BOBGRAM issued 16 June 2013

WEATHERGRAM
YOTREPS
Issued 16 June 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, reaching 1.2 from 9 to 16 June. So the signal coming from the Atmosphere has been erratic and at present is tending towards La Nina.

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus the Sea surface temperatures SST across the equatorial Pacific may be thought of as a factor in the running of planetary weather engine. Its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet. In the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is a possible sign of a trend towards La Nina conditions, but we still do not have enough evidence to say this trend will continue.

Tropical cyclones
No tropical cyclones around at present, but there is a tropical Low in NW Pacific, and this is the region with the highest risk of formation during the coming week.
The outlook for later in June is for the heavy rain that is now over India/Singapore to migrate eastwards.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is stretched thinly along 10S, with a small branch between Vanuatu and Fiji that is fading, and another branch over French Polynesia that is linked to a Low that is expected to form near 30S 140W and then move southeast.

Sub-tropical Ridge STR
Highs within the STR are weak at present. The high that is likely to be over Tasmania on Wednesday 19 June is expected to weaken away in the Tasman Sea on Thursday.

Roaring 40s and New Zealand
Warm front of a large low system crossed NZ over the weekend. The Low is expected to have multi centres and it crosses NZ mid-week, bringing SW squally gales to the area north of NZ.

On Thursday the Low centre is expected to be east of NZ and this should combine with the weakening high then in the south Tasman Sea so that these systems act like an eggbeater focusing cold strong southerly winds fresh from the chill of the southern ocean to the midriff of NZ.

Route weather picks
Tahiti to Tonga:
Looks like a good week to go, with a quiet SPCZ, but will need to take the northern route via Suwarrow to get a useful trade wind.

NZ to the Tropics.
The mid week gale over northern NZ makes this a 'stay-at-home' week. Maybe ok to depart northern NZ again on Friday or Saturday 21/22 June.

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