BobBlog 31 Aug 2014

WEATHERGRAM
YOTREPS
Issued 31 August 2014
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

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one number. It is
based on the standardised difference in the barometer readings between
Tahiti and Darwin.
It has been negative since July and has been hovering around -5 (Australian
units) for the past month - enough to be called a weak El Nino. --

TROPICAL TOPICS
After a busy period, activity has relaxed for the moment in the tropics and
there are no Tropical Cyclones around the planet at present.
The weekly rain maps for the past fortnight show a continuing burst of rain
around India. Activity between Mexico and Hawaii has eased, and there has
been a well-defined band of rain across the SW Pacific. It also continues to
be wet in the northern Tasman Sea.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is now well established expected along 10S from 165E to 165W and is
likely to drift southwards towards Samoa later this week. A weak branch of
the SPCZ is drifting eastwards across French Polynesia this week and is
expected to help in the formation of a low near 30S 135W from Wednesday 3
Sep UTC.

Departing westwards from Tahiti:
The convergence zone that has been lingering over French Polynesia over
recent days should travel southeast off the group allowing trade winds to
move onto the area by local Tuesday, good enough for departure.
The SPCZ may travel slowly south towards Samoa by late this week. If this
track continues into next week then it may reach northern Tonga after 7
Sept, so keep a watch out for that A squash zone of strong SE winds is
likely to form between Tahiti and Samoa on Thur/Fri/Sat UTC and then drift
to the southeast- this squash zone is associated with the northeastwards
travel of a large high along
40 to 30S in the southern ocean. Uncomfortable.

STR= Sub-tropical Ridge
The STR is well south of its normal position this week. The intense high
referred to in my blog last week did indeed linger over southern NZ at over
1034hPa, with lots of low cloud and drizzle under its inversion affecting
various coastal parts of NZ (a dirty high). It is now east of NZ and
travelling eastwards along 45S. This high is well supported aloft and slow
to move, a good example of what is sometimes called an omega
block.

Between NZ to the tropics
Not a good week for travelling south to NZ.
There is a squash zone of enhanced NE to easterly winds between the high
east of NZ and a Low tonight over northern NZ.
On Monday, Low is expected to move slowly across Northland (at 1008 it isn't
very intense but because its rain bands are slow-moving they have a high
accumulative affect).
On Tuesday, that Low should finally move off to the east allowing a day of
SW winds that may be good enough for setting sailing off to the north (but
may encounter a period of strong winds on Thursday night near 30S).
However another Low is expected to form in western Tasman Sea on Tuesday,
rapidly deepening to 990hPa on Wed/Thu and then traveling onto North Island
on Fri/Sat. Its associated warm front is likely to cross North Island on
Thursday and be preceded by strong northerly winds and followed by strong
squally westerly winds near NZ. Avoid.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
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