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

06 March 2010

BOBGRAM7 issued 7 March 2010

WEATHERGRAM
YOTREPS
Issued 07 Mar 2010
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
South pacific Convergence Zone SPCZ is active along 10S and has a zone
of equatorial winds from west or northwest on its northern side
extending from Solomons to Tarawa. No good for fishing, diving or
mooring.

This is likely to culminate in a new Tropical system forming somewhere
near Tokelau. Computer output all differs at this stage as to timing,
but most likely track of this system would be somewhat to the southwest
into the Wallis/Futuna/Rotuma area by around Thu Fri 11/12 Mar and then
about or west of Fiji over weekend of 13/14 March or maybe early next
week 15/16 March. Double check with updates and plan accordingly.

SUBTROPICS/TASMAN

The subtropical ridge is still well south of its normal latitude and the
high cells are getting further apart, allowing the troughs in-between to
bring doses of northerly then southerly winds to the mid latitudes and
almost into the tropics.

That Low in the subtropics between Raoul and Chatham Islands or just
east of the NZ North Island today should fade away near Chathams by
Tuesday and has meant that its partnering high cell has had to skirt
around it along 50S.

This has weakened the subtropical ridge and upset the normal westerly
flow in the southern ocean a bit and so the next trough rolling in from
the west is likely to turn into something of a polar blast as a
consequence. Well, OK, maybe not a true polar blast but one that will
allow the cold air near 55S to get to something like 35S. Cold
southerlies/southwesterlies and huge swells are likely to hit Tasmania
on Tuesday 9 Mar and then follow an active front onto New Zealand on
Thu/Fri 11/12 Mar and Chathams on Fri 12 March.

So forecast for Auckland Boat Show is for a showery SW change around
about late Thursday, clearing and easing for the weekend. There may
well be some mountain snow on southern facing hills of South Island
Thu/Friday, but soil temperatures should be warm enough to melt it on
Saturday.

This cold blast just doesn't seem to have the right timing to get
into the subtropics. If it did, that would cause a rapidly deepening
low. Maybe next time.

Another factor feeding this 55-to-35S blast is the next HIGH which is
likely to expand to 1036 over Tasmania on Friday, shovelling the air
ahead of it northeast wards. This high should move over central NZ
around the middle of next week, good for Saint Paddy day revelry.

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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