BOBGRAM issued 7 July 2013

WEATHERGRAM
YOTREPS
Issued 07 July 2013

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.

Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin. It rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, maintaining a value over 1 for most of June. By 7 July the SOI relaxed to 0.74 .

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet. So far this year NINO3.4 has been on the cool side, but not strongly so, and during June it has relaxed closer to normal.


GLOBAL TROPICAL TOPICS
Off the Mexican west coast, TC DALILA has weakened and TC ERICK has formed and is heading for Baja California and expected to weaken.
Over Asia, Monsoon rain has had an average week after a few abnormally wet weeks.
ITCZ is north of its normal position and has had some active rain recently in Micronesia, after several dry months.

WEATHER ZONES

Blocking anticyclones have been the features which kept Lows over NZ last month producing abnormal wind rain and snow. The blocking index was strong then and is weak now, setting the stage for a good period of disturbed westerlies over southern NZ in recent days.

South Pacific Convergence Zone SPCZ
The SPCZ has retreated north to the Solomons area is the past week, and has second branch at present from Samoa to Suwarrow to Tuamotu. This second branch is expected by Wednesday to shift south from Samoa to Tonga and to weaken around Suwarrow/Tahiti. Around Friday and Saturday 12/13 July part of the SPCZ may wander southeast across Vanuatu and Fiji. Otherwise it is expected to have a quiet week.

Sub-tropical Ridge STR
Next anticyclone to follow the path set by the STR is currently over central Australia and should go east along around 30 to 35S. However, it is likely to be delayed in its progress across the Tasman Sea and finally 'squeeze' its way across central NZ on Sat/Sun 13/14 July.
Between this slow-moving high and the SPCZ, a squash zone of enhanced SE winds is expected to form in the Coral Sea. Avoid until Tuesday 16 July.

Roaring 40s and New Zealand
A series of deep Southern ocean Lows have been travelling east past the south of NZ recently bringing bursts of wind and swell, as is typical during what we call high index weather with the absence of blocking. .The coming week is likely to be different with a swing to what we call low index weather, thanks to the southerly flow on the eastern side of the High approaching from Australia. This southerly has already managed to close the roads out of Queenstown with snow and slips, and is expected to travel north across NZ on Monday and Tuesday, deepening into a slow-moving low over northern NZ by Wednesday that is expected to dance around there until there until Sunday 14 July with strong to gale SE winds on its south side. Avoid.

Tahiti to Tonga:
The challenge on this voyage is to go through or around the SPCZ. This is expected to weaken by over Tahiti by end of Tuesday but some lingering showers and fickle winds may linger in the region on Wednesday 10th, and the SPCZ over Suwarrow should weaken from the 10th. So voyages may be more comfortable for Thursday 11th July (rather than before then).

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