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

08 May 2011

BOBGRAM7 issued 8 May 2011

WEATHERGRAM
YOTREPS
Issued 8 May 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
La NINA is now starting to weaken in the atmosphere -the Southern
Oscillation Index 30day running mean was 2.55 on 23 April, 1.94 last
week and 1.21 on 8 May .

South Pacific Convergence Zone SPCZ spent last week in active mode from
Solomons to Fiji (some flash flooding in Fiji). There has also been a
rain band hovering around from Niue to Southern Cooks to Gamblers.
And over past week there has been a convergence zone along 5S from 100
to 140W --- this maybe some twinning of the ITCZ, something which
usually happens around the equinox (that was ~6 weeks ago now) and may
be linked to lingering positive SOI. Should fade soon, and puddle
jumpers from Galapagos to Marquesas should wait a few more weeks.

Low is expected to form near Southern Cooks on Tue/Wed UTC with gales,
and then move off to SE, deepening to 998 near 30S 160W by Thursday UTC,
then move further off to S and SSE and slowly weaken. Avoid.

SUBTROPICAL RIDGE:
Intense blocking HIGH (called H1 last week) has been centred east of NZ
and south of Southern Cooks for past week, with pressures rising to over
1038. Fiji Met have had a marine gale warning covering the enhanced
trade winds on north side of this high for past few days 142W to 173W,
20-25S. High has been well supported aloft and has been blocking a
series of lows over NZ area forming abnormal unstable weather. A tragic
tornado over northern suburbs of Auckland last Tuesday faded within 5km
of my place.

This week, this High is finally expected to weaken and more off.

The next High from the west is now NOT expected to be able to get into
the Tasman Sea until Mon 16 May--- so this is expected to be a week when
the subtropical ridge is weak over the South Pacific, allowing
mid-latitude fronts to reach the tropics, and a burst of 'roaring 40'
westerlies to march across NZ and further east.

NZ/TASMAN SEA AREA
After a Monday of light winds and a Tue /Wed of increasing NE flow,
there is likely to be a front Wed night/ Thursday morning (local time)
Then another interlude of light winds, then another front on Friday 13th
(maybe local daytime), and then a burst of strong and squally westerly
winds on Sat 14 May, the wind turning to be more of a southerly on
Sunday 15 May
- this S/SW change gets to the Tasmanian area by Tuesday 10 May and
marks the end of this series of fronts/troughs as it heralds a high
moving into the Tasman Sea.

WHEN TO DEPART NZ FOR FIJI/TONGA?
Again it is not a good week to depart NZ. Either too little wind
between the fronts, or two much NE wind ahead of the approaching fronts.
What we want to look for is the front that is followed by a SW change
that leads in a new High across the Tasman Sea so we can ride these SW
winds to 30S (once they have settled). At this stage, it looks as if
this MAY be the case on Tue 17 May --- and as an added bonus such a
voyage will have some moon-lit nights. More on this, next week.

Heading east from NZ has been hassled by that intense High for past week
or so, but this hassle is now fading.

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

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