Bobgram

Bob McDavitt' ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are
from the patterned world.
Compiled 25May 2025

Tahiti to Tonga
Now that many yachts are about to travel west from Tahiti to Tonga, it
is time to review the weather, in general terms, for this route.

The main players:
1. Getting from the Tahiti area to Tonga/Fiji means crossing the South
Pacific Convergence zone, SPCZ
This zone may be weak or may contain squally showers. In the models it
is portrayed as a zone of light winds. To see a forecast for this
zone, use windy.com and rain accumulation for a period over five days.
This zone may linger in the north (it does this in an El Nino, or it
usually hovers between Samoa and Southern cooks. To avoid it choose a
different latitude or aim for gaps in the zone.
The SPCZ goes thru periods of activity and inactivity and is active
when there is the positive phase of an MJO (Madden Julian
Oscillation).
Occasionally a tropical low/passing trough will travel east/southeast
along the zone, making for a burst of wind/rain travelling east from
Fiji to Niue then SE across Southern cooks. These occur on occasions
even at this time of the year and can be forecast beforehand and
avoided.

2. Another thing to avoid is a SQUASH ZONE when a large High travels
east along 30S.
When the central pressure in the High is 1030hpa or above, it gets
dirty (above ten thirty is dirty) and squeezes the isobars north of
the high in the trade wind zone closer together making a "squash zone"
of enhanced winds and rough seas. These usually occur around 20 to 25
South and may last for several days. They are reasonably easy to
forecast and are worth avoiding. If caught in one, change your
latitude rather than your longitude.

3. The third thing to avoid is the PASSING TROUGH or passing low.
The systems tend to form on the eastern end of the SPCZ in which case
they travel southeast, propelled by upper NW winds as a steering
field. OR they may form on the NW (back) corner of a travelling HIGH.,
in which case they may get shunted south or sometime to the southwest.
They often have their own mini-squash zones (on their polar/southern
side), and calm zones (on their equatorial/northern side). Isobars
give a clue about the intensity of a passing trough: those above
1010hP are usually weak and those below 1007 are worth avoiding.

There are three routes between Tahiti and Tonga, the northern, central
and southern:

Sometimes the northern route via Suwarrow/Samoa is favoured, and
sometimes not.
The northern route goes via Suwarrow to Samoa. Often this is covered
by the SPCZ, but when the SPCZ is further south the northern route and
its northeast winds may be best.

The middle-route offers stopovers such a Palmerston Island and Niue
and is a good alternative when the SPCZ is further north. Watch out
for passing troughs and squash zones … these come and go. The middle
route allows short hops.

The southern route tends to get higher swells from the Southern Ocean

 SO, all in all, the "best voyage" changes every few days … sometimes
it's worth waiting for something that suits, sometimes it isn't.

Basically, avoid passing troughs and squash zones, and go thru the
SPCZ when it is weak. This may mean taking short hops rather than
getting direct from Tahiti to Tonga in one go.

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Onset of this year's Indian Monsoon
This week the monsoon is advancing onto southern Indian (blue lines
below)

It is around 7 to 10 days earlier than normal
From https://mausam.imd.gov.in/imd_latest/contents/monsoon.php

TROPICS


Last week an unnamed tropical storm spun up for a few hours along the
southern coast Another quiet week but note the potential areas around
India due to the early Monsoon, and hints of potential off Mexico west
coast. Also, a strange blob of potential around Niue, where L1 forms
this week.

WEATHER ZONES

Weather Zones Mid-week GFS model showing isobars, winds, waves
(purple), rain (red), MT (Monsoonal trough), STR (Subtropical Ridge),
SPCZ (South Pacific Convergence Zone) CZ (Convergence Zone)

Wind and rain accumulation this week from Windy.com

Rain accumulation this week from Windy.com shows weak SPCZ and blobs
of rain with L1 and near Niue. Also -- Jet driven rain with L2
especially over South Island. A large DRY zone over New Caledonia,
Vanuatu and Fiji.

The wind accumulation shows a windy zone attached to L2 when it is
east of New Zealand. After the recent squash zone over Minerva, this
week is looking quiet especially around New Caledonia, Fiji and Tonga.

LOWS and HIGHS

HIGH H1 spent last few days over Tasman Sea and New Zealand. It helped
form last week's EASTIE which turned BEASTERLY over New South Wales.
H1 is this week expected to exit stage right and migrate off to the
east.

Low L1 and its associated troughs is expected to form on the NW corner
of HI, forming east of Niue and travelling south to southwest around
H1. Avoid.

Low L2 bred in the Southern Ocean and isa well supported feature
driven by jet streams feeding moisture into it from the Indian Ocean.
After going thru the normal 3-day life cycle of a low in the
Australian bight it is expected to do another three-day life cycle as
it crosses NZ Tuesday to Thursday dropping to below 980. Avoid.

Gulf of Panama: Just light winds Healthy current going to South then
west.

Panama to 2N: Squally convergence zone. South of 6N there is a strong
east to NE going current.

2N to Galapagos: Southerly winds, and sea drifting to the N or NW.
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