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

29 July 2012

BOBGRAM issued 29 Jul 2012

Issued 29 July 2012
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.

The Southern Oscillation Index or SOI continues to relax. Its 30-day running mean started July at a year-to-date low of -1.21, and had reverted t0 +0.27 by 28 July.

Monsoon continues to rage over India /Malaysia and Singapore. There are also some deluges in northern China.

In the South Pacific, the Convergence Zone SPCZ has had a fairly quiet week, as a slab of dry air (possibly dropping down from the stratosphere, downstream of the monsoon) has been making its was slowly eastwards across the South Pacific.
There have been squalls in a convergence zone across Solomons but these have been rather weak over Vanuatu and to south of Fiji. A trough on this zone is likely to deepen into a low between Vanuatu and Fiji by Thu 2 Aug and then deepen as it moves SE past Minerva on Fri 3 Aug. Avoid.

There has been another line of convergence from Tuvalu to central Cooks and the trough associated with this is expected to cross Southern Cooks on Mon/Tue and weaken over French Polynesia on Wednesday/Thursday 1/2 Aug in association with a deepening low moving from 25 to 30S.

High east of NZ near 170W tonight is expected to reform a cell of high pressure between 20 and 30S at 160W on Monday. This new cell should go east and fade away on Tuesday and Wednesday.

High is southern Bight is having a difficult time to get east. Its normal path into the Tasman Sea is blocked by a Low and so it is likely to extend a tongue southwards across Campbell island on Thursday and then off to the east of NZ along 45 to 50S.
This High is expected to move onto central Australia by Thursday and another tongue of High pressure from this high is likely to extend along 20 to 25S across New Caledonia on the weekend 4/5 Aug and early next week.
This all helps maintain strong SE winds in the Coral Sea this week.

NZ/Tasman Sea
That low which is tonight and on Monday pushing a frontal trough across northern NZ is also deepening and likely to get its central pressure below 980 to west of NZ. By then the Low will have expanded vertically and the upper part of it will trap the lower portion so the system is likely to look around the Tasman for the remainder of the week, finally crossing the North Island around Mon 6 August when the upper low weakens.

The original frontal zone does have a link with the subtropics, but that slab of dry air mentioned above is interfering with the supply of moisture so it isn't as wet as the frontal trough which brought flooding a few weeks back. However the low itself is a breeding ground of instability, so there may well be some severe thunderstorms especially over the North Island during the week.

Traveling towards New Zealand this week:
Lots of squally westerly winds to contend with around the northern side of that low, so take care.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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