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

27 November 2011

BOBGRAM issued 27 Nov 2011

Issued 27 November 2011
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.

There was a solar eclipse with the New Moon on Friday and this is a Perigean moon. The perigee (= moon-closest-to-earth) occurred 30 hours before new moon. This means that the tides over weekend and on Monday are more extreme than normal - but not quite as extreme as this time last month (Oct perigee was within 7 hours of New moon).

We are having a minor La Nina. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 18 Nov it was 0.90 and on 26th 1.0. Computer modelling suggests it will continue for next few months, possibly making for a quieter than normal cyclone season. At this stage the first Madden Julian oscillation- a period of enhanced convection –may be starting to appear to west of Australia over next week or two, and so this is an early indication it may reach the South Pacific around mid December.

Last week was reasonably quiet on the South pacific. The South Pacific Convergence Zone SPCZ has two branches: one from Solomons to northeast Vanuatu to Fiji/Wallis/Futuna to Samoa, and the other from Tokelau to Northern Cooks. It is expected to stay is much the same position for the next week or so.

High that is crossing to east of northern NZ today should move smartly along 35S over the next few days. Another HIGH is expected to develop at around 40S to 45S in the Tasman Sea on Monday and to spend this week travelling sedately eastwards, crossing the Chatham Islands on Tues/Wed 29/30 Nov. There should be a humid and muggy northerly flow over NZ (and an easterly flow for Northland) for several days.

Another High is expected to travel across Tasmania on Thu /Fri 1 and 2 Dec. This is expected to travel east along 45S, crossing South Island on Wed/Thu/Fri 7/8/9 Dec. More on this in the next weathergram.

Between the Highs there are troughs. One of these is moving across NZ Sunday night and Monday, and should bring some heavy rain to western South Island and some strong to gale NW winds to southern and eastern districts, but just a moderate wind change to Northland.

Next trough should cross NZ on Sat 3 to Mon 5 Dec, and bring a change in Northland from Sun 4 Dec to West then SW then SE winds.

There are still some cruising yachts sailing to NZ, and indeed this looks to be a good week to approach from the north, with the high now at 35S pulling off to the east at last, so that there is a northerly flow over Northland on Fri/Sat 2/3 Dec. Even the trough crossing Northland may be OK for anyone arriving Sun 4 Nov, but it may deepen into a low on Mon/Tue/Wed 5/6/7 Dec and that may be worth avoiding.

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