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

08 April 2012

BOBGRAM issued 8 April 2012

WEATHERGRAM
YOTREPS
Issued 8 Apr 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 had an interesting week starting off around plus 0.5 (30-day running average) and ending up on 8 Apr at minus 0.6. We are at the time of year when overhead sun in moving into the northern hemisphere, and any change in SOI at this time tends to set the trend for the next year, so maybe we are in for an El Nino (consistent negative SOI). Still too early to say fully, but if so then trade winds may start easing, making tropical travel over next few months….interesting.

South Pacific Convergence Zone SPCZ has been active from north of Solomons to Fiji. There has been another branch mainly along 10S across Tuvalu and Northern Cooks/French Polynesia – this is the northern extent of the SPCZ, and a favoured position in an El Nino event, so maybe another pointer in that direction.

This recent MJO that brought extremely heavy rain to Fiji has now done its dash, and we are likely to go through a period of less than normal activity in the tropics over the next few weeks so this is an indication that we are that's the end of the southern hemisphere cyclone season. Voyaging sailors can now start thinking about sailing to the tropics.

Galapagos to Marquesas: Showers and convergence zones have eased away so now so it's time to go. Indications are OK this week for ignoring the equatorial current and taking the southern route—get quickly to 5S to catch the easterly winds to go west. There are stronger Easterlies further south with bigger seas so not so comfortable. 5S is a good compromise.

SUBTROPICAL RIDGE STR
High east of NZ has helped to keep the Low that has been NE of NZ away from the most of the mainland over Easter. High cell over Tasman Sea should spread over northern NZ on Monday and Tuesday and the main high in the east should migrate east along 40S, reaching 1032.

A big Fat high BFH is moving across the Aussie Bight. It is a slow-mover and should peak with central pressure around 1038 near 40S on Tue 10 April. It should spread across Tasman Sea/NZ along 37S from Fri 13 to Mon 16 April.

There are zones of enhanced trade winds on the north side of these highs, especially the BFH- avoid Coral Sea from Wed 11 to Sun 15 April.

TASMAN SEA/NZ AREA
Trough between the highs is expected to cross NZ on Tuesday 10 and Thursday 11 April. If you want to take an 'early' departure from NZ to the tropics, weather is looking Ok from Thursday 12 April – a good SW for departure. Avoid Fri 13 if you are tridentophobic.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

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