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

28 June 2009

BOBGRAM7 issued 28 June 2009

Issued 28 June 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.

The South Pacific Convergence Zone SPCZ stretches across the South
pacific in two main bands ... one is a continuation of the northward
moving band which has been around for a few weeks and was mainly
stretching form Solomons to Tuvalu to Tokelau to Northern cooks this
wee. Another band has been forming from around Rotuma to Samoa. The
recent activity over New Caledonia and Loyalty Island sis due to an
incoming trough that mixes the tropics with the subtropics and mid
latitudes, and isn't part of the SPCZ.

During the next week these two branches of the SPCZ are likely to merge
into once band from Solomons to Tuvalu to Tokelau, with a side branch
extending south to Fiji. According to some models Fiji is likely to
bear the brunt of some accumulated high rainfall by early next week.

The Galapagos to Marquesas route is all quiet now, but easterly winds to
Marquesas are less than normal and this trend may continue, yuck.

Last weeks high has turned into a sausage along 30S between 130 and
170W, and this is expected to pull out further east and wander to 25S
this week, ending over south-eastern Islands of French Polynesia. Its
associated squash zone of enhanced trade should also move north and
reach Marquesas and extend as far as 90W - so those on the Galapagos to
Marquesas voyage should have a good week if the get south of 5S. What
with the warming winds in that area, it will be wise to take these winds
while they are there, for the outlook is that they will not last.

Subtropical ridge is going thru a weak period this week in the Tasman
Sea. This allows sub-polar ridges to form instead--- that HIGH
currently over Campbell Island area is expected to head for the Chathams
by Tuesday and then fade as another High cell to the south buds off the
ice shelf - this is likely to feed chilling southeasterlies onto
southern NZ on the 4-5 July weekend.

Next subtropical ridge is forming in the Aussie Bight this week, it is
expected to get to Tasmania around 7 July and NZ around 10-11 July.

So this week is going to be a troughy week in the Tasman Sea/NZ area.

First Low formed between Lord Howe and Norfolk Islands today, on cue,
and its front and easterly-wind squash zone is over central NZ. Low is
forecast to move east along 35S, crossing northland on Monday. Wind and
rain over central NZ should ease by Wednesday as this low goes away.

Second Low is forecast to deepen over Tasmania on Thu/Fri and then cross
the Tasman Sea /Central NZ on Sat/Sun 4-5 July - bringing heavy swells
to western North Island and followed by chilling SE then S and SW winds.

Associated front comes first and should cross Tasman Sea/NZ on Wed/Thu
preceded by a period of strong NW winds, accompanied by a burst of rain
for western areas, and followed by showery westerlies-but this front
might trigger a brief secondary low off Wairarapa on Friday.

All these troughs crossing the mid latitudes of the Tasman Sea are
making for generally westerly winds between Brisbane and New/Caledonia/
Tonga, not the other way. Use the flow, Luke.

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