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

12 September 2010


Issued 12 September 2010
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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

I've been asked to supply some details about the strength of this La Nina... The Oceanic Nino Index ONI, which measures the difference from normal of Sea temperatures in the target area of the eastern equatorial Pacific (Steep) now has a 90 day funning mean of -0.6 == roughly the sea is on average that much cooler than its norm. The Atmospheric Southern Oscillation index SOI, taken from the pressure difference between Tahiti and Darwin, has a 90 day running mean of +1.64 (standard deviations). Both indicate moderate to strong La Nina conditions and the ocean and atmosphere are coupled in this.

South Pacific Convergence zone is beginning to return to normal now , showing up as bubbly convection over Papua New Guinea and Solomons, and as a shear zone of vertical convergence (a layer of trade winds near the surface topped by some vigorous W to SW winds) around Tokelau/Samoa to Tonga/Niue to Southern cooks/ Tuamotu.

The upper trough the marks the boundary of this shear zone is expected to drop to the surface around the Southern cooks/ Tahiti on Tue/Wed 14/15 Sep and then to move south-maybe-Southwest across the Southern Cooks Thu 16 to Sat 18 Sep. Anyone planning the west jump from Tahiti should take this into account - maybe delay a while.

The northern extension of a mid-latitude front that moved off North Island of NZ today is affecting New Caledonia tonight Sun 12 Sep, and should fade away over Kermadecs around Tue/Wed 14/15 Sep.

A BFH big fat high which is well supported aloft continues to dominate proceedings. Its central pressure is around 1038 tonight Sun 12 Sep near 36S 155W and should slowly wander east and ease to be around 1028 at 31S 133W this time next week 19 Sep. So the squash zone of enhanced easterly winds on its northern side is bringing vigorous easterly winds and rough seas between Tahiti and Niue - at least until that trough drops down around Tue 14 Sep, and even after that it will take a while for things to settle down.

Not the best week then for venturing westwards from Tahiti.

Next high cell in the subtropical ridge doesn't have much vertical support - it is budding off eastern Aussie now and will cross the northern Tasman Sea on Mon 13/Tue 14 Sept and fade over Kermadecs on wed 15/Thu 16 Sep.

The high after that is expected to wander east across the Aussie bight along 40/45S on Sat/Sun 18/19 Sep.. shovelling cold air from 55S to Tasmania .. not quite a polar blast but it'll be a wintery entrée to the equinox for SE Aussie.

A brief break on Monday and then another front on Tuesday 14 Sep UTC, with some heavy rain for western districts, and followed by a cold SW in southern districts.

After another brief break late Wednesday and then some roaring westerlies of spring, enhanced by a deep low in the southern ocean -- these should reach NZ for Thursday 16 and much of Friday 17 Sep, followed by cold SW flow on Sat/Sun 18/19 Sep.

So a voyage from Tonga to NZ departing early this week may encounter a weakening trough/ridge couplet around 30S and then some strong west to SW winds over Northland around 20 Sep, reasonably adventurous.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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