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

07 November 2009

BOBGRAM7 issued 8 Nov 2009

Issued 8 November 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.

There were 22 yachts at north Minerva last Tuesday as this spring's
yacht-migration hits peak. The wind flow around Northland has been
generally from the southwest over the past week - occasionally from the
south or southeast following a few fronts, and there has been a large
zone of calm along the subtropical ridge neat 30S, so weather has been
suitably challenging and produced some stints of analysis paralysis at
the Minerva marina.

Northern Hemisphere is still producing tropical cyclones: IDA started
in central America and now expected to build briefly over Gulf of
Mexico, and a TC called TWENTYFIVE is wandering northeast away from

On our side of the equator, our South Pacific Convergence Zone SPCZ is
now entering a quiet period. During the past week it extended from
Solomons to Wallis/Futuna to northern Tonga to Northern Cooks. During
9-12 Nov UTC a piece of this zone is expected to go southeast to south
of French Polynesia FP and briefly become a low system, stealing the
wind from FP for a few days.

Another piece is likely to drift southwards towards Fiji and Tonga on
10-13 Nov. Otherwise I think this is a week where the main SPCZ will be
hugging 10S.

Because of HIGH pressures in the Tasman Sea, strong winds are expected
to continue all week across the Coral Sea. This squash zone covers
central parts of Vanuatu and extends to Fiji at times.

During a brief gap between Highs a few days ago, the Queensland heat
trough was able to ingest some moist air from the Tasman Sea and dump it
onto Coffs harbour area on Friday: their fifth flood this year.

Highs are continuing to linger and fade in the Tasman Sea. It is as if
the New Zealand's southern door has been left open allowing
fronts/troughs to roll onto NZ from the southern ocean: one on Monday 9,
one on Wednesday 11 and one of Friday/Saturday 13-14, each slightly
cooler than the one before. There seems to be some Antarctic melt water
around Macquarie Island at present- a large iceberg was seen passing by
there late last week.

Watch the wind: There is likely to be a deep low passing by in the
southern ocean on Sat 14, and that should make for very strong winds in
central and southern NZ areas this weekend.

As for the HIGHS, today's Tasman High got over 1036 but should move into
the north Tasman Sea during the week and fade away by Thursday 12 Nov.
In the gap-between-the-highs the forecast, at present, is that we don't
get a repeat of last week's Coffs harbour flood. This time a low is
likely to form off the southern New South Wales Coast on Thursday 12
Nov, then deepen a little between Lord Howe and Norfolk Islands on Fri
13 Nov, then fade as it goes northeast.

This low should be followed by another High crossing Bass Strait on Fri
13 Nov, then heading northeastwards.

Next week, from around 16 Nov, this high should be east of Northland and
a NW flow is finally likely over Northland, making landfall slightly
easier for a few days.

Note that I'll be attending to the MetService display at Royal Show in
Christchurch this week, so may not be able to respond easily to emails.

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