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

30 November 2014

Bob Blog

Issued 30 November 2014
Bob McDavitts ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world

I've just been introduced to another website presenting the weather data in
an interesting way

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, relaxed for a while in October, and dived below -
10 early in early November, but is now relaxing again.
We know that 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.
Well, as November draws to a close we can look at the average weather map
mid-month, thanks to NOAA and this shows Highs have been stronger than
normal in the north Tasman sea, and just south of South Africa, and lows in
the southern ocean have been deeper and more north than normal in south
Indian Ocean, and to SE of NZ, and to SE of South America. The extra highs
in north Tasman Sea and lows to SE of NZ have combined to give extra SW
winds over NZ.

NIWA have issued a news release about New Zealand's November weather at they say there has been
more SW winds than normal over New Zealand during November. The rainfall, as
measured by the SPI (Standardized Precipitation Index) has been close to
normal in most places but the extra SW winds show as patches of extra
rainfall in the W and south of the South Island.

The extra SW winds are causing soils to dry out faster than normal in
eastern North island places from Northland thru Coromandel and Bay of Plenty
to Gisborne/Hawkes Bay/and the southern quarter of the North Island, and the
eastern South Island.

They also say, and I quote It should be noted that this rainfall pattern is
commonly associated with El Nino events and while El Niqo has not been
officially declared, it may be an indication that such an event is

Last weekend Cyclone Adjali was the first of the new season in the Indian
Ocean and was heading towards Mauritius before it weakened. The Volvo Race
fleet diverted clockwise around another system now near 20S 60E and one of
fleet, Sailing Vessel Team Vestas Wind, ended up sailing onto a reef (at
night) in the Cargados Carajos Shoals around 400 km to northeast of
Mauritius. Ouch.

At present the remains of Cyclone SINLAKU are fading over China.
Another area of interest is developing in the NW Pacific and is expected to
become a Cyclone over Micronesia by Tuesday and travel west towards
Philippines late this week.

There are a couple of areas of interest in the Indian Ocean, one near 20S
60E and another near 11S 90E  I suppose we can declare the Southern
Hemisphere cyclone season to now be officially open.

Weekly rain maps over the past fortnight show an increase in convection over
the past week in the Indian Ocean and in the zone from north of Solomon
Island along to Samoa. There was flooding in Samoa early last week, but the
feature responsible then faded as forecast in last weeks Weathergram.

This increase in convection in the Indian Ocean is possibly part of a MJO
cycle that will gradually make its way into the Pacific Ocean over the next
few weeks. By mid December this is expected to increases the risk of a
tropical cyclone generating in the area north of Vanuatu.

SPCZ=South Pacific Convergence zone.
SPCZ may not be as strong as it was last week, and is expected to be located
near the north end of its normal range, stretching from north of Solomon
island to northern Vanuatu and the east across Tuvalu to Samoa/Tokelau, with
scattered convection also over French Polynesia.
A burst of northerly swell reaching 2 to 3metres is expected to spread south
and east over the next few days  it should be most noticeable about and
just north of the SPCZ and roughen the sea there. Not the best for good
diving or fishing between northern Vanuatu and Samoa this week.

STR= Sub-tropical Ridge
The STR is also strong and well defined and mostly at its normal latitude
for the time of year, but now north of normal in zone east of NZ. It has
been north of normal in the NZ area but is expected to be closer to normal
latitude this week as the HIGH tonight in the south Tasman Se is expected to
travel NE across the North Island on Tuesday night/Wednesday and then be
slow-moving off the NE of NZ until next week, so that approaching troughs
stall in the Tasman Sea.
HIGH near 35S 130W, to the south of Pitcairn Island, is expected to be
slow-moving and peak in intensity over 1032 hPa by Tuesday UTC and is
expected to stay put until Wednesday and then slowly weaken and move NE.
There are likely to be strong easterly and NE winds on the northern side of
this high, especially between 15 and 20S until Friday.

Between the tropics and NZ
Good weather this week for sailing from the tropics to NZ.
No good arriving on Monday 1 December because thats when a cold trough is
expected to cross the North Island. Then there is an easterly flow expected
on the north side of the high that is expected to cross the North Island on
Tuesday night/Wednesday.
After Wednesday there is expected to be a useful N to NW flow over northern
NZ, good for arrival. This NW flow may get strong and unsettled on Tuesday 9
December, as a trough travels east across southern NZ. The NW flow should be
Ok again on Wednesday 10 Dec for arrival, and then northern NZ may become
unsettled as troughs start to arrive from the Tasman Sea.

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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1 comment:

Sappa Lama Gomso said...
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