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 31 August 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
www.youtube.com/watch?v=DQheG_B2I24
Lows crossed NZ on1,8,17 and 27 August, with healthy Highs in-between ..
The Low on 27 August was especially squally and thundery:

A back trajectory analysis shows that the air then arriving over NZ was a
clash between air from the north and air from the south:

The 500hPa heights and anomalies shows three healthy HIGHS crossing NZ in
August.
From http://www.cpc.ncep.noaa.gov/products/intraseasonal/z500_sh_anim.shtml

Sea Surface temperature anomalies from psl.noaa.gov/map/clim/sst.shtml


North Pacific just continues to war. North Atlantic has cooled. And there
seems to be some strong cooling along the southern polar circle.
There is now a NEGATIVE IOD (Indian Ocean Dipole) (west cool/east warm).
this may bring some rain to central/southern AUSTRALIA.
Also, the SAM/AAO index has gone negative over last few days , consistent
with a burst of polar air onto NZ. However, this is NOT expected to last
long.


The monthly pressure pattern and anomalies are
from http://www.psl.noaa.gov/map/images/fnl/slp_30b.fnl.html

During August the Subtropical ridge in both hemispheres weakened in
intensity. However, it resumed over the Tasman Sea after lower-than-normal
isobars there in July.
The monsoon is moving steadily east across Asia.

Pressure anomalies for past month
Shows a major zone of Higher isobars over NZ and lower isobars east of NZ.
with strong cold SW winds in-between.

And a zoom in on the SW Pacific
There is a new 1020 isobar over Australia and NZ , but the 1025 isobar has
faded away.
The 1015 isobar is drifting NORTH over north Australia.

TROPICS
. At least three people perished as Typhoon Kajiki battered northern and
central Vietnam with fierce winds and torrential rainfall that collapsed
homes, felled trees and turned streets in the capital of Hanoi into rivers.
. Tropical Storm Fernand formed over the North Atlantic, while Juliette
passed well off Baja California.

WEATHER ZONES
Rain accumulation this week from Windy.com below shows well defined SPCZ
between Solomon Islands and Tulavu (north of Samoa).Also a convergence zone
around the Niue area.

Wind accumulation from windy.com below shows squash zones in the tropics one
to north of Fiji and one from Tonga to the Southern Cooks. These are
associated with the passage of HIGH H1 from 35 to 30S after Wednesday.

LOWS and HIGHS
LOW L1 crossed central NZ today and is travelling quickly off to the
southeast.

HIGH H1 is expected to cross the Tasman Sea on Monday and Tuesday and NZ on
Wednesday, then travel from 35S to 30S when east of NZ. This should produce
squash zones of enhanced Se winds further north. between Tonga and Southern
Cooks late in the week. Be aware.
A Low L2 should travel quickly along 50S mid-week.
Another Low L3 is expected to form west pf H1 and then the trough of L3
should cross NZ on Friday, followed by High H2 coming from central
Australia.

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