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

07 September 2014

BobBlog 7 Sep 2014

Issued 07 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

To help keep you advised of my blogs and other weather details I have opened
a twitter account. My twitter handle, if you want to follow it, is @MetBob2

For readers with Internet access: Lets start with an oldie but goodie
experiment to prove that 1000hpa= 10,000 kg (10 tonnes) of pressure: also read the comments, (those
Sydney physicists make physics fun)

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 in the past week the 30day running mean
has dived below -10 (Australian units)  if it holds this for a month then
we can call this a full-blown El Nino.
Interestingly, the amount of heat being stored in the ocean in the eastern
equatorial Pacific Ocean is also on the rise now, as measured by the weekly
NINO 3.4 index.

So some of you may be asking, IF an El Nino is kicking in , THEN how come we
are NOT getting lots of strong SW winds over NZ/Tasman Sea area? Well the
answer to this can be gleaned by looking at the variation from the average
weather map for August  the monthly isobar anomaly map shows an anomalous
HIGH over Australia New Zealand, but shows typical El Nino type weather
across the remainder of the South Pacific Ocean.
These anomalously high pressures have been thanks to some local blocking
around the 180th meridian recently. Blocking is an interesting factor that
plays around on a monthly scale with the seasonal pattern. The current
blocking has been around for six weeks. So , it is probably due to soon
move off and is there is pattern in the weather, then we can expect  after
this weeks generous looking Tasman High- a return to a month or more of
disturbed westerlies that are more El Nino-like over NZ area. Probably
about time too, as there hasnt been any decent rain in several parts of
western NZ (including Westland/Buller) for 3 weeks. And we all know pasture
grass dies after 3 dry weeks.

Hurricane NORBERT is an interesting cyclone off the Pacific NE coast , which
some computer models are picking may remain as a meteorological entity all
the across north America. Wow.
The other feature around at present is tropical storm FENGSHEN off eastern
And there is a tropical depressions in the South China sea and a couple in
the tropical Atlantic ocean.

The weekly rain maps for the past fortnight show that rain burst that was
near India is shifting east across Burma/The Republic of the Union of
Myanmar, and there has been a weakening in the convection in the South

SPCZ= South Pacific Convergence Zone
The SPCZ is slightly weaker than last week, and a branch of it is expected
to affect the area between Vanuatu and western Fiji on Wednesday
10 September, along with strong SE winds on its southern side. Avoid.
Indeed there is likely to be a zone of strong SE winds on the south-side of
the SPCZ across the northern Coral Sea for the next week plus.

Departing westwards from Tahiti:
A trough in the southern ocean/mid-latitudes is expected to travel east this
week weakening the trade winds between Niue and Southern Cooks. It is
expected to deepen over and to the SE of the Austral Island (south of
Tahiti) late this week.
SO those in Tahiti intending to go west this week should take this trough,
and its associated convergence zone, into account.

STR= Sub-tropical Ridge
The STR has a wave-shape in its latitude across the map, dipping south over
NZ at present, but is north of normal over Australia and Tahiti.
The High that is expected to cross North Island next few days should travel
east along 40S to east of NZ from Wednesday. After this high, a return to
disturbed NW/South island W winds is expected to reach South Island on
Thursday and North Island on Friday.

Between NZ to the tropics
Good weather for getting north out of NZ on Monday.
Also good for departing tropics early this week to get to NZ: the next
period of SW head winds around Northland is expected to occur next Sunday
14 Sep 14 September, and these is not expected to be strongso come on in if
you are ready to go now.

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