it is a measure of the strength of the dam that walls the cold air over the Antarctic. When positive this wall is strong, and when negative this wall is weak and polar air can make be diverted outwards and northwards, maybe affecting New Zealand.

When SAM drops suddenly from positive to negative, there is an increased possibility of a “polar outbreak” somewhere in the southern hemisphere.

 

A light-hearted and animated presentation of SAM as one of the climate dogs that affect New South Wales may be seen at www.youtube.com/watch?v=G-S-YmE-Lkc . To meet the whole dog pack see www.dpi.nsw.gov.au/climate-and-emergencies/droughthub/information-and-resources/seasonal-conditions/climatedogs

 

A good proxy for SAM is the Antarctic Anomaly Oscillation (AAO) as seen at www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html ,this shows that SAM is forecast to dip negative this week.  It's not a huge dip, but does make SAM as negative as it was in mid-October, and if we look at the air temperature maps on a polar stereographic projection using earth.nullschool.net we can see cold air spreading north between last Thursday and tonight, turning all NZ dark green.

This helps explain the coldness of the front crossing New Zealand early this week. The abrupt temperature change helped form a tornado over the Canterbury Plains this afternoon:

see www.newshub.co.nz/home/new-zealand/2018/11/watch-witnesses-in-awe-of-massive-tornado-north-of-ashburton.html

 

THE TROPICS

Latest cyclone activity as seen at tropic.ssec.wisc.edu

GAJA continues west after crossing southern India during the past week.

There is also BROCHRA near 12S 84W going WSW.

Looking at the weekly rain maps, last week’s shows a split in activity over the Indian Ocean away from the equator, an easing in the activity over the South Pacific and an increase in activity over the tropical Atlantic.

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 and extend from east of the Solomon Islands to between Fiji and Samoa. It is likely to produce a trough that may visit Tonga mid-week and Niue area on Wed UTC, fading over Southern Cooks on Thu UTC.

 

Subtropical ridge (STR)

Since SAM is going negative , the Subtropical ridge is retreating to the north.

A High is expected to form near 25S between New Caledonia and New Zealand by mid-week and then travel eastwards to fade over Southern Tonga by the weekend.

 

Tropics to New Zealand

A series of trough and lows are expected around New Zealand this week. There should be an interlude of relative calm on local Friday.

 

Between Tropics and Australia.

OK at first, but maybe a trough travelling from Australia interior across the coast and into the Tasman sea on or around late local Thursday followed by west to southwest winds.

 

From Tahiti to Tonga

Weak trough passing over Tahiti on local Sunday, then looks Ok to go with eastly wind that may last over a week to north of 18S. Further south there is another passing trough to take into account.

 

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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

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

11 November 2018

Bob Blog 11 Nov

WEATHERGRAM

YOTREPS

Compiled Sun 11 Nov 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.

 

HOW TO ACTIVATE A FRONT

I once send a wedding telegram along the lines of “Beware: whatever promotes upward motion can lead to frontal development”. I was reminded of this last Thursday when the front crossing the South Island turned into a monster, dropping around 700mm of rain into Cropp River (Southern Alps , between Mt Cook and Hokitika). The torrential rain mainly fell over the Southern Alps. Rivers flooded, and  the main highways were closed by slips and a washed out bridge.

 

Meteorologist know that the most important equation they need to understand is the omega equation. Basically, if you tap your barometer and it dips the you know you have falling pressure, and this is linked to rising motion/increased wind/ heavier rain. The equation explains how: it relates omega (change of air pressure over time) with changes in vorticity advection and thermal advection (I won’t explain these in much detail tonight, except that CVA (cyclonic vorticity advection) and WA (warm advection) result in negative omega (upward motion/ thicker clouds), and AVA (anticyclonic vorticity advection)/CA (cold advection) result in positive omega (sinking motion/clearing sky).

 

The recent weather event over New Zealand shows what happens when a well-defined upper trough with a strong jetstream travels eastwards faster than the associated front near the surface.

 

1. On Wednesday night, the jetstream was west of the surface front

as seen in an Isobar map 1am Thursday courtesy of MetService with red arrow showing jetstream.

 

2. Then by 1am Friday the jetstream moved on top of the front.

This was the day the front dumped its heaviest rain. There was also a “tropical pipeline” or moist conveyor belt feeding the front with moisture from the heat trough over Australia. Our own version of the pineapple express.

 

3.  And  by 1am Saturday the upper trough buckled, so that the jetstream stopped advancing over the South Island, and the front travelled off to northeast of the jetstream and decayed to a trickle.

 

Between 1 and 2, the CVA and WA reached high levels over the front, turning it into a monster (being over the Southern Alps also helped promote its upward motion). Then between 2 and 3 CVA and WA dropped off and the front faded.

 

Our global weather models have the ability to watch all these atmospheric equations with precision and can handle the forecasting of these events well in advance. Interestingly, if we add an extra 0.5 C to the air temperatures and re-run the global model, we can end up with as much as an extra 10% in the resulting rainfall.

 

Note that the subtropical ridge over the South Pacific was sufficiently north of the North Island to allow this front to spread over all of NZ. This is typical of EL NINO and is likely to become a trend over the next few months (but not the coming week). One positive response to the incoming El Nino is the hydro lake levels in NZ. Lake Pukaki was dropping in past few months but is now rising nicely: see www.meridianenergy.co.nz/about-us/our-power-stations/lake-levels

 

THE TROPICS

Latest cyclone activity as seen at tropic.ssec.wisc.edu shows that it is busy at present in the Indian Ocean.

ALCIDE is expected to fade or divert to the north of Madagascar.

GAJA is forecast to travel across southern India.

 

Looking at the weekly rain maps, last week’s shows high intensity rainfall in the Indian Ocean, and an increase in activity over the Solomon island to Samoa/Niue. and a relaxing of convection in the Intertropical convergence zone across the Pacific.

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

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is active from Solomon Islands to Tuvalu and is expected to produce a trough that should visit Fiji on Wednesday, Tonga on Thursday, Niue on Friday UTC and then go off to the SE.

 

Subtropical ridge (STR)

Unlike last week, the STR for the coming week is expected to be around New Zealand until Thursday. Another High is expected to travel across the South Tasman Sea from Saturday 17 to Mon 19 Nov.

 

Tropics to New Zealand

There is expected to be a squash zone of enhanced SE winds between Fiji/Tonga and NZ especially near 25 to 30S from Tuesday to Thursday. Further south, it looks Ok for arriving in NZ over next ten days. Trough from the south Tasman Sea is expected to spread onto South Island on Thu and then fade over the North Island on Friday.

 

Between Tropics and Australia.

Looks mostly OK, but maybe a trough offshore Coffs on Friday/Saturday with southerly winds near the coast.

 

From Tahiti to Tonga

Trough over Tonga on Thu 15 UTC and Niue/ Southern Cooks on Friday 16 UTC then travelling off to the SE. North side of this trough may reach Tahiti around Sun 18 November. Trough is preceded by NW winds and followed by a period of W /SW winds.

 

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

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

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

04 November 2018

Bob Blog 4 Nov 2018

WEATHERGRAM

YOTREPS

Compiled Sun 4 Nov 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 LAST MONTH WEATHER PATTERNS

Sea Surface temperature anomalies as at start of November 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 steady warming trend. Cyclones have been stirring the seas between Mexico and Hawaii, making avenues of cooler surface water. There is a zone of warmer seas from Solomon Island to Samoa. Temperatures around Australia and the Tasman sea remain below average, a possible indicator of drier than normal conditions in the next month or so. Also, this should cool the southwest winds that reach New Zealand.

The Gulf Stream off the east coast of North America and the Kuroshio current off Japan still stand out but not as much as last month.

 

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

The subtropical ridge in the southern hemisphere has intensified during October

 

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

The rain map shows extra convergence around equatorial Indian Ocean.

 

THE TROPICS

Latest cyclone activity as seen at tropic.ssec.wisc.edu

shows XAVIER off the west Mexico coast, and it is expected to stay offshore.

Looking at the weekly rain maps, last week’s shows an increase in activity across the equatorial Indian Ocean, and a relaxing of convection around Papua New Guinea and the Mariana Islands. Not much change elsewhere.

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

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is sitting over northern Coral sea and northern parts of Vanuatu across Wallis and Futuna to northern Tonga, and part of it may travel southeast to Southern Cooks this week.

 

Subtropical ridge (STR)

High is travelling east along 25 to 35S past northern NZ from Monday to Friday and the expected to linger to east of New Zealand until early next week.

 

Tropics to New Zealand

Trough from the Tasman Sea is expected to spread onto central New Zealand by Fri 9 Nov and then stall, so that a zone of cloud and rain with light winds spreads onto northern NZ by Tue 13 to Thu 15 November. It looks OK to arrive in NZ with this trough.

 

Between Tropics and Australia.

Avoid a passing trough across the northern Tasman Sea on Wed and Thursday with light variable winds.

 

From Tahiti to Tonga

There may be a trough forming near Niue around Sunday 11 November, otherwise the forecast is for useful easterly winds.

 

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

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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