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

05 December 2009

BOBGRAM7 issued 6 Dec 2009

Issued 6 December 2009
Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Sadly the Satellite that has been helping us out with sea winds
(Scatterometer) over the past decade has died--- the bearing which spin
its mirrors were only made to last 5 years, and they seized after 10
years , so no more data. I have not been informed of any replacement
plans at this stage, so in the meantime my ability to check wind over
the ocean is not as good as it was.

The wettest part of the planet last week has been along the Pacific
equator, between 160E and 180. There is a ship near by this region
reporting strong westerlies, so we can assume that is a surge of
equatorial westerly winds. Hold on to your hats in Kiribati, the swell
will possibly keep you in side harbour this week.

This could be an active week near the dateline:
Rain in the South Pacific convergence zone has also been active between
Solomons and Vanuatu and around the North Coral Sea. The SPCZ also lies
along a zone from Samoa to French Polynesia. Computer models are
latching on the area between northern Vanuatu and Tuvalu as a likely
breeding ground for Tropical lows this week, but lows breeding in this
region are likely to be slow-moving. There is already a large zone
below 1005 in that region. This is likely to feed off the equatorial
westerlies and bring strong NW winds across Tuvalu, Tokelau and Samoa.
Once these NW winds reach Tokelau-around Tue 8 Dec UTC, that may fire up
a rapidly deepening tropical low system between Tuvalu and Samoa on Wed
9 Dec, and then that system will likely travel SSE near Wallis /Futuna
on Thu 10 Dec and then across Northern Tonga on Fri 11 Dec where it may
well earn a name for itself. Avoid.

A good sized high is dominating the subtropical ridge along 30S from 180
to 140W at present and this has enhanced trade winds on its northern
side. This high is expected to fade by Thu 3 Dec, allowing a trough to
spread east. Following this trough there is expected to be a new high
spreading east of NZ along 40S, thus taking the subtropical ridge
southwards this week.

Subtropical air brought some welcome but briefly intense rain to our
North Island last week. The SW Pacific subtropics are now shifting back
to around 30S again for at least the next fortnight.

SO it is back to the disturbed westerly pattern over NZ, but the fronts
are likely to weaken as the move across the country-one on Tue 8 Dec
just raining on southwest fringes, another on Wed 9 Dec more noticeable
over South Island, and a third on Fri/Sat/Sun 11/12/13 Dec bringing a
southerly wind change to eastern areas.

The fronts 8, 9 and 11 should be preceded by strong NW winds
concentrating rain on Alps and bringing some hot dry winds further east.

Around Sun 13 Dec, a High is likely to cross Tasmania and this should
shovel a cold front north on Mon/Tue 14/15 Dec feeding in a cold
southerly blast onto NZ and further north. If you are trying to sail
into Northland from the north then winds will be more useful for
landfall before Sun 13 Dec rather than after.

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