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

20 September 2009

BOBGRAM7issued 20 Sep 2009

WEATHERGRAM
YOTREPS
Issued 20 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.

TROPICS/SUBTROPICS
Last week's Low between NZ and Fiji was blocked form its normal exit
route to the southeast by a retrogression of a long wave ridge around NZ
--- This meant NZ had settled weather (after a strong NW last Monday)
and Fiji to Niue got a concentrated burst of rain from that blocked
trough. Yep, that's what flooded that Nadi bus station.

We have the equinox on Tuesday UTC, and finally the overhead sun will be
getting into the southern hemisphere. Around this time of year we often
get a mirror image of the ITCZ along 10S for a few weeks, and this does
seem to be forming now-at least from Solomons to Tokelau. In some years
the equinoctial convergence zone has tropical cyclone twins - one
forming on ITCZ triggering another in southern hemisphere and "out of
season". Not much is known about what triggers this pattern, but it is
something to add onto to list of possibilities to watch out for over
next few weeks


SUBTROPICAL RIDGE ZONE
HIGH cells migrating eastwards along 25/30S this week will keep the
South Pacific convergence zone off to its northern most position,
merging with that equinoctial convergence zone. One High will be on the
date line around Wednesday UTC and the next Sun/Mon 27/28 UTC. North of
these highs will be an area of enhanced trade winds, mainly at 10 to
15S. Because these periods of enhanced winds are no goof for fishing or
diving, and because all you need to time them is to watch the passing
Highs , I recommend that's what you do using the Fiji Map website at
http://www.met.gov.fj/aifs_prods/0641.html or our 7-day site at
http://www.metservice.com/public/maps/rain-forecast-7-day.html.

There's a trough between those two highs and it will be crossing Fiji
and Tonga around Friday/Sat 25/26 Sep. This brings fluky winds and
tropical showers so not the smoothest for sailing.

TASMAN SEA/ NZ AREA
It's too complicated and changeable for this bulletin. In this area
there is no blocking at present, so it's a ridge/trough/ridge/trough
pattern, but some of the troughs are building into deep LOWS in the
Tasman Sea, lapping onto NZ but not quite doing their worst there (yet).

For settled periods use the ridges--- one over NZ Tuesday UTC and next
is Friday UTC. There's a mid Tasman LOW between these systems on Thurs
UTC that models are deepening briefly to 975 hPa to west of Fiordland
travelling southeast. Avoid.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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