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

06 April 2014

Bob's Blog issued 6 April 2014

Issued 06 April 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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

SOI. The Southern Oscillation Index SOI 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.
SOI (30 day running mean) has been in the pink (below minus 10 in the graph for the past three weeks. One more week and we
can start calling this the beginning of a new El Nino episode.

Sea Surface temperatures
The warmer the sea, the faster it evaporates water into the atmosphere and
this is what helps weather (cloud and rain) to form. Areas of sea that are
warmer than normal (positive anomalies) are breeding grounds for tempestuous

Recently the sea surface temperatures in the target area (the equatorial
Pacific between 180 and Galapagos) has warmed to a slight positive anomaly.

During March the Sub tropical ridge (STR) became stronger than normal across
the South Pacific along 40 to 50S. This knocked the westerlies of the
roaring 40s well to the south and also managed to shield NZ from the fronts
it normally gets. This pattern is NOT expected to continue, and by the end
of April, after Easter, the fronts should become more noticeable especially
over the South Island.
The clouds that brought the rain that hit the Solomons (see have now congealed to make up Tropical Cyclone ITA
in the Coral Sea. At this stage the future track of ITA is difficult to
deduce. The GFS ensemble is picking it may re-curve into the Tasman Sea but
the Canadian model is picking it should go west into the Gulf of
If you are in the region then keep watching.

Panama to Galapagos: There are mainly light and variable winds between now
and Thursday 10 April, then there may be sufficient NE winds to allow a good
enough voyage from Friday.

SPCZ South Pacific Convergence Zone
This is rather disjointed this week and appearing as two distinct zones,
with one from Coral Sea to Loyalty Islands and another from east of Solomons
to Fiji area, spreading to Tonga late in the week.
There is still a weak convergence zone along around 5S from around 100 to

STR Sub-tropical Ridge
The STR continues to be strong between 40 and 50S, well south of its normal
This is so far south that there is room for lows to form in the sub-
tropics, and one of these is forecast to form to SE of Pitcairn Island by
Wednesday 9 April, traveling away to the SE.

New Zealand area
The squally Low that hit Rarotonga last Sunday night (NZ Date) (see has been moving to the SW for the last week and is
now moving SSW along the east coast of the North island. This abnormal track
is associated with the strong STR. The warm air associated with the Low
brought air temperatures up to 28 in the North Island today, a new April
This low and a front that has stalled in the Tasman Sea are both expected to
fade over the North Island area whilst a HIGH cell moves east across the
South Island mid-to-late week.

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