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

11 May 2014

Bob's Blog

Issued 11 May 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) was in the pink and below minus 10 in the graph for much of April but started May in the blue
and is now falling again.

The World Meteorological Organization now says that a recent weakening of
trade winds has significantly warmed the sub-surface waters of the central
Pacific. This warm anomaly is now rising to the surface. When this warmth
reaches the surface we shall have an El Nino episode, and that maybe be
within a month or two. The amount of extra heat currently store in the
tropical Pacific has been measured to be on a par with the amount that gave
us the last big El Nino in 1997/8. We shall see/sea.

In the past week there has been some intense pre-monsoonal rain over Sri
Lanka and southern India. An intensive convergence zone moved over Sri Lanka
last Monday. A squally shower formed a waterspout over a shallow river,
resulting in a rain of fish on nearby fields. See http://www. for more details.

No tropical cyclones on the planet for the past two weeks, we are in that
interesting hiatus of changing seasons. During the past week the ITCZ has
been very active off the west coast of Central America. Later this week a
tropical low is expected to form near 9S 90W around Wednesday and this
should then travel out to the west and northwest.

Galapagos to Marquesas
No problems with any convergence zones. To get the best benefit of the
available current go for 3S96W then 6S121W then direct to Marquesas.

SPCZ South Pacific Convergence Zone
This is now weak and about to spend a week or so at the north end if its
A small trough in the trade in belt is expected to form over the Loyalty
group between New Caledonia and Vanuatu on Thursday and then move SE
crossing the Minerva area on Friday night. This system may carry a zone of
strong easterly wind and near 3 metre swells on its south side, a small
squash zone. AVOID. This feature may then stall and hover a few days and
deepen into a low around Tuesday 20 May near the Kermadecs. Too far away to
Some of the more erratic weather models (see last week weathergram for more
explanation) are making more of this trough than the more reliable weather

STR Sub-tropical Ridge
High in the Tasman Sea tonight should fade away as it crosses northern NZ on
Monday, and then redevelop as it travels east along 30 to 40S.
High moving off New South Wales on Monday is expected to take its time
crossing the Tasman Sea this week and then cross NZ on Friday.

Departing from NZ to the tropics
A trough is expected to cross NZ on Tuesday (stay put) followed by a showery
W/SW flow over northern NZ on Wednesday (might be OK for departure).
This should ease and turn more southerly on Thursday (OK for departure), and
lighter more variable winds on Friday.
There may be another trough approaching NZ over this coming weekend.

Over the past week and into the next week, there has been a noticeable
increase in the number of westerly isobars in the Southern Oceanthe lows at
60S are now regularly reaching 980hPa.

See my yotpak at for terms used.
Weathergram text only (and translator) is at ;
Weathergram with graphics is at, click FOLLOW at bottom
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