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

17 June 2018

Bob Blog 17 June



Compiled Sun 17 June 2018

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.


CAPE  addendum.

I was asked to try and track down who originated the CAPE index. I‘ve looked and not found—maybe someone who reads this can fill us in. The earliest stability indices where when we started to use weather balloons to measure the vertical temperature profile and plotted this data on what we call tephigrams.


See a tephigram at this us a Meteorologist’s lunch: 27  parameters listed on the right, and CAPE is the red area .

For teh latest tephigram see ( select New Zealand and  Gif Stuve  then NZWP )

Spot indices such as the Lifted index  LIFT (difference between Temperature of environment and temperature of adiabatically-lifted parcel at , say, 500hPa),  and Showalter index  SHOW (much the same but at 850hPa) can be computed quickly on a Tephigram and were the first ones used. CAPE can also be shaded-in on a Tephigram and takes into account the whole vertical range of the temperature profile (what we call the “sounding”) and so carries more finesse. When computer models started to go global we needed a good way to parametrise the  vertical temperature profiles  so they could  be compared around the planet. That’s when CAPE became popular.


The earliest paper I could find using it is M. W. Moncrieff, M.J. Miller (1976). "The dynamics and simulation of tropical cumulonimbus and squall lines". Q. J. R. Meteorol. Soc. 120 (432): 373–94. Bibcode:1976QJRMS.102..373M. doi:10.1002/qj.49710243208. At


CAPE is good, but not a universal convective index. You can see how it is always highest in the tropics. This is because when the air is warmer, CAPE can be higher.   However, thunderstorms in the mid latitudes triggered by air being forced to rise over a mountain range--- such as the Southern Alps or the Andes --- can have updrafts just as powerful as their tropical cousins –even if they have much lower CAPE values.


As we approach the SOLSTICE this week(Thu 21June 1007UTC), it is time to check the state of the ENSO = neutral


The Atmosphere:

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.


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.


From June 2017 to May 2018 the SOI has been mostly between plus 0.5 to plus 1, sometimes higher, consistent with a weak but rather persistent La Nina. The subtropical ridge line was further from the equator than normal, and trade winds were stronger than normal.

However, over the past month, the SOI has settled into a near zero state, and has a NEUTRAL status.

Neutral conditions as seen at

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


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 warm seas of 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/Aug 2017, and then a cool period. That cool period seems to be coming to an end now, and the sea surface temperature are near normal.

Fading 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 during the rest of this year, but the average of the predications has only warming to 0.5 above normal--- not enough to be called an El NINO event (but closer to it that we have been for a while).Then again 3 of the models are hinting at an El NINO by end of the year.

CPC/IRI predictions are at


Latest SST anomaly map shows a zone of slightly warmer sea from Galapagos along equator to dateline. Worth watching, if it grows we can expect the trade winds to weaken. The remains of the cooler seas of the recent La Nina seem to be further south.  Sea surface temperatures across the Pacific can be seen  at



There has been an MJO in the North Pacific over past week or so, triggering several developments: MALIKSI bypassed Japan, followed by EWINIAR

AT present we have TC GAEMI off the south of Japan, also expected to peel off to the northeast, and this is likely to be followed by another tropical depression. Can be seen at


Off the western Mexico coat we had tropical storm ALETTA and BUD over a week ago and now we have TC CARLOTTA pounding the Mexican coast with potential flodding rain for Acapulco.. It’s the second-earliest fourth tropical cyclone in this region during the satellite era (since 1966), missing out on being the earliest by 6 hours.

Can be seen at


If we compare the past week’s rain map with the previous week we can see a build-up of activity over yanmar and in the China Sea, and a focussing of activity off western Mexico. Activity has eased across the South Pacific.

– see



SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from Solomon island to north of Vanuatu, and be weaker than last week. There may be some rain from a passing trough other Austral island of southern French Polynesia.


Subtropical ridge (STR)

A High is expected to travel east along 25 to 30S from 180 to 150west this week. There is likely to be a small squash zone of enhanced trade winds and swell on its northern side, affecting mainly 17S from 150W to 170W /Niue mainly on Tue/Wed UTC.

Next HIGH from the west is expected to enter the south Tasman Sea around Thu 21 June and then spread across NZ on Sat/Sun 23/24 June.


Around Tasman Sea, NZ to tropics /FP

LOW1 is expected to cross central NZ on Monday /Tuesday, and Low2 is expected to travel from northern Tasman Sea on Tuesday to east of northern NZ by Thursday.

After Low2 there is a reasonable-looking weather pattern for heading off north to tropics or east to Tahiti. Might be able to depart on Thursday, but that voyage may have some strong winds off shore on its first night.


Australia to tropics

There is a good opportunity this week, on the back end of Low1/Low2.

However, strong SE winds are forecast in the Coral sea this week.


Tahiti to Tonga

South Pacific convergence zone is not really a factor of concern this week.

Passing trough over Australs on Monday 18June UTC and gambier Islands on Tuesday 19 June UTC.

There is expected to be a zone of enhanced ESE winds over 20kt and swells over 2.5m along around 17S from 150W to 170W /Niue mainly on Tue/Wed UTC.

Next trough from the west is expected to reach New Caledonia on Tue 19 June UTC, Fiji/Tonga on Thu 21 June UTC, and then Rarotonga around Sun 24 June UTC.


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