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

01 February 2015

Bob Blog

Issued 1 February 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: The extra heat that has been stored in the eastern equatorial
Pacific Ocean is now decreasing and no longer pointing to an El Nino

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 up and down over past few

Cyclone OLA has appeared in the area west of New Caledonia on Saturday and
is expected to go south then southwest and weaken and then be erratic.

In the Indian ocean, to south of Diego Garcia, TC EUNICE is travelling ESE
and soon expected to curve to the SE and fade.

The weekly rain maps over the past fortnight show heavy rain associated with
tropical depressions and TC EUNICE in the Indian Ocean, and also TC OLA.
There has also been heavy rain in past week from Samoa to Niue.

SPCZ=South Pacific Convergence zone.
SPCZ is expected to weaken and retreat to the north and west this week to an
area between Vanuatu and Tuvalu. It should shed the clouds associated with
TC OLA into the Tasman sea.
A Low is shown in some model weather maps over Fiji by Wednesday. This has
less than a 50% chance of forming (other models do not have it forming)
anyone in the area should remain vigilant and get updates.

STR= Sub-tropical Ridge
The STR spent January over the NZ area thanks to a series of blocking HIGHS
near 180.
The Blocking index time-longitude graph shows this strong blocking near
180 has now faded, and now indicates that it is shifting more towards
160E/Australian Bight. This suggests we can expect more Highs over SE
Australia during February and fewer building over NZ in other words a swing
to a more SW wind flow over the South Island.

Over NZ/Tasman
By mid-week a deep low is shown on the forecast weather map in the south
Tasman Sea, and this shows how this change of weather pattern may work:
with a low expected to deepen rapidly as it crosses the south Tasman Sea
so that it delivers strong SW winds to most of NZ on Wednesday. This new
regime will be wetter and winder for southern and western NZ than the
January blocking regime, but might only bring occasional showers to Nelson
and Canterbury-Hawkes Bay.

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