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

24 May 2015

Bob Blog

Issued 24 May 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. It was near and slightly southeast of the apex of a Jet
Stream as shown at the FL340 level map. At this position air may be sucked
upwards and outwards from the low in a process similar to how we can use a
hose to spray out garden chemicals. This causes the central air pressure (as
measured in hectoPascals) of the low to drop quickly, creating more and more
isobars near the centre, making for stronger winds around the system.
Many meteorologists monitor the 500 hPa (FL 160) map as it is in the
middle-atmosphere and combines the features of the upper air (jetstreams)
and the surface. The flow at this level is a good approximation of the
steering flow for surface features, and this map last Thursday showed strong
NW winds above the surface low, so that is was no great surprise that this
surface low travelled so quickly across northern New Zealand on Saturday.
The low has already been blown to the far SE of NZ and thus dislodged a
chunk of air out of the Southern ocean onto New Zealand as shown in the
latest plot of the current snow expected for the next 24 hours over NZ on

The Atmosphere:
The Southern Oscillation Index SOI is based on the standardized difference
in the barometer readings between Tahiti and Darwin (30 day running mean)
and sums up the weather pattern over the South Pacific as one number. It has
dived very negative in the past few weeks, possibly due to troughs near
Tahiti. We haven't seen it this negative since Jan 2010.
SOI is shown at
If this index remains lower than -10 for four weeks in a row then we have a
fully-blown El Nino (both ocean and atmosphere). This is NOT a typical El
Nino. When compared to the past it seems to be mimicking the 1986-87 event
rather than the biggest ever 1997-1998 event: see

The Ocean
Here is an interesting theory:

Theory is that heat is being taken from atmosphere over the Pacific Ocean
and then driven by wind into the Indian Ocean. SO the Pacific Ocean
temperature indicators may take on the values of an El Nino, but might not
be delivering an atmospheric El Nino.

TC DOLPHIN has moved off to the NE of Japan, and at present we are having a
quiet period between cyclones. Next one is likely to form near 10N 105W by
Thursday, off the Southwest coast of Mexico.
In an El Nino year the Indian monsoon is usually late, slow, and not-so-
wet. Well, this El Nino hasn't really got into the atmosphere yet, and the
Indian Monsoon is starting up already and is maybe a day or so EARLY.
Latest Indian Monsoon index is a few days early, and so far is wetter than
normal NOT a typical El Nino-Monsoon at all. See

The rain map for the last two weeks shows how quickly the South Pacific has
dropped in rainfall since that early May low over the Solomons. It also
shows maximums of rainfall in Southeast Asia as the Monsoon kicks in.
And troughs over southern French Polynesia. see:

Galapagos to Marquesas:
Better winds for departing are on local Monday/Tuesday this week.
There is still signs of a good west-going current along around 5 to 6S from
95W to 130W, so use this if you can. There may be some convergence shower
activity around the Marquesas this week.

SPCZ=South Pacific Convergence zone.
SPCZ is expected to be weak this week and stretch from Tuvalu to Suwarrow,
with a relaxation of the convergence zones that have been bothering French
Polynesia over last few week.
Late in the week a trough is expected to form between eastern Fiji and south
of Tonga (Minerva area). This trough is likely to have a squash zone of
strong easterly winds on its southern side.

STR= Sub-tropical Ridge
Next HIGH is expected to move east into central Tasman sea along 35S by
Wednesday and then across the North island on Thursday and off to the east
by Friday, and hug 35to 40S as it travels off to east of NZ next week.

Departing from Australia to the tropics his week:
Once that HIGH starts travelling east from Wednesday this will make for
strong SE winds in northern Tasman Sea, and there is a zone of light winds
within the HIGH for anyone motoring.
Big trough is expected to move into southern Tasman Sea from Thursday,
bringing strong SW winds and heavy swells as far north as Lord Howe Island
by Sun 31 May. Avoid.

Departing from Northern NZ going north.
Southerly blast is still strong on Monday and should ease on Tuesday.
Then easterly winds and light winds over northern NZ on Wednesday and
Thursday, and then the NE winds ahead of the next front are expected from
Friday. SO best day to depart is Tuesday 26 May.

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