BOBGRAM7 issued 24 July 2011

WEATHERGRAM
YOTREPS
Issued 24 July 2011
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.

TROPICS
Southern Oscillation Index SOI continues to hover slightly above zero; its 30day running mean was 0.5 on 16 July and 0.6 on 22 July.

Galapagos to Marquesas: The sea around Galapagos now seem to be reverting to normal temperatures. If you are still looking at this voyage, it seems that the direct path is as good as any other. Some showery activity is occurring on-and-off around Marquesas.

South Pacific Convergence Zone SPCZ is having a weak period. There is one band that is active over Solomons and then weakens as it extends SE to east of Vanuatu and about or to Southwest of Fiji. There is also a weak convergence zone in the French Polynesia FP region.

Not much change with the SPCZ during the coming week, except that a trough may form on it around or to south of Fiji/ New Caledonia on 31 July.

SUBTROPICAL RIDGE STR
The high that is east of Tasmania tonight 24 July is shoveling a polar chilled southerly flow onto eastern NZ. It is showery stuff but with enough snow and ice to impede traffic. This High is expected to travel NE across Tasman Sea on Mon and then, when it gets to 30S on Tuesday, to extend itself east along 30S from 160E to 150W, intensifying the trade winds in the tropics and the westerly flow on its south side.
The SE trade winds in the tropics are likely to intensify over the whole South Pacific and reach a peak around Wed/Thu 27/28 July due to this rebuilding of the STR.
The STR may be reinforced by another high cell making its way across central Australia and budding off at 30S around Thu /Fri 28/29 July.

This is a typical weather pattern.

Next weekend: as this STR goes east and when air-pressures start to fall between Fiji/ New Caledonia and NZ: a trough is likely to form – GFS has this by 31 July, and EC has it, at this stage, around 1 Aug. For the EC – see http://bit.ly/ecoz .

'Roaring 40s and a deep low' is the expected pattern for Aussie Bight next week —no sign of the next High until end of next week.

TASMAN SEA/NZ AREA
Polar chilled southerlies should turn to be from the SW and weaken on Monday as that high migrates across the Tasman Sea.

A disturbed westerly flow is then likely for Tue 26 to Sat 30 July, with an embedded front on Wed and Friday.

SAILING TO/FROM NORTHERN NZ.
Useful winds on Tuesday to Saturday--- but check again on the possible trough to north of NZ next weekend, and maybe avoid the fronts on Wednesday and Friday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz