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

05 April 2009

BOBGRAM7 issued 5 April 2009

Issued 5 APR 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.

The "low from Rotuma" that was mentioned in last week's Weathergram
turned out to be LIN which blossomed when it moved south-southeastwards
across southern Tonga earlier today giving Easterly then NW gusts to
over 50 knots for around 5 hours. It is expected to continue to peel
off to the southeast.

That strong wind and heavy rain which hit Coffs Harbour early last week
was associated with a small trough in tango with the strong squash zone
on the north side of big fat High.. the High was over 1040 hPa in the
south Tasman Sea, and when its over 1040 it's going to get naughty.

The South pacific Convergence Zone SPCZ is still very active in the
Tuvalu/Tokelau/Samoa/Tongs/Niue but it looks as tho' LIN will take much
of this activity with it off and away to the mid-latitudes.
Indications are that the tropics are about to go thru a period of more
settled weather , starting in the Coral sea and spreading east, over the
next week or two . This settled wetaher has already arrived in the
Coral Sea.

There is a zone of convection between 9S 120W and EQ 105W about 300
miles wide. This is the ITCZ that normally lives at 3 to 5N and has
been knocked across the equator by the equinox. Well it is a few weeks
after the Equinox now so the overhead sun in the Northern Hemisphere and
this zone of convection should soon fade. Meantime, anyone who is going
from Galapagos to Marquesas should head south of 9S in those longitudes
to avoid the squalls.

There is a branch of the SPCZ between northern Cooks and Marquesas.

The HIGH that crossed NZ over the past week or so hit over 1040 in the
south Tasman sea last Tuesday and tossed dry cold southern ocean air
onto NZ, producing as much as 6 degrees of frost to harass the grape
growers at the start of harvest. This High is expected to continue its
migration east between 40 and 45 South from 160 to 120W this week. The
Low that belongs to LIN is well positioned to travel south "down the
western shoulder" of this High. Beware the squash zone between High and

Next HIGH is already moving into the Australian Bight and is also taking
the near 40S latitude that the subtropical ridge prefers near the
equinox. It should cross Tasmania on Thu 9 Apr and NZ on Good Friday 10
April. It is not likely to get as much vertical support as its
predecessor, so will likely fade to a slither along 35S as it crosses

Use these Highs to make your travel plans; they have more cohesion than
the lows and fronts. Also remember the squash zone of enhanced trade
winds that exists on the northern side of these Highs.

NZ will be "between Highs" this week, and thus will have a series of
The first front is already in the Tasman Sea and Fiordland. On Monday,
its rain clouds may need to stall over the SW of the South Island, as
another front approaches. On Tuesday, both these fronts should reach
the North Island and, on Wednesday, a secondary may low form on these
fronts over the South Island, as some cold air arrives aloft. Avoid
sailing these days.

On Thursday a SW flow should settle over NZ leading in the Friday High.
So it looks like a period of settled weather in NZ for the main part of
Easter, except for a disturbed westerly flow over the South Island.

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