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

24 June 2012

BOBGRAM issued 24 June 2012

WEATHERGRAM
YOTREPS
Issued 24 June 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 has been steadily dropping into the negative during June. It has fallen from -0.14 on 27 May to -0.96 on 24 June.

Indian Monsoon is about a week behind its normal position at present, and seems to have reached its peak intensity which is near normal. It is taking a while to penetrate NW India.

In the South Pacific, the Convergence Zone SPCZ is slowly rebuilding in the north after a few weak weeks. It is active across Solomons to Tuvalu and Tokelau then weak over the Cooks. Several lines of convergence are lingering around French Polynesia FP. There is a slab of very dry air over New Caledonia to Samoa with its heart over Fiji. The SPCZ is expected to start heading south this week so that the dry slab should slowly fade.

There is a Low developing south of the Cooks near 25S 160W by Monday and this is feeding off some of those convergence lines over French Polynesia, and slackening the breeze over the Cooks. It should wander off to the south-southeast on Wednesday and Thursday allowing a southerly wind to clear conditions over FP and SE trades to return to the Cooks.

As the SPCZ drifts south into the Coral Sea, a Low may form there on Wednesday and Thursday, mature on Friday and then move south of Saturday and southeast on Sunday when it may weaken over New Caledonia. Avoid this low when it is mature.

SUBTROPICAL RIDGE STR
Large High between 35 and 40S to south of FP is the one that left NZ last week and it continues to move east this week. It has expanded so that its central pressure in now over 1036, and it is expected to hold this intensity until Wednesday and then start to fade.

Next high is likely to move from central Australia into the Tasman Sea at around 35S on Tuesday/Wednesday and this should travel east across northern NZ on Friday and then continue further east.

NZ /Tasman Sea troughs
Between those two Highs a series of fronts are crossing the NZ area. One of these did so over the last two days, greasing the way for others to follow. The front that is expected to cross NZ on Tuesday should usher some polar chilled into the Tasman Sea, producing a rapidly deepening Low that is expected to drop to below 960hPa just east of Campbell Island, making severe gales over Southern NZ. The SW winds that follow the accompanying front are likely to bring heavy SW to Southerly swells all the way to 20S between New Caledonia and Tonga on Thursday and Friday. This will be long period swell, so slope may not be much of problem expect around the Lord Howe Island area where there may well be a squash zone of enhanced winds between the Tasman High and Coral Sea Low. Avoid that.
Another trough is expected to cross Tasman Sea on Sat 30 June and then NZ on Sunday 1 July; this front may be thundery and is likely to be followed by strong showery SW winds on Monday 2 July.

Traveling between NZ and the tropics:
Best days to depart from northern NZ this week are Monday and Thursday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

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