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

28 September 2014

BobBlog 28 Sep 2014

Issued 28 September 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.
Barometer jumps
A yacht skipper at anchor in a river on the NE end of Baie de Prony (at SE
end of New Caledonia) last Wednesday morning noticed
his barometer jump up 3 hPa in an hour and then down in the next hour, and
once again.
These jumps were superimposed upon the normal semi-diurnal atmospheric tide
that the barometer was recording as a series of bumps
that look like Sine waves. The STRANGE thing is that nothing happened to
the wind or cloud in the area (usually a change of this
magnitude is associated with an incoming gale or a nearby squall). Nothing.

Later he found out that another boat anchored 10 miles further south
observed much the same thing.
Does anyone have an explanation of such an observation??

Background influences
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 standardised 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.
The amount of heat that is being stored in the sea in the Eastern Equatorial
pacific has also increased during September, as measured by the
During an El Nino episode, weather patterns tend to be drawn closer to the
equator. The subtropical ridge in the southern hemisphere tends to be north
of its normal position and this weakens the trade winds.
The South Pacific Convergence zone tends to be tugged north and east of its
normal position. If this happens during the cyclone season then it increase
the risk of cyclone formation about and east of the dateline, especially
during February and March.
In itself an El Nino episode doesn't have much impact on cyclone activity
about New Caledonia, but it may reduce the risk in the Coral Sea.

Sea surface temperatures are currently warmer than normal in a zone from
Samoa to French Polynesia
so this is likely to be a zone of extra convection when visited by the SPCZ
in the next few weeks.

In the East Pacific we have RACHAL and in the west Pacific we have KAMMURI
The weekly rain maps show a shift of the heaviest rain from 100E to 140E.

SPCZ= South Pacific Convergence Zone
The SPCZ is more homogeneous and more intense this week than it was last
week and is averaging its position across Solomons to Tuvalu, but there has
also been convection over Vanuatu last week. SPCZ is expected to shift
southwards of Solomons towards Vanuatu on 4/5 Oct. The convergence
zone/trough which is crossing Vanuatu tonight is expected to weaken over
Fiji on Monday and fade over Tonga on local Tuesday.

STR= Sub-tropical Ridge
The STR is now mainly along 25S across the South Pacific, perhaps somewhat
north of its normal position, and certainly allowing the roaring 40s to roar
over the mid-latitudes.

Departing westwards from Tahiti:
There may be some scattered convection about Suwarrow this week, nothing
organised. Otherwise all is looking good for sailing west from Tahiti to
Tonga this week.

Between NZ to the tropics
A jetstream is triggering a rapidly developing low to NE of North Island
tonight and this low is expected to travel SSE over the next few days,
sideswiping eastern North Island. Not good for approaching NZ but maybe Ok
for getting north. This development wriggles the upper air so that another
jetstream forms an even deeper low near 130W by Wednesday, It is when a
jetstream is switched from zonal to meridonal that we get these surface lows
The next trough in the Tasman Sea is expected to cross northern NZ on
Thursday, followed by strong SW winds and big SW swells in the north Tasman
Sea on Fri. Avoid.
There may be some big SW swells left over in the Tasman Sea around Norfolk
Island on Saturday. Then the next High from Australia should bring in
settled conditions on Sunday/Monday/Tuesday, so try and time your trip

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