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.

=-=-=-=-=-=-=-=-=-=-=-

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.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 
If you would like more details about your voyage, check metbob.com
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com
(subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at
weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Bobgram

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 18 May 2025

The QBO (stratospheric Quasi Biennial Oscillation)
Last week in my weathergram I showed that our recent South pacific
cyclone season was one of the quietest ever. Maybe the QBO can partly
explain this
The Quasi-Biennial Oscillation (QBO) is a natural climate pattern
that causes a shift in the direction of the equatorial stratospheric
winds, switching between easterly and westerly winds. These winds are
read twice daily by radiosondes released from Singapore. This
oscillation, with an average period of about 28 months, can have an
impact on weather patterns and is considered a valuable component of
long-range weather forecasting.

From acd-ext.gsfc.nasa.gov/Data_services/met/qbo/qbo.html
During the last cyclone season the equatorial stratosphere had
westerly winds. more than normal, and there was a slightly lower
troposphere than normal. These factors make it harder for tropical
cyclones to develop.
TROPICS

This week the monsoon is advancing onto southern Indian (blue lines
below)
From mausam.imd.gov.in/imd_latest/contents/monsoon.php

WEATHER ZONE

Wind and rain accumulation this week from Windy.com
The wind accumulation shows a windy zone attached to that "Eastie"
which briefly forms off the New South Wales coast, and a lot of
southerly winds onto Chatham Island east of NZ, on the back end of L1
There is also zone of enhanced trade winds mainly between 20 and 25S
south to south of Fiji.

The rain accumulation also shows a rain band associated with "Eastie"
and a rather uniform SPCZ north of Fiji
LOWS and HIGHS
Low L1 and its associated troughs has got east of New Zealand today
and Monday and is expected to travel steadily eastwards this week.

HIGH H1 is over Bass strait tonight and expected to take its time
travelling east across the Tasman Sea then cross central NZ next
weekend. By then it will have a weak squash zone in the trade winds on
its north side. GFS has a trough/low forming near Minerva reef
Mon/Tue 26-27 May but other models disagree. Watch that space and be
prepared to brace accordingly.

Gulf of Panama: Just light winds Health current going to SW.
6N to 2N: Not as showery as last week South to SW winds. Variable
currents
2N to Galapagos: Winds from S/SE and sea drifting to the WNW.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more details about your voyage, check metbob.com
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com
(subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at
weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Bobgram 11 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 11 May 2025
A review of the cyclone season

We can compare the timing of these cyclones with the active phases of
the MJO on the South Pacific shown here in Blue.

The 2024–25 South Pacific cyclone season was one of the least active
seasons on record within the South Pacific Ocean to the east of 160°E.
Just four named storms formed (compared with an average of 7). None
intensified into severe tropical cyclones, the first such failure
since the 2008–09 season.

TROPICS
A second quiet week for tropical cyclones world-wide.

WEATHER ZONES

Rain accumulation this week from Windy.com shows well defined SPCZ and
blobs of rain with L1
The wind accumulation shows a windy zone attached to L1 as it moves
out of the tropics. Only patches of wind in the Tasman but a period NW
winds late in the week for eastern North Island.

LOWS and HIGHS
Low L1 has formed tonight just south of Fiji with an active wind and
rain band over Minerva and Tonga tonight and Monday then moving ESE to
reach Southern Cooks on their local Monday and Tuesday. Stay put for
that.

A FRONT with strong westerly winds is crossing south end of NZ on
Monday and then travelling east to merge with L1. This gives a SW
flow over NZ on Monday and a weakening southerly on Wednesday.
Large HIGH H1 in Tasman Sea expected to stay put until mid-week then
cross central NZ on Wednesday and travel off to the east from
Thursday/

The Trough following this HIGH should reach Tasmania on Wednesday and
NZ on Sat/Sun as a well-formed Low.
HIGH H2 in Aussie Bight by mid-week is expected to spread east onto
Melbourne area this weekend.

Gulf of Panama: Maybe some useful northerly winds on local Monday
else light winds or southerly winds. Health current going to SW.
6N to 2N: Showery, South to SW winds. Variable currents
2N to Galapagos: Winds from SE and sea drifting to the WNW.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more details about your voyage, check metbob.com
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com
(subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at
weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Bob blog

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 4 May 2025

A review of last month's weather

Here is a link to a YouTube clip giving an animated loop of the
isobars and streamlines in the South Pacific for the last month at
youtu.be/M72SdfJhSnk

In Mid-April Cyclone TAM formed near Vanuatu and travelled south into
the Tasman Sea where it was held in place for several days by a HIGH
east of NZ, affecting NZ on the approach of Easter.

Then at the end of the month another LOW from the Tasman Sea
explosively deepened near Cook Strait bringing floods to Christchurch
area and damaging winds to Wellington

The 500hPa heights and anomalies shows last month's typical ridge
/trough pattern, with the Highs showing either wave 2 to 3

Sea Surface temperature anomalies as at 2 May
from psl.noaa.gov/map/clim/sst.shtml
show that The Galapagos area is now showing some warm anomalies.
And there is a cold anomaly along the Canadian east coast.
Around 85percent of the globe is showing warm anomalies.

From http://www.psl.noaa.gov/map/images/fnl/slp_30b.fnl.html

A ridge has turned into a trough in the Tasman Sea, else just small
differences from last month.

Pressure anomalies for past month shows deep anomalous lows in Tasman
Sea and mid-South Atlantic, and a switch from trough to ridge over
USA.

The 1010 isobar has retreated off Australia.
The northern 1015 line has moved north across Australia, but south in
the Tasman Sea.
The southern 1015 line is in much the same position.

TROPICS
A quiet week for tropical cyclones

WEATHER ZONES
Rain accumulation this week from Windy.com shows well defined SPCZ and
blobs of rain with L2 and L4
The wind accumulation shows a squash zone between 20 and 25S from near
Vanuatu to Southern Cooks. It also shows strong northerly winds near
NZ with L2.

LOWS and HIGHS

Low L1, the remains of the low that affected wellington and
Christchurch last Thursday, is travelling off to the east

HIGH H1 travelling east across central NZ next few days is expected to
form a squash zone of strong east to SE wind on its northern side
between 20 and 25S. Avoid.

Low 2 is expected to form over Vanuatu area late in the week and might
travel as a trough towards Northland this weekend as low L3 travels
east across southern NZ.

Since L2 might turn the winds between NZ and Tonga to be from NE late
this week it may be more comfortable to wait until early next week or
when this threat passes away before departing Northland for the
tropics.

now south of Southern Cooks is moving slowly SE.  Associated trough is
expected travel from Niue to Southern Cross this week, and the
southerly swells on its backside are expected to reach French
Polynesia late in the week.

HIGH H2 is expected to move into Tasman Sea by end of this week.

And there is expected to be a low L4 forming south of Tahiti by end of
the week.

Gulf of Panama:  Maybe some useful northerly winds from 8 to 15May,
else light winds.
Some light winds next few days then useful NE winds after Tuesday.
Showery.
6N to 2N: Showery, often westerly or SW winds and a sea drifting to
east
2N to Galapagos: Winds from between SW and S and a sea drifting to the
NW.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 
If you would like more details about your voyage, check metbob.com
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com
(subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at
weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>