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

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.

 

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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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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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