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

11 July 2009

BOBGRAM7 issued 12 July 2009

Issued 12 July 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.

Rather than a South Pacific Convergence Zone this week, there is, at
present, a straightforward tropical trough almost-north-to-south and
moving east across the South Pacific---it passed over New Caledonia last
Thursday/Friday and is over Fiji today - and should be over Tonga on
Monday, Niue on Tuesday, Southern Cooks on Wednesday and, in a weaker
and more drawn-out state, may cross the southern Islands of French
Polynesia FP on Thursday and Friday UTC.

This trough is forecast to be followed by a HIGH, zipping east along 30S
this week - the High should be south of New Caledonia on Monday, reach
180 on Tuesday, and be south of FP on Saturday.

As the trough approaches a place, winds turn northerly and become fitful
and the weather becomes squally; and following the trough are southerly
winds and clearing weather.

This trough is only moving along places south of 15S---further north are
healthy trade winds over the entire Southwest pacific. But these trade
winds weaken when the trough travels by to the south, and then
strengthen when the following high zips by to the south.

Another tropical trough is expected to form over the Coral
Sea/Vanuatu/New Caledonia area on Thursday and maybe reach Fiji on
Saturday and Tonga on Sunday 19 July. There seems to be a rhythm here,
but the following high is likely to behave differently for this week's
high, so don't get too settled into this pattern.

Not much rhythm in the Tasman.

This weekend's low was formed by interaction of a jet stream and a
tropical trough - it brought damaging wind/rain to Northland last night,
but a dense surface-hugging layer downwind of the Coromandels stopped
the southeast gales from reaching sea-level at Auckland harbour.

Next front should roll in from the south, reaching South Island late of
Tuesday and eastern North Island on Wednesday.

Then a trough should linger over the North Island on Thursday. On
Friday, this is likely to join forces with that tropical trough already
mentioned over New Caledonia/Vanuatu and form a new LOW to north of NZ.
This new LOW should swing ESE past northern NZ on Late Friday/Saturday,
deepening as it goes... Avoid.

New High is likely to cross south of Tasmania on Thursday/Friday and
move into Tasman Sea on 18/19 July weekend, strengthening the trade
winds in the Coral Sea. Avoid the squash zone between this high and the
low over northern NZ.

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