Bob Blog 6 Aug

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 06 August 2023

SPCZ + Jetstream = wonky weather.
SPCZ, the south Pacific Convergence Zone is well known to Pacifica peoples.
It is where the easterly winds on the north side of the Easter Island High
bumps into the SE winds found on the north side of the Highs that migrate
across the Tasman Sea,
In previous blogs I've talked about how it has "moods" depending on things
like long term influences such as El Nino/La Nania, and the annual cycle,
and shorter cycles such as the MJO.
The recent weather over Vanuatu has been so wonky it prompted the request to
"explain the (for us) strange weather pattern that we experience now around
Vanuatu. It is not a ' normal' low, but what is it then?"

The surface map last Thursday shows a large 1039 High in the central Tasman
Sea and a zone of enhanced SE winds over Vanuatu. I call this a SQUASH ZONE.
The well-formed upper trough with Jetstreams is directly above the squash
zone and over Vanuatu. This helps cause strong upward motion leading to
squally weather. This upper trough has now deformed and Vanuatu weather is
now relaxing, The SPCZ stays lingering over northern Vanuatu much as it
usually does. It was the upper trough/Jetstream that produced the wonky
weather.

TROPICS
Former super typhoon DOKSURI brought the heaviest rain to east Chimba since
records began 140 years ago. Super Typhoon KHANUN followed and skirted
southern Japan. Cyclone and is now heading for Korea. DORA is travelling
west along 12N south of Hawaii, and EUGENE is travelling NW parallel to Baja
California coast. There is now high potential for development in the
Atlantic. As the old saying goes "July, stand by. August, a must".

WEATHER ZONES
The South Pacific Convergence zone is expected to have a quiet week across
the Coral Sea to north of Fiji. From the recent wonky weather over Vanuatu a
LOW L1 is forming near Fiji and expected to track to the SE and deepen. A
convergence zone is expected to linger between Northern Tonga and Austral
Island. It has some passing troughs, but these are mediocre, and weather
seems Ok for sailing from Tahiti to Tonga afterL1 has moved off. Northern
route avoids most of the convergence zone.

HIGHS and LOWS
There is a strong subtropical ridge along about 32S
Low to NE of Chathams Islands tonight is moving quickly off to the east and
High H1 should form east of NZ by mid-week.
Low L2 over southern NZ tonight is expected to travel NE and linger near
Chathams until Thursday, bringing a cold south/southwest flow to NZ.
After a ridge over NZ on Friday/Saturday the next trough is expected to
deepen into a slow-moving low over north Island early next week. Avoid.

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