BobBlog

WEATHERGRAM

YOTREPS

Issued 29 June 2014

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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Kiwis can brace for a real bit of winter on Wednesday and Thursday this week. This isn’t going to be “southerly eggbeater” but is expected to be a polar outbreak, good for snow for the ski-fields, just in time for the school term 2-week break from 4 July.

The polar vortex is the name given to that westerly spin of wind around Antarctica, holding the cold in the south. Sometimes the vortex weakens and that allows high cells to snap free from the ice shelf and travel to the northeast, dislodging ice-chilled air and shoveling it northwards—that’s what I mean by a polar outbreak,

NIWA use a parameter called the Sothern Annular Mode, or SAM, to measure the strength of the polar vortex. When this dives suddenly negative the chances of a polar outbreak increase. Sometimes, such as this week, the polar outbreak gets into the Tasman Sea. I don’t have any real time access to the SAM index, but a good proxy for it is the AAO or Antarctic Oscillation, and its expected future trend is shown at

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html

and this shows a sudden dive into the negative on 1 to 3 July, but not for long.

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SO by noon Wednesday most of the inland South Island is likely be have air temperatures below zero (purple line), and in many places be wet enough to snow.

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MetService 3-day outlook for noon Wednesday as at

http://www.metservice.com/maps-radar/rain-forecast/rain-forecast-3-day

TROPICAL TOPICS—

There is a lot of convective activity this week off the west coast of Mexico.

And there is still less than normal convective activity over India, Reports are that the monsoon is over 40% deficient over much of central India.

All of this shift of convection is consistent with an observed warming of the sea in the eastern equatorial Pacific Ocean as measured by the NINO3 index

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NINO3.4 as shown at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34

NINO3.4 index measures the difference from normal of sea temperatures in the EEP (eastern equatorial Ocean) and this has been on the rise in recent weeks, but still hasn’t swung the SOI index (in the atmosphere) towards El Nino.

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Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing isobars, Lows and Highs, wind, waves, rain, current, SPCZ and STR . Purple lines are upper flow, light brown arrows are wave directions. .

SPCZ= South Pacific Convergence Zone

This continues to intensify in the Coral Sea to Tokelau area and is expected to drift south towards Vanuatu /Futuna and maybe New Caledonia on 5/6 July. A weak convergence zone is likely to linger over the west end of French Polynesia during Monday and Tuesday UTC.

STR= Sub-tropical Ridge

The STR is in its normal latitude position of 30 to 40S this week. One High cell that was over northern NZ on Sunday is expected to travel east along 30S this week. The next high leaving Australia on Thursday is likely to be weaker than its predecessor and diverted to 30S to north of NZ by Saturday 5 July.

Departing from NZ to the tropics

Large trough is crossing Tasman Sea and NZ over next few days, the first front is expected to cross NZ on Monday, preceded by N /NE flow, and followed by squally NW/W winds. Then the cold SW/S winds from the ice at 60S are expected to spread across NZ on Wednesday and Thursday.

SO Friday looks to be the best weather pattern for departing Northland this week.

Departing westwards from Tahiti:

There is a lingering weak convection zone around Papeete on Monday/Tuesday UTC and another is expected around Sat/Sun 5/6 UTC and these zones weaken the trades and may provide a few tropical squalls, Between these the weather pattern is looking good to go.

See my yotpak at boatbooks.co.nz/weather.html for terms used.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz

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