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

31 May 2015

BOB Blog 31 May 2015



Issued 31 May 2015

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.


The Atmosphere:

The Southern Oscillation Index SOI sums up the weather pattern over the South Pacific as one number and is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean).  It has dived very negative in the past few weeks, possibly due to troughs near Tahiti. We haven’t seen it this negative since Jan 2010.

SOI is shown at

If the next reading of this index remains lower than -10 then we have a fully- blown El Nino (both ocean and atmosphere). .


TC ANDRES is now well off the west coast of Mexico and slowly fading. There is another tropical depression already set to follow it this week.


In an El Nino year the Indian monsoon is usually late, slow, and not-so-wet. Well, this El Nino hasn't really got into the atmosphere yet, and the Indian Monsoon is starting up already and is maybe a day or so EARLY.

The Indian Monsoon is now about 5 days BEHIND schedule.  Its failure to bring ocean-moistened air into the heart of India has led to extremely high temperature to inland India:

From  - last Friday’s reading was more likely to have been 41 rather than 14 (to err is human).  Human fatalities in India related to this pre-monsoon heat wave for 2015 are so far around 2000.—it is interesting to note that the previous high was 2541 in 1998  (the last mega El Nino).

Advance of Monsoon is seen at


The rain map for the last two weeks shows ANDRES blossoming the rain SW of Mexico and also a build-up of heavy rain over Papua New Guinea.

Weekly rain signatures for past two weeks, as seen at



 Galapagos to Marquesas:

Poor light winds for departing on local Wed/Thu this week.

There is still signs of a good west-going current along around 5 to 6S from 100W to 125W, so use this if you can. There may be some convergence shower activity around the Marquesas until 6 June UTC. 


SPCZ=South Pacific Convergence zone.

SPCZ is expected to be weak again this week and well north of its normal position. A rare week of good weather for sailing from Tahiti to the west.


STR= Sub-tropical Ridge

STR is also north of normal—an El Nino trait.  IT is strong over dry Australia and the along 25S across the entire South Pacific.

Next HIGH is expected to move a cell into SOUTH Tasman Sea on Thursday and combine with a HIGH being knocked off Antarctica, there-by shovelling chilly winds onto NZ from the Southern ocean, with traceable trajectories off the ice shelf. The snow from this is expected to be mainly alpine.


Departing from Australia to the tropics this week:

LOW is expected to cross central Tasman Sea from Monday to Wednesday with strong SW winds and rough seas.  After Wednesday these winds ease and it should be Ok for sailing to Noumea again.


Departing from Northern NZ going north.

Front on Monday and LOW affects NZ weather with gales and heavy rain from Tuesday to Thursday. You can see its Thursday position in the above map, so forecast for Thursday in NZ is alpine snow.

Then it MAY be Ok for a departure on Friday from NZ to tropics—however there is likely to be a small low forming between Lord How and Norfolk Islands on Sat 6 June and then going SE .  This small low might affect those Friday departures—too far away to tell much detail at this stage.


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