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

14 September 2014

BobBlog issued 14 September


Issued 14 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.

1) One of my yachts encountered the South Pacific convergence zone that was
lurking to north of Fiji last Wednesday night/Thursday. He reported
I'm currently at 15 06S, 179 41E, on passage from Samoa to Port Vila,
Vanuatu. We left Savai'i Island on Sunday 7th Sep after hanging around for a
bit waiting for a window where the winds weren't going to be too strong.
After a day or so of 10 - 15 knot winds, they began to build from the SE
quite significantly.
2 days ago at around 7am, end of night watch, we were at 14 12S, 176 00W,
and with a GFS GRIB forecast of 12 knots we had winds around 21 knots. It
continued to build steadily from there, yesterday at 7am we were at 14 48S,
179 06W with winds around 28 knots (forecast 13), and at the moment we have
winds touching the bottom of the gale force range at a steady 36 knots
(forecast for here and now is 15 knots) with gusts to 44 or so. I'm looking
at the GFS file that I picked up a few hours ago, and I can see that just
ahead, about where we will be at 1pm today, there are forecasts around 22
knots. If I was a betting man then I would be putting my money on the actual
winds being steady in the 50+ knots range with perhaps peaks touching 60
I'm running at the moment with essentially a tri sail and storm jib and
although it's tempting to douse the tri sail there is also a swell that's
been building from 4 metres and the tri sail is helping to reduce the roll a
bit. I have suffered a bit of minor rigging and structure damage out at the
aft deck but nothing too serious.
I guess that my main concern is that in amongst all of this there is nothing
from the various weather bureaus warning of this. I think that if any sailor
is venturing out into waters where the wind is blowing 50 knots or more,
especially where we are which is in reasonably well trafficked waters just
north of Fiji, they would expect at least a gale warning if not a storm
warning. There are no warnings at all from Fiji Met Service and none from NZ
MetService for this area, despite the actual winds having been blowing in
the "strong wind warning" range for over 24 hours now, and we're now in the
range where I would expect a gale warning to be issued, right now, for the
area we are in.
I also have no real idea if the anomaly between forecast wind and actual
wind (about 100% difference or a bit over as of the last 36 hours) is due to
some kind of local conditions.
So, a few things.
Can we get a gale warning issued for these waters at the first available
opportunity please? As a gale warning would be expected for any area where
gale force winds are likely to happen, and as there are gale force winds
blowing right here and now I think it's fair to warn other boats.
Are these conditions likely to continue? We're going to try to hold a course
close to 245M / 257T to get us to Port Vila, Vanuatu. I'm now treating the
GFS GRIB data as a work of fiction, but if you weather boffins can give me
some idea whether these winds are likely to build up to and past 50 knots
that would be appreciated.
Are these conditions entirely local or is there somewhere we can go that's
likely to give us a bit of relief, if not from the winds then at least from
the swell which is continuing to build (with a lot of white caps and a
significant amount of foamy spray as well)? With the amount of swell we have
at the moment, not to mention waves breaking over the base of the mast and
as high as the lower part of the HF antenna, it's a wonder I can hear
anything and unsurprising that I can't be heard.
He was caught in some squalls on the south end of the South Pacific
Convergence zone and you can sense his frustration with these gale-like
winds and increasing swell. In this case the way out of the SPCZ was to go
south, and I was able to check the WINDSAT satellite and the animated
satellite imagery from to show him (when he finally got good Internet at
Vila) that this was indeed isolated squall activity and thus unlikely to be
given a gale warning on a high seas forecast, even though the local winds
were gale-force.

Let this be a warning to us  there are limitations to weather forecasts:
the weather computers that produce those GRIB files just average the
weather out, and DO NOT resolve the details in the South pacific Convergence
zone, also the High Sea forecasts do NOT issue gale warning for squalls
that come and go along the SPCZ.
2) There is a rare double solar flare passing by our planet at present, with
stunning aurora and also some possible disruption to satellite
3) For readers with internet access: If you enjoyed last weeks experiment,
here is another go from some college students:,
and another showing how to get an hard-boiled egg into and out of a bottle
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) a
week ago, and relaxed a little in the past week  if it holds below -10 for
a month then we can call this a full-blown El Nino.

In the ATLANTIC we have EDOUARD, off the Mexican coast there is ODILE and a
tropical depression, and in the Pacific NW we have KALMAEGI.
The weekly rain maps show that a rain burst has been hovering around
northern India for the past fortnight. Indeed, Kashmir seems to be the worst
affected area, with an estimated 200 deaths in India and another
200 in Pakistan.

SPCZ= South Pacific Convergence Zone
The SPCZ is slightly weaker than last week, and by Wednesday is expected to
be hugging the 10S line from PNG to east of Samoa. It may shift a little
further south later in the week.

STR= Sub-tropical Ridge
The STR has reverted to its normal latitude for this time of the year and
this pattern may last for a week or two, with no sign now of the blocking
that was affecting the pattern over the past month or so. This week the High
cells with the STR are expected to travel east along 30S.

Departing westwards from Tahiti:
A convergence zone is expected to slide east across the southern parts of
French Polynesia during local Monday and Tuesday, followed by a period of
easterly winds. These easterly winds should be well maintained by a high
cell traveling along 30S, and should be good for going west.
The SPCZ may affect Suwarrow from time to time. Also there may be a small
squash zone of SE winds near 16S late in the week. And the north end of a
trough is likely to cross Southern Cooks late in the week. Otherwise it is
looking to be an OK week for sailing westwards good for travelling west.

Between NZ to the tropics
A trough is expected to be crossing Northland on Tuesday, followed by
another on Thursday night/Friday/Saturday. Avoid.

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 unsubscribe.

No comments:

Blog Archive