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

25 February 2018

Bob Blog 25 Feb 2018



Compiled Sun 25 Feb 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.


Progress report from Greg Just in Vava’u and his program for Tongan weather stations:

“– the unit I had purchased has proven unreliable as It only lasted a year before having to replace the unit as all but one sensor died.”

“I have selected Davis Vantage Vue as a better model, based on an expat in Vava’u that has one that only broke when it fell from his windmill (rust in the mounts) after 10 years. The unit is roughly 50% more expensive. I am getting a unit up to trial in May on a yacht. This will be based in the bottom of the Vava’u group on an island resort so I can get feedback.”

“I am also waiting to trial 8 AIS units across Tonga with TCC (Tonga Commnications Corp), and can then host the weather stations / webcams at the same sites with UPS and Internet services provided free.”

“The local emergency assist project/NZ Police are also working with NZ Coastguard to mount sensors on Floating Buoys in Vava’u. Which will provide info on weather and Tsunami.”

“I was hoping to get the whole project funded by an Asian Development Bank Grant scheme. I am applying for another grant shortly through GCF as the project will be a lot easier if 100% funded all at once.”

  Greg went on to describe the Damage from GATI:

“No damage for Vava’u, a little damage in Ha’apai. Tongatapu/’Eua:  I’m guessing around 40% roof damage and 10% loss of buildings based on unofficial observations, huge crop and coconut damage. They have not had a high CAT cyclone for 60 years so had many flying objects especially.”

If you would like to make a donation to Tongan Weather stations go to


The state of the ENSO = confused, La Nina ocean and neutral atmosphere.

The Atmosphere:

El Nino and La Nina are opposite ends of an identifiable tropical influence on our seasonal weather: either a La Nina, caused by cooler than normal seas along the equatorial eastern Pacific, which shifts the subtropical ridge away from the equator. Or an El Nino, with warmer than normal seas, which draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year,  and sometimes the state is in-between or neutral, and so their status may be used to forecast the weather for the coming season.


The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) for 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.


After switching into a weak to moderate La Lina around last October, the SOI indicator turned negative last week, a sign that the La Nina may be  over and we are in neutral territory, as far as the atmosphere is concerned.


Neutral territory in the atmosphere 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 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 this cool period of a La Nina. In the ocean the coolness is continuing, reaching around -1C at times, showing no easing in this La Nina as far as the ocean is concerned

A continuing La Nina is seen at


The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The mid-February probabilistic forecasts show this La ina is expected to revert to neutral territory by April/May. 

CPC/IRI predictions from


The sea surface temperature anomaly map shows a marked warmer-than-normal Gulf Stream of North America—so they may have an early spring, perhaps.   La Nina shows itself as several cooler eddies along the equator of a typical La Nina.  And there are several yellow/flecked with red zones of WARMER ocean, with one continuing around New Zealand.

New Zealand has just had its warmest summer on record thanks to this warm zone of sea temperatures.  We think it formed thanks to lighter winds and fewer clouds over the region last spring, but this is just a theory at this stage.


Sea surface temperatures across the Pacific are at



The MJO has moved out of the Pacific and is now reforming in the Indian Ocean.  This means the risk of tropical cyclone formation in the South Pacific over the next few weeks is lower (but not negligible). 

There are no cyclones around this evening. We are having a welcome break.

The liquid remains of KELVIN have merged with the wet front crossing southern NZ tonight. Milford Sound have had over 300mm of rain so far today.

Looking at the weekly rain maps from last week and the week before, we can see an easing of the intensity of the rain in the South pacific.  There is an increase in activity around northeastern Australia. And in the ITCZ across the equatorial Atlantic.

See for last week's rain


Weather Zones

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to be weaker than last week, and is spread rather thinly from Solomons to Vanuatu to Fiji/Tonga, and then as a narrow band across Samoa and Cook Islands, affecting parts of French Polynesia at times.


Subtropical ridge (STR)

HIGH is expected to travel east across the south Tasman Sea on Monday and then ‘squeeze’ around southern NZ on Tuesday and then travel NE to east of NZ and build as it travels east along 45S to east of NZ for the remainder of the week.

Next HIGH is expected to form east of New South Wales on Thursday and travel across southern NZ on Friday and weekend of Sat 3/Sun 4 March.


Around Tasman Sea

Front that was active over South Island today/Sunday is expected to fade over central NZ on Monday. A secondary low is expected to deepen on its western edge in the central Tasman Sea by Tuesday. This Low should deepen and approach South Island on Wednesday then weaken as it crosses southern NZ on Thursday.

From Thursday, a LOW may form between Fiji and New Zealand, with brisk easterly wind son its southern side.


Panama to Galapagos /Marquesas

If you are ready now to do this trip, then the next two weeks are looking OK with N to NE or East winds and a tail current as far as 3N.  After that there are light ESE winds to Galapagos, and a weak convergence zone near the equator.

As for going to Marquesas there seem to be moderate Southeast to East winds to west of 105W, and these should last until at least first or second week of March.



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