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

12 November 2017

Bob Blog 12 Nov 2017



Compiled Sun 12 November 2017


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.

Correction to my Facebook entry last week: Tonga is NOT having Summer time this year .


The state of the ENSO = La Nina trend now showing

El Nino and La Nina are opposite ends of an identifiable tropical influence on our seasonal weather. La Nina brings cooler than normal seas along the equatorial eastern pacific, is associated with stronger than normal trade winds and shifts the subtropical ridge (and its jetstream) away from the equator. El Nino with its weaker than normal trade winds and 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 forecast the weather for the coming season.


NOAA have declared this to now be a (weak) LA NINA event with a good chance of lasting next 3 to 6 months. See

Bureau of Met, Australia, is on a LA NINA watch and says the La Nina remains possible, but the effect on Australia’s climate may be less than recent events.

Implications for NZ area in the next 6 weeks is that we should have an early start to summer, with anticyclones crossing the Tasman Sea and New Zealand mainland bringing periods of sunny weather and light winds. After New year, there may be periods of strong easterly winds over Northland. In the South Pacific, convergence zones should be further south than normal, and trade winds may be stronger than normal, especially along the equator.


The Atmosphere:

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 the placement of isobars 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.

Since August the SOI has been more than 0.5, and two weeks ago it was over the +1 threshold. In the past week it has relaxed a little. Weak La Nina is 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 that since August there has been a cool trend for NONO3.4, and a few weeks ago the seas were almost 1-degree cooler than normal.

Weak La Nina is seen at


The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The mid-October edition shows NINO3.4 temperature may keep dropping (a little) until JFM (Jan Feb Marc) 2018.   CPC/IRI predictions are at



It has been busy around Vietnam recently, and Tropical Storm HAIKUI is currently heading west across the South china Sea after visiting the Philippines, but should soon fade.

Looking at the weekly rain maps at from last week and the week before, we can see a burst of heavy rain travelling east along the Indian ocean equatorial region towards Indonesia, and a burst of rain about Cuba (passing cyclone).



SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay from Solomon Islands to northern Vanuatu to Fiji. A Low is expected to form on this zone by Thursday between Vanuatu and Fiji by Thursday. This low is expected to deepen to around 1000 hPa near 27S 174E by Friday.

A secondary convergence zone is expected to hover from Samoa to Southern cooks.

Any voyage from Tahiti to Tonga will need to sail/motor thru this secondary zone —it has several weak spots, so this may be worthwhile with a little guidance.


Subtropical ridge (STR)

HIGH H1 in the south Tasman sea on Monday is expected to move towards central NZ by Thursday and off to the east of NZ on Friday. It may be a useful HIGH for anyway intending to cross the Tasman Sea

Weak trough should cross southern NZ on Thursday, followed by another HIGH H2 spreading slowly into South Tasman Sea from Friday to Sunday (19 Nov). This may be a useful pattern for anyone intending to cross the Tasman Sea next week.


Between Tropics and NZ

Models are picking LOW to form between Fiji and NZ and deepen on Friday with strong, maybe gale, easterly winds near 30 to 33S (a squash zone with H1). Something to avoid.

The Low is expected to move off to the east/southeast on Sat/Sun 18/19 Nov, or maybe early next week--- its future depends on sneaking in-between H1 and H2.


Between New Caledonia and Australia

H1 allows a good voyages from New Caledonia to Australia this week and H2 next week.


It is the set of the sails, not the direction of the wind, that determine which way we will go

- Jim Robin


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