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

17 May 2015

Bob Blog

Issued 17 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.

Wellington and Kapiti s Thursday deluge:
As an active front rolled under a jet stream last Thursday it was able to
enjoy some enhanced upward motion. The arrangement of winds, with Jet
stream on top, prefrontal northwest winds underneath and converging winds
along the front, allowed rain to be collected from a wide area and then
focused so that it fell in a narrow zone. The rainfall occurred in narrow
bands, or as thunderstorms, and so some parts of the region were affected
far worse than others. MacKay s crossing had 145mm, and Otaki, just a
few miles further north, had insignificant rain. Avalon in the Hutt Valley
had 42mm in one hour.

The Tephigram for Paraparaumu (data from MetService and see display from around midnight during the
event it is a graph of the temperature and dew point trace as measured by a
weather balloon ascending through the atmosphere and is one of the main tool
s used by meteorologists to examine these sort of storms.
The winds are calculated by the actual drift of the weather balloon,
followed by GPS transponder, and are plotted on the right hand side of the
graph so that NW winds are drawn as coming from the top left. A barb is
worth 10 knots, half a barb is 5, and a pennant is 50. Notice how these
winds show a jet stream on top at 80 knot and winds curving with height so
they sculpt and focus the falling rain.

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
went negative, to be -10 units for a few weeks in late March/early April,
then relaxed for a while. It has dived very negative in the past week, and
I think this is in response to a low that formed near Tahiti and has since
then moved off. Another trough of low pressure is expected to affect Tahiti
this week, and maybe this is going to be the trend as El Nino settles into
the atmosphere.
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.

The Ocean
More than half of the world s oceans are warmer than they should be at
present, and this is true along the entire Pacific equator. This means the
ocean is storing extra energy, not just in the target area for El Nino, but
also in many other places. When this excess gets into the atmosphere we can
anticipate extreme storms. Sea surface temperature anomaly is shown at

Super Typhoon DOLPHIN is managing to stay offshore as it swings past the
Philippines this week. There has been a tropical Low over the Solomons for
the past week and we have been watching it wondering if it would develop
deeper. Well it now looks as if that system is now on its last legs and
should fade away in next few days.
This year s Atlantic Hurricane Forecast: has now been issued. This region
has already had ANA, and the forecast is for BELOW NORMAL NUMBERS= 8 named
storms. 4 hurricanes and 1 major hurricane are predicted for the Atlantic
basin, with only 2 or 3 USA landfall.
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. You can see
it as these SW winds in the Bay of Bengal on, and when you
switch to rain and accumulation for next 10 days, then the
amount that is being predicted is indeed under-whelming.
( I like to check out the FL350/450 winds at this time of the year and
spot Easterly Jet Streams they are so weird and I'm sure there is a Phd
waiting for anyone who can dare try and explain them!)
If you would like a website to track this year s monsoon then try

Weekly rain signatures are seen at
The rain map for the last two weeks shows heavy rain with DOLPHIN and that
low over the Solomons. Also rain from a low near Tahiti. And some bursts
of rain continuing from Nepal to the China coast.

Galapagos to Marquesas?
Better winds for departing are on local Wednesday/Thursday this week. There
is now a counter-current to avoid near EQ to 4S at 95W so go to south of 4S
95W, then chase the west-going current which is still along about ^S from
100W to 130W a free ride. There may be some convergence shower activity
north of 5S, another reason to stay south of 5S.

SPCZ=South Pacific Convergence zone.
SPCZ is expected to relax this week over Solomons remain active across
Solomons and part of it may travel south cross Coral Sea and reach new
Caledonia by the end of the week. The main part of SPCZ is likely to stay in
the northern position from
Tuvalu to Samoa to the Cook Islands/ Australs. One friend from Bora Bora
has emailed me to say he hasn't seen the sun in weeks.

STR= Sub-tropical Ridge
HIGH in central Tasman Sea is expected to be a frosty one as it crosses NZ
on Tue/Wed/Thu. With its central pressure easing to 1022 or less it does
not have a squash zone of enhanced easterly winds on its north side. From
Thursday it is expected to move off the east along 25S.

Tasman Sea /NZ
Trough is expected to move onto South Island from Wednesday and develop a
LOW in central Tasman Sea by Thursday that is expected to deepen as it
crosses northern NZ on Fri/Sat 22/23 May.

Departing from Australia to the tropics this week?
May still be too much onshore wind over Queensland on Monday, but OK to go
on Tuesday/ Wednesday. Take care with the wind changes and rain from that
Low on Thursday, and the southerlies and big swells that follow it on
Friday. A squash zone of strong SE winds is expected in the Coral Sea by
end of the week.

Departing from Northern NZ going north?
As the HIGH goes East there are some SW/S/SE winds that are good for going
north Best departure date was today, and Monday departure is OK but likely
to be engulfed by Wed/Thu and then will need to contend with prefrontal
Northerly/NW winds on Friday and the FRONT on Saturday 23 May. FRONT
should be followed by OK SW/S/SE winds for the remainder of trip. By
Tuesday the current window closes. Next window might open around Sun 24
or Monday 25.

Departing from Northern NZ going east?
Good departures on Monday and Tuesday. After that it may be better to wait
until next Sunday or Monday.

See my yotpak at for terms used.
Weathergram text only (and translator) is at
Weathergram with graphics is at, click FOLLOW at
bottom right to subscribe.
My website is at Feedback to
To unsubscribe send a reply email saying LEAVE.

No comments:

Blog Archive