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

13 September 2009

BOBGRAM7 issued 13 Sep 2009

Issued 13 September 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

This week a MJO, which encourages extra convection near the equator, is
expected to weaken around the Solomons area. There was an interesting
tropical low that briefly formed in the Coral Sea to NW of New Caledonia
mid last week, and some of the energy of this is now wrapped up in the
LOW today between NZ and Fiji. This MJO is also in the India Ocean and
one computer model has come up today with the scenario of a Tropical Low
deepening near the Diego Garcia (near 5S 70E) in the Indian Ocean this
week. However the MJO is expected to be fading in this area by then, so
this will maybe just be a small low.

We are approaching the equinox, and that's when the sun gets overhead at
the equator. Around this time of the year a line of convergence
sometimes forms across the South Pacific along 10S-something like a
mirror image of the ITCZ... sort of an equinoctial convergence zone. We
might see this starting to form during the next week or two.

The South Pacific Convergence Zone SPCZ has been mainly located from
Solomons to Wallis/Futuna in the past week, and I expect its eastern
edge to wander north towards Samoa this week, and then stretch southeast
to be active in-between the Cooks and French Polynesia.

The LOW that is today between Fiji and NZ formed as an upper wave ridge
was arriving over NZ, masking NZ a place of rising rather than falling
pressure-this has blocked that low, causing it to do a loop and knocked
it a little North. Soon it will wander east (accompanying tough is over
Fiji today and should cross Tonga on Mon/Tue 14/15 Sep) and fade with a
little of it moving south well east of New Zealand.

A new trough is expected to form over Fiji and Tonga on Thu/Fri 17/18
Sep and then, on sat 19, deepen into a low near 30S 170W, to south of
Niue, and then wander off to southeast. People in Tonga looking to
sail early to NZ should wait for that trough to move off on Sat 19,
however there is a day between troughs-wed 16 Sep-which may be an OK day
to depart.

HIGH crossing Bass Strait on 14 Sep is expected to slowdown as it
approaches NZ, should be over North island on Thu /Fri 17/18 Sep, and
linger to north of North Island Sat/Mon 19-21.

There is a disturbed westerly flow south of this ridge, with one front
crossing the South Island on 14 Sep, next on 16 Sep, next on 18/19 Sep,
next on 21/22 Sep. Each front should be preceded by a burst of NW winds
and followed by west/southwest winds.

HIGH near 30-40S 165W today is expected to wander slowly east along 35S
all week, and the squash zone of enhanced easterly winds on its northern
side should cross French Polynesia around Thu 17 Sep UTC. Good for
surfing but no good for diving or fishing.

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