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

10 July 2016

Bob Blog 10 July

WEATHERGRAM
YOTREPS
Issued 10 July 2016
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.

The weather patterns averaged over the last month show an interesting
enhancement of the subtropical ridge from Africa to SW of Australia and from NZ
to southern South America. The resulting anomalous anticyclonic northerly flow
over New Zealand led to June being a warm month. The lower than normal pressures
around Antarctica has been giving a strong positive Southern Annular mode (SAM)
- strengthening the westerly winds and swells around the Southern Ocean. And
there has been a breeding ground of Lows west of UK in the North Atlantic month
since March -- no wonder Wimbledon has been so wet this year.

Averaged isobars for June, and their deviation from norma are seen at
www.esrl.noaa.gov/psd/map/images/fnl/slp_30.fnl.anim.html and
/slp_30a.fnl.anim.html

Tropical cyclones are becoming damaging. NEPARTAK was cat 5 when it crossed
Taiwan and is now weakening over eastern China.BLAS is still travelling west
near 20N 135W and now fading, and is being followed by CELIA which may travel
all the way to Hawai'i between local 17 and 19 July. Brace if it does.

Last fortnight of weekly rain profiles, as seen at
trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif,
shows how the Intertropical convergence zone is remains strong over the eastern
pacific. Also there is intense rain over south China and Southeast Asia.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain draped from Coral Sea to north of Vanuatu to Samoa
and at times to the Palmerston Island /Niue area. There is also likely to be a
developing trough near the northeast end of Australia by the weekend.

STR= Sub-tropical Ridge
The HIGH which has formed to east of the South Island is held dynamically
together is the LOW on its northern side between NZ and Tonga. There is a strong
squash zone between these systems. These should travel east this week to 160W as
a tied-pair relationship, with HIGH near 45S and low near 30S until end of the
week when the low weakens.

New HIGH is expected to travel east along around 36S across Bass Strait area on
Saturday and then slowly cross the Tasman Sea by Tuesday 19 July.

Voyage Outlooks:
Tahiti to the west
A PASSING TROUGH offers a challenge with NE/N winds on its eastern side, and a
period of S/SW winds on its western side, and some squalls in-between. The winds
associated with this trough are expected to be less than strong, so this may be
an acceptable challenge perhaps. OR what until after Friday 15 which is when the
trough should go by the Borabora area, for a less complicated passage.

Between NZ and the tropics
The Low between NZ and Tonga is travelling slowly east along 30S, with a strong
squash zone on its south side. Too much SE swell for comfort on Monday.
Tuesday is looking to be a good compromise departure, and possibly Wednesday.
On THURSDAY an active front is expected to cross NZ from the Tasman Sea followed
by a day or so of strong and squally SW winds maybe until Saturday or Sunday,
stay put for that. Then there may be a good weather pattern departure by Mon 18,
perhaps Sun 17 July.

Between Australia and New Caledonia/Fiji
An active trough is expected to affect south of 30S on Monday to Friday, and
weather may be OK for a departure from Brisbane to New Caledonia on Monday or
Tuesday, but after that may as well stay put.
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See my yotpak at boatbooks.co.nz/weather.html for terms used.
See my website www.metbob.com for information on tailor-made voyage forecasts-
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