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

18 October 2008

BOBGRAM7 issued 19 Oct 2008

Issued 19 Oct 2008
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 below are given in UTC unless otherwise stated.

TROPICS The South pacific Convergence Zone, SPCZ, went through a period
of activation in the Coral Sea last week. This was induced by a passing
of an upper level trough and the corresponding 'blob' went over Loyalty
Islands on Friday, to south of Fiji on Saturday and is now weakening
over Minerva reef. The latest Quikscat imagery conforms a reflection of
this trough at surface level, with a zone of northerly winds and
clearing weather passing over southern Tonga today and Monday (UTC).
May be useful for Island hopping.

The SPCZ this week is expected to mainly extend from northern Coral Sea
to Vanuatu to south of Fiji and occasionally to Southern Tonga, with
another branch from Tokelau to Northern Cooks

For the Port2Port this week there is an unavoidable squash zone in the
Coral Sea, it marks is fed by a rather weak HIGH cell crossing the
Tasman Sea from 19 to 21 Oct. Another HIGH is likely to cross the Tasman
Sea from 23 to 27 oct, and its squash zone may weaken from 26 Oct as the
SPCZ goes through another period of activation.

The main challenge this week comes from a LOW that is likely to form
near Lord Howe Island on 21 Oct. By the way, this low is expected to
form between highs in the subtropical ridge, a standard pattern. The
conflicting densities of air feeding into this low from the warm moist
north and the cooler south is not being handled well by the computer
models. The main model that you may have access to in your GRIB files
is the GFS, and it and the NOGAPS model deepen this low and take it
across the North Island on Friday, just in time for our COASTAL CLASSIC
from Auckland to Bay of Islands. The EC and Canadian models do not
deepen it so much and may track across the north of Northland, a
different outcome. At this stage the best pick is somewhere in-between
and nothing more definite can be written until the models start to

One thing that can be said about this low is that it will be forming
whilst a front, today in the South Tasman Sea, moves north across New
Zealand. That front is followed by some grunty swell from the southern
Ocean and the LOW will help enhance that swell, especially along the
western part of the Tasman Sea along the Australian coast as far north
as Brisbane to Lord Howe on Thursday, and also on the western side of
the Low as it crosses whichever part of northern NZ it does around 24
/25 Oct. Avoid this swell.

Tonga to NZ
I like the idea of taking on a front at around 30S--- they are weak at
that latitude and there plenty of ocean to play with. Fronts this time
of the year are about 5 to 7 days apart on average crossing NZ, and the
voyage, over 1000nm will take you over a week so meeting a front is
unavoidable. Doing so mid-way maximises your chances of making landfall
in NZ "between fronts". I think this is a better strategy for a
cruising yacht than attempting to make landfall in NZ as a front is
approaching (but some may prefer this).

Anyway: this week THE FRONT associated with that Tasman Low is
expected to be crossing that magic point-pretty well half-way between
Raoul Island and 30S 180- on 24 Oct. So my best idea is that you depart
from where you are in Tonga to reach that point around 24 Oct and head
sufficiently off to the west to allow for the SW winds that will follow
that front, and the SW winds that will be on the western side that that
low, and you should have a reasonable voyage. As for the swell on the
western side of that Low, you will meet it near 30 to 35S over the
weekend, but by then it will be around 3m and on the decline /long
period, so not too bad.

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