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

24 April 2011

BOBGRAM7 issued 24 April

Issued 24 April 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.

La NINA is still active in the atmosphere.
The Southern Oscillation Index 30day running mean has been on a roll
upwards : from a low of 1.85 on 25 March to a high of 2.55 on 23 April.

That recent pulse of active convection, known as a MJO period, which
brought unsettled weather to North Australia earlier this month, faded
away upon reaching the Coral Sea last week. This was contrary to our
thinking a week ago, showing that the atmosphere is always capable of
nice surprises.

This suggests then that South Pacific is going through a relatively
settled period for the coming week.

The International Tropical Convergence Zone ITCZ lives near 3 to 5
degrees North across the Pacific, and there are signs of a "mirror" to
this near 5S, especially along 100 to 180W. This tends to happen
around the equinox, but that was a month ago so the mirror zone should
break up over next few weeks and then it should be ok to puddle-jump
travel from Galapagos to Maquesas. No rush :)

South Pacific Convergence Zone SPCZ is near the southern edge of its
realm, across the Coral Sea where it is entwined with trough in the
Tasman Sea. Another branch of SPCZ stretches from Vanuatu across Fiji
and Tonga to Southern Cooks, coming and going.

The STR has been knocked south of normal as well.
One High starts this week near 45S 150W, perfectly placed to develop a
LOW on its left shoulder near 30S 160W - close enough to bring a
westerly flow to Southern Cooks and Australs for a few days. There is
an intense squash zone between Low and High to avoid, and the Low is
expected to go SSE and the High goes east, so that the winds in this
squash zone are likely to start out as being from the east and end up as
being from the north.

Of more interest in the NZ area is another HIGH that starts the week at
43S to east of Tasmania. This HIGH is stuck by troughs trafficking over
NZ and finally should be able to swing around southern NZ on Thu 28
April, and then expand over Chatham Islands area on Fri 29 April and
wander further east next week.

With low pressures to the north and high pressures to the south, an
easterly flow is expected to linger over northern NZ this week. One
rather shallow low is likely to cross Northland on Monday and the marine
part of Bay of Plenty on Tuesday causing an intense squash zone of
southeast winds over central NZ. This low should fade on Wednesday and
Thursday, but the squash zone is likely to build a heavy swell on
Gisborne/Hawke's Bay Coast by Thursday 28 April. Avoid.
On Thursday and Friday another low should deepen-this time in Tasman Sea
near Lord Howe Island --- this Low should move SSE and may cross
southern NZ around Sun 1 May, but there is some uncertainty at this
stage on this track so please update.

These troughs are expected to bring unsettled weather between Northland
and the tropics this week and much of next week, so it is NOT a good
time to plan a voyage north just yet. Patience will be rewarded in May.

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