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

10 September 2011

BOBGRAM issued 11 Sep 2011

Issued 11 September 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.

Three cyclones (KATIA, MARIA and NATE) in the North Atlantic and some areas of interest to wets of Mexico…so Northern Hemisphere Cyclone season is still raging.

The South Pacific this week offers trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S. The daily maps look reasonably close to seasonal norm, but just for this week.

The tropical ocean temperatures in mid pacific are near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos. This has prompted USA's Climate prediction centre to call this a new La Nina, but Australian and NZ climatologists are happy enough to call it neutral - let's compromise with neutral + bursts of La Nina at times.

From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.46 on 2 Sep and .49 on 10 Sep. There have been some lingering La Nina weather patterns in the atmosphere--- the STR was further south than normal earlier this month, but is near its normal latitude this week.

No large highs this week in our area, just a weak STR along about 25S. Next high should be moving off Australia and along 30S into northern Tasman Sea from Thu 15 Sep to crossing NZ on 22/23 Sep. This provides a good window for sailing westwards across the north Tasman or eastwards across the mid Tasman Sea.

Rising pressures this week over eastern Australia are helping to enhance strong SE winds in the Coral Sea.

The Low that crossed southern NZ today in followed by a period of squally SW wind and heavy swells in the Tasman/NZ area until at least Wednesday 14 Sep, and these then spread eastwards of NZ as a disturbed westerly flow. Avoid.

Another trough is likely to deepen in the central Tasman Sea on Friday 16/ Sat 17 Sep and cross North island on Sat 17/ Sun 18 Sep then move off to east of NZ - but this feature is still somewhat uncertain, so if it concerns you then get updates on it .

The Island Cruising Association ICA fleet are starting to think about their All Points Rally from Tonga/Fiji/Vanuatu/New Caledonia to Opua. Well, my idea about the best time to leave from any of these places is so that you encounter a front at 30S – one that is followed by a mediocre ridge. The reasoning is that this voyage takes around a week and fronts in this part of the world have a frequency of at least once a week, sometime more frequent. The disturbed westerlies over Northland this week do not offer much opportunity for this pattern. So let's just enjoy another week of tropical bliss this week, and check in again next Sunday/Monday.

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