Bob Blog

WEATHERGRAM
YOTREPS
Issued 4 January 2015

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 Ocean: extra heat has been stored in the Pacific Ocean for a while now
and all the Oceanic indices have crossed the threshold into El Nino
territory.

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 standardized difference in the barometer readings between
Tahiti and Darwin. It has been negative since July and dived below -10
(Australian units) for much of September, and again for a week in November,
then relaxed in early December, and has been stuttering down and up again
over last few weeks.

In an El Nino event the latitudes of the normal weather zones are drawn
closer to the equator, causing the South Pacific Convergence zone to shift
further north and east of its normal position.

TROPICAL TOPICS

There are no tropical cyclones around at present, but a squally and intense
tropical low is expected to form over NW Australia by Thu 8 Jan and then
travel SE across Australia and reach the Tasman Sea by Tuesday
13 January. Avoid.

The weekly rain maps over the past fortnight show that rain is easing over
South Pacific Convergence Zone and, finally, in Malaysia. There is also a
recent build up in rain over NW Australia.

During December the averaged isobars show that in the Northern Hemisphere
the roaring 40s have been stronger than normal and the sub-tropical ridge
and trade winds weaker than normal, BUT in the Southern hemisphere the Highs
in the sub-tropical ridge have been picking certain longitudes to be
stronger than normal.--- Thus those in the Northern Hemisphere are getting
typical El Nina seasonal weather, but us in southern Hemisphere are having a
mixed bag of seasonal weather lately.

A look at the Standardized precipitation index map over NZ for the past
month shows a wet area around Opua/Whangarei and dry to almost drought
conditions near Dunedin/Balclutha.

WEATHER ZONES

SPCZ=South Pacific Convergence zone.
SPCZ is expected to weaken this week.

STR= Sub-tropical Ridge
A new High cell is expected to cross Tasmania on Monday and then travel east
across the Tasman Sea and the on to NZ on Thursday 9 Jan.

Over NZ
A Trough is expected to follow that High on Friday and the coming weekend
bringing unsettled weather mainly to the South Island.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
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