Bobgram 3 Aug 2025

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 3 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
youtu.be/gjWlhQmSeTs

Lows crossed NZ on 4-5, 11, 17 and 29-31 July …. Almost continuing a
weekly cycle, but also a blocking HIGH between 20 and 29 July.

The Low on 29-31 brought heavy rain to Tasman/Nelson/Marlborough area
and Bay of Plenty area.

The 500hPa heights and anomalies shows last month's typical ridge
/trough pattern,

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

The build-up of heat east of Japan an in the north Atlantic is
continuing.

The cooling /upwelling off Peru has finished.

The Sothern Annual Mode /AAO spent first half of July positive,
accentuating the polar vortex.

The monthly pressure pattern and anomalies are

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

During July the roaring 40s in the southern hemisphere moved north.
The Subtropical ridge across the south Indian ocean intensified.

Pressure anomalies for past month Shows the pressure rise in south
Indian ocean to be part of a roller coast with lows over Australia and
larger Highs SE of NZ.   Deeper lows over Asia.

The 1015 isobar has shifted north to cover the south half of Tasman
Sea.
A 1025 Isobar has formed in the Indian Ocean.

TROPICS

The latest cyclone activity report is at zoom.earth and
tropic.ssec.wisc.edu and Tropical Cyclone Potential is
from http://www.ospo.noaa.gov/products/ocean/tropical/tcfp/

After lashing the far northern Philippines as a Category-1 cyclone,
Tropical Storm Co-May drenched Okinawa before bringing heavy rain and
gales to the Chinese coast around Shanghai.

 • Typhoon Krosa skirted the Japanese coast near Tokyo as a tropical
storm.

• Hurricane Iona and Tropical Storm Keli churned the open Pacific
Ocean, well to the south of Hawaii.

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)

Rain accumulation this week from Windy.com shows well defined SPCZ
between Solomon Islands and Tuvalu.  Also, a frontal zone from Tonga
to Austral Islands (avoid).

Wind accumulation from windy.com shows zones of strong wind around L1
and L2 and a frontal zone along 20S (avoid).

LOWS and HIGHS

Low L1 near 40S 160E, well south of Tahiti is moving off to the
southeast but leaving behind lots of southerly swell getting as far as
20S.

HIGH H1 has been travelling across New Zealand last few days and
should continue a steady eastward migration along 45S

Low 2 is the one that affected the third rugby test Wallabies beating
the British and Irish Lions in Stadium Australia, Sydney … a game
delayed by lightning and influenced by rain.  This Low is being
steered east along 28S… an uncommon path for a low. So, there is a
squash zone between it and H1 with strong easterly winds and large
swells at 30S. Avoid. 

Further west over Australia several troughs should travel southeast
across Tasmania and the South Tasman Sea reaching southern NZ by end
of the week and a large High H2 moves into the Aussie Bight and then
follows those troughs.

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