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

31 October 2010

BOBGRAM7 issued 31 Oct 2010

WEATHERGRAM
YOTREPS
Issued 31 October 2010
Bob McDavitt's ideas for sailing around the South Pacific.

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

Welcome to the official start of the South pacific Cyclone Season.
Nothing expected this week. The South Pacific Convergence Zone SPCZ has
been going through an active period recently. It has cleared off Coral
Sea and New Caledonia but is still being strengthened by an upper trough
near Vanuatu and lies almost west-east between 12 and 15S from Rotuma
across Samoa and to Suwarrow, it then extends SE across Southern Cooks.
The upper trough is expected to cross Fiji on Mon 1 / Tue 2 Nov where it
may form a surface low for a time, and then it should weak and it moves
over Tong on Wed 3 Nov. This will likely encourage the SPCZ to come
southwards this week.

SUBTROPICAL RIDGE: STR.
We did indeed get a High well over 1044 east of NZ this weekend. It is
now weakening and migrating NE to east of NZ and that area should become
more zonal and high index by the end of this week with just a ridge left
over along 30S and a disturbed westerly flow further south.

The next High is wandering along 45S across the Australian Bight. Its
nose end is shovelling cold air north, but the showers are expected to
mostly clear in time for the Melbourne Cup on Tuesday. Anyway, this
High should edge NE wards, crossing Tasmania / 40s on Fri 5 Nov, and
tonguing itself onto North island /35S on the 6-7Nov Weekend, then
dividing into two parts on 8-9 Nov - one part moving off to east of NZ
along 40S, and the other stalled in central Tasman Sea at 35S. There
are squash zones of enhanced easterly winds on the north side of this
high.

NZ AREA
Between those highs, there is a replacement trough. This is currently
over Tasmania and, at present, includes a rapidly intensifying low
centre that is rushing off to the South. It's the same trough which
deluged the Melbourne derby yesterday. Anyway it is expected to cross
NZ on Thu 4 Nov, preceded by a strong NW flow over central and southern
areas, and followed by a crisp cool SW with showers that will last into
the start of the fireworks on Fri 5 Nov.

TASMAN SEA
As indicated in my previous weathergram, on 5 Nov a LOW is expected to
form off Queensland coast, it should move SE and deepen, reaching peak
intensity SW of Norfolk on 7 Nov, and then it should slow down and
wobble near Northland on 8-9 Nov - then wander to the east to north of
NZ by around 10 Nov (uncertain). Strong squash zone of enhanced winds
between this LOW and that HIGH to the South especially from 9 to 12 Nov.

SAILING TO NZ???
The High east of NZ will maintain an easterly flow until 4 Nov.
That trough crossing Northland on 5 Nov is followed by a period of SW
winds, but for yachts coming from Fiji or Tonga these winds will likely
only briefly get to 30S and so are just a minor irritation (they will
also bring SW swell - reducing the comfort factor).

Of more interest is that LOW from the Queensland Coast. The computer
models are not yet is agreement as to what it may do, so updates will be
needed. Avoid this LOW neat Norfolk especially on 6-7 Nov as that's
likely to be when it peaks.

As for the Northland coast; most likely (not yet certain) idea is that
LOW will bring a period of enhanced ENE on Mon Tue 8-9 Nov and then
enhanced SE to S winds on 10-11-12 Nov. This complicates all voyages
that may be planning to arrive in NZ in the period from 9 to 12 NOV.
The impact of this Low should clear away so that a new weather window
may start opening for good voyage from New Caledonia /Fiji/ Tonga to NZ
after 13th Nov.

A further complication is that I will be UNAVAILABLE during 9-12 Nov, as
I'll be attending to a MetService display at the Canterbury Show. So,
if you want an update for your arrival, maybe it's a good idea to avoid
arriving on those days.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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