Bob’s Blog 26 Aug 2018



Compiled Sun 26 AUGUST 2018

Bob McDavitt’s ideas for sailing around the South

Disclaimer: Weather is a mix of
pattern and chaos; these ideas are from the patterned world.

The state of the ENSO = neutral

El Nino and La Nina are opposite ends of the swing of
an identifiable tropical influence on our seasonal weather: the La Nina, caused
by cooler than normal seas along the equatorial eastern pacific. shifts the
subtropical ridge away from the equator, and the El Nino, with warmer than
normal seas, draws the subtropical ridge closer to the equator. Their comings
and goings can last several months, maybe over a year, and so their status can
be used to help forecast the weather for the coming season.

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific
Ocean that acts as a heat storage area during an El Nino or becomes cooler than
normal during a La Nina. This plays with the heat budget of the atmosphere and
thus with the weather patterns.

At the farmonline web site we can see the trend in the
sea surface temperature in the NINO3.4 area. The diagram shows the weekly
temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a
hump on a camel. Since then there has been a cool period late 2016/early2017,
then a warm period until July 2017, and then a cool period until May 2018.
 Ove the last three months we have
been having another warm period.


Weak La Nina as seen at

The International Research Institute of the Climate
Prediction Centre compiles data from several ENSO prediction models. The model
predictions for the Nino 3.4 SST anomaly is that the seas ae likely to
gradually WARM another half a degree during the rest of this year, and then
relax slowly next year.  The most
likely period for this parameter to reach +1 and be called an EL NINO is NDJ
(November December January).  And
that is the early part of our South pacific cyclone season.


CPC/IRI predictions from


The Atmosphere:

ENSO = El Nino/Southern Oscillation. The main parameter
we watch from the atmosphere is the Southern Oscillation Index SOI (30 day
running mean) as it sums up the whole weather pattern over the South Pacific in
one number. It is based on the standardized difference in the barometer readings
between Tahiti and Darwin, in other words it counts the average number of
isobars between them on the weather map. When the SOI is more than plus one
(standard deviation from its mean) for more than a month we call it a LA NINA
event, and when it stays more than minus one we call it an EL NINO event.

For the past year the SOI has been mostly around plus
0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. However,
there have been several brief periods of negative SOI over the past six months
and we are having another of these at present, indicating that there is a
tendency towards an El Nino.


As seen at

(Note that in this graph on the vertical axis 10= 1
standard deviation)


Latest SST anomaly map shows the remains of a large
blue pool of cooler water across the central equatorial Pacific. Also, there are
warmer yellow waters appearing around the Galapagos.  The
 Australia/Tasman Sea is getting
colder than normal at the coldest time of the year, and so is the Humboldt
current off western South America.


Sea surface temperatures across the Pacific on 23 Aug


In Summary: We are neutral but watching as parameters
trend towards an el Nino. There is a good chance of a weak EL NINO for the turn
of the year.  This should weaken
the trade winds, allowing the South pacific Convergence zone and subtropical
ridge to shift to the north a little— this may disrupt cyclone formation to
the east of the norm, somewhat lessening the likelihood near Australia and
increasing it in the Fiji /Samoa area.  Still too early to be sure of this, so just keep it on the backburner for
now, and don’t make any decisions based on that idea just yet.




Map of current storms is from


TC LANE brought wind and rain to Hawaii, much as
forecast. There are a couple of tropical depressions that seem to be vaguely
moving westwards or north-westwards.

After a week of heavy rain over Japan and Korea, the
North Pacific is relaxing now,  and
the Atlantic continues to remains quiet.


Looking at the weekly rain maps we can see that the
Asian monsoon is active between India and the Philippines, and the ITCZ is
active across the Pacific and around Panama. South Pacific Convergence zone is








Weather Zones (see text) as expected Wednesday 00UTC
showing isobars, winds, waves (magenta). STR, and SPCZ. Pink area = lightning
likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon
Islands to Samoa and from there to the southeast.

Broad trough to east of NZ is expected to travel
eastwards and associated passing trough should reach Society Island around local
Tuesday and Tuamotu Archipelago around local Wednesday.

There is a jetstream over New Caledonia tonight and
this is expected to combine with a trough deepening east of Australia on local
Monday to form a trough over Vanuatu on local Tuesday, reaching Fiji on local
Thursday and southern Tonga on local Friday.


Accumulated rainfall for next week from


Subtropical ridge (STR)

HIGH crossing NZ tonight is expected to is expected to
move east along 35S to 40S this week and intensify to over1040hPa by end of the
week. A squash zone of enhanced
trade winds on the north side of this high is expected to peak along 12 to 20S
between Tahiti and Tonga from local from local Tuesday to Saturday.  Avoid.

This squash zone detracts from a comfortable voyage
from Tahiti to Tonga this week., unless  you skirt around it via Suwarrow and
avoid its peak on local Thursday.

From Thursday to end of the week, Another HIGH is
expected to travel east from Australia to South of Fiji along 30 to 20S.  Small squash zone is expected in Coral
Sea on the northwest side of this High.


Around Tasman Sea, NZ to tropics.

Passing High with light winds on Monday.    Low is expected to
deepen in Tasman Sea bringing strong northerly winds to northern NZ on Tuesday,
then a front on Wednesday, and then the LOW over central NZ on Thursday and
Friday.   It’s
a stay at home week for NZ yachts, but there are good voyages from Australia to New
Caledonia.   If sailing to NZ avoid the Wednesday front.



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