Bob Blog 7 May

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.
Compiled Sunday 7 May 2023

SQUASH ZONES

When sailing from Tahiti to Tonga there are strange weather features to
avoid.
A couple of weeks ago I introduced the SPCZ.
This week is a good week to talk about SQUASH ZONES
I got an email a few days ago asking:
"This is next week's forecast for Society and Tuamotus.
How is it possible to have 50kt winds (gusts) so close to absolute calm?
What on earth will be happening at that interface??"


And here is my reply
The satellite imagery and isobars give clues as to what's going on :
A BFH (Big Fat High) well south of French Polynesia with central pressure
near 1040hPa has been dominating the weather map for past 10 days.
It is now being nudged east and Isobars on its northern side are coming ng
closer together to allow the BFH passage.
This is not a frontal system nor a convergence zone nor a weather bomb (a
weather bomb requires pressure to fall 24hPa in 24hr).
It's what I call a SQUASH ZONE .
This squeezing of the isobars is occurring most within the cloud canopy seen
on the satellite image and a small low is expected to form near Papeete,
producing a lull just north of the squash zone.
Note the interesting thing that in the lull-of-no-wind the baro is going
down slowly, and in the zone of maximum gusts the isobars are stationary, so
the baro is steady.
Further south where the wind is increasing the baro is rising.
Welcome to the south Pacific and its wondrous weather systems.

TROPICS
South Pacific Convergence Zone SPCZ
The South Pacific Convergence zone is expected to be divided into sections
this week. The Main zone should rebuild over Coral sea and stretch across
Vanuatu towards Fiji. It might help form a low near Fiji early next week. A
Convergence zone is coming and going around Northern Cooks.

HIGHS and LOWS
A BFH (big fat High) H1 is lingering east of NZ and south of FP and this
week is finally expected to be nudged to the east. It has a squash zone on
its northern side near the Austral Islands. Avoid.
Low L1 is deepening off the Sydney coast tonight and expected to curve NE
then E across central NZ mid-week so that its wet and windy front crossed
North Island on Tuesday. Avoid.
It should then form L2 between Cook Strait and Chathams and that is expected
to deepen as it and L1 go off to the southeast.
In the wake of these Lows a vigorous southerly is expected across the
Tasman Sea reaching South Island on Wednesday and North Island on Thursday.

Another High H2 should reach Tasmania late Wednesday and then travel east
along 45S reaching South Island by Saturday.

The best day this week for departing NZ is Friday. However, there are early
indications that a Low may form near Fiji early next week and that could put
a spanner in the works. Check that before departure.

Panama to Marquesas: Good northerly flow fades mid-week, then we have
missed the bus and may as well wait for at least another week.

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If you would like more detail for your voyage, then check metbob.com to see
what I offer.
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at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or txt 64 277762212
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