Bobgram 12 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 12 MAY 2024

Snapshot of marine traffic show that this year the tropical migration
has started last week from Aotearoa/NZ but not yet from Australia:

Tahiti to Tonga - Beware the squash zone.

When planning to sail from Tahiti to Tonga you either go direct or
sail a path north of the direct path (to avoid uncomfortable swells
further south) or sail a path south of the direct path (to avoid the
squalls and convergence zones further north). Sometimes your best path
is partly north and partly south of the direct path.

This week I'll take a closer look at squash zones. These are easy to
forecast and avoid when you know how, yet every year we hear the
stories of yachts getting caught by them unawares.

The thing to look for is a HIGH passing over Aotearoa /New Zealand .
There is one doing that tonight:
Often (as is the case this week) these HIGHS migrate to the east and
intensify and expand to have a central pressure isobar over 1030 hPa
As the HIGH intensifies and accommodates some extra isobars around its
centre, the isobars on its northern periphery get squashed together.
Look at my illustrated blog for an animation of this.

So there is a zone of enhanced trade winds on the northern side of a
migrating High. In this squash zone conditions quickly become
uncomfortable due to increasing wind and swell, even if the baro
remains unchanged. Since the waves are and being compacted, they
become short and choppy. Squash zones cover more longitudes than
latitudes so if caught in one then go north or south to escape, rather
than west or east.

As the High moves further away there is a zone of lowering pressure on
the western end of the squash zone. This becomes a breeding ground for
squally rain and maybe a new trough, as is also something to avoid.

A tip for those doing voyage planning is to select the wind
accumulation map on windy.com and put this on 5 days accumulation.
This will help you identify the zone to avoid.

TROPICS
Last week, Coastal Tanzania was drenched by heavy rainfall from
Cyclone Hidaya, which formed unusually far north off the coast of
Africa. Again, there are no named storms. Tonight. The place of
greatest potential is skirting the equator around New Guinea.

WEATHER ZONES
The South Pacific Convergence zone is active over New Guinea and from
Solomon Islands to northern Vanuatu. to Samoa. A Low L2 is expected to
form on the SPCZ near Tonga at end of the week.

HIGHS and LOWS

HIGH H1 east of the North Island is expected to travel east from 35S
to 40S and intensify.

A squash zone of enhanced trade winds on northern side of the High is
expected between Tahiti and Tonga along around 20S (as illustrated
above).

Low L1 has formed off Sydney tonight and it and associated trough is
expected to cross the North Island on Wednesday and Thursday followed
by a westerly flow. Avoid.

High H2 is expected to move from Australian bight across NSW during
the week and into the central Tasman Sea by end of the week.

L2 provides an opportunity for sailing eastwards from Australia to New
Caledonia.
Also, when this trough gets east of Northland that should make a good
opportunity for sailing from NZ to the tropics.

Panama to Marquesas:
Light winds for starters. Squally doldrums from 6N to 2N, intense
between 5N and 3N.i

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If you would like more details about your voyage, then check
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