BOBGRAM 7 issued 26 Sep 2010

WEATHERGRAM
YOTREPS
Issued 26 September 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.

LA NINA
SOI (90 day) is now 2.02, up 9% from last week, now over 2 standard
deviations from its mean so La Nina is just getting stronger.
During October I'll explain what this means for coming migration/
cyclone season.

TROPICS
South Pacific Convergence zone is showing signs of hugging 10S... This
seems to happen around any equinox and so is likely to stay this way a
while. However there are signs of it spreading southwards in the Coral
Sea.

A pulse of extra convection is making its way eastwards from Indonesia /
North Australia into the North Tasman Sea, assisted by a subtropical jet
Stream --- this is producing cloud at high and mid levels and these
should thicken into a surface trough that is likely to cross the North
Tasman Sea on Wed/Thu 29/30 Sep and fade over Kermadecs around Sat/Sun
2/3 Oct.

A similar trough may come and go around Southern Cooks during the week.

SUBTROPICAL RIDGE: STR.
High 1026+ between Northland and Kermadecs at present (26 Sep) is
expected to move slowly SE to 38S and get south of French Polynesia by
the 2/3 Oct weekend and intensify to 1038+. This will strengthen the
trade winds on its northern side into a yucky squash zone. So if you
intend puddle jumping from Tahiti westwards this week, set off early
before the squash zone arrives.

Next High is crossing Australia at present and should move across the
Tasman Sea just as a weak 1025 ridge along 34S from Thu 30 Sep to Sun 2
Oct, and then be reinforced and get to a 1032 Cell and wander along 40S
from 2 to 3 Oct and cross central NZ 3-4 Oct. A welcome change from
past few weeks. Should be good for anyone wanting to come to NZ so long
as you arrange arrival before the next trough strengthens in the Tasman
Sea, and that, at this stage, is looking to be around 7 Oct. Also,
there is likely to be a squash zone building over Tonga/Kermadecs from 3
Oct.

NZ AREA
Monday 27 Sep : NW strengthening
Tuesday 28 Sep : Front embedded in NW flow.
Wednesday 29 Sep : Front stalls over North Island, Low whizzes past
Southland.
Thursday 30 Sep : Front lingers North Island, SW flow establishes
elsewhere.
Friday 1 Oct : Southwest flow ahead of a High.
Saturday 2 Oct : Dying southwest flow ahead of a High.
Sunday 3 Oct: Lighter variable winds around High in central/E
Tasman.
So, at this stage looking OK for the annual CANANZ Kowhai cruise in
Hauraki Gulf.

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