Bob Blog 25 Aug



Compiled Sun 25 August 2019

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.


There was an eruption of a undersea volcano at 18.325S 174.365W or around 27nm NW of Vava’u early in August that has drifted SW to be around 60nm west of Late island off west coast of Vava’u, affecting voyages from Vava’u to Fiji.

It seems to currently be drifting north slowly. and dispersing.

A write-up on this pumice is at

and an excellent sailor report about this pumice appeared on facebook on 15 Aug at



Your may have heard that a “Sudden stratospheric warming (SSW) is occurring over the Antarctic south of Africa to Australia, and wondered what that means. It may buckle the Polar vortex so that some cold streamers from the Southern Ocean may travel onto Australia/NZ in September.

This has only been recorded in the Southern hemisphere twice before (Sep 2002 and Sep 2010) and both times saw temperatures in New Zealand plunge.

In the Southern Hemisphere during winter, a ring of stormy and freezing weather encircles Antarctica, spinning clockwise. This is called the Polar Vortex and extends into the stratosphere, 30 to 50km high, and it is usually very good at locking the cold close to the Pole.


Stratospheric winds at 70hPa (near 18km) on 27as seen at showing zone of calm distorting the Polar Vortex.

Sometimes the winds in the stratosphere temporarily weaken, or even reverse, leading to a zone of sinking air, which warms the stratosphere as it compresses downwards, and this is called a SSW. It may cause the Polar Vortex to buckle, which then filters down towards the surface and send out streams of Antarctic chilled air northwards. In this case the most likely time is early September into Australian bight, , but there may be ramifications throughout September for the Tasman Sea. .


Stratospheric warming mid-last week as seen at


The latest cyclone activity report is at and TCFP tropical Cyclone Formation Potential at

There are three active tropical cyclones., DORIAN heading for the Caribbean , IVO staying offshore of west coast of Mexico and BAILU fading on the coast of China.

There are several areas for potential development, especially around NW Pacific and in he North Atlantic— as expected at this time of the year.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from south of the Solomon Islands across northern Vanuatu to Tuvalu, and may visit north of Fiji early in the week.

Passing trough over Palmerston/Aitutaki on local Sunday (tomorrow) , and bora Bora on local Monday/Tuesday with mainly light winds.

Passing trough over Niue on local Tuesday, to Palmerston/Aitutaki local Wednesday, and Bora Bora/Papeete on local Friday/Saturday, with a swing to NE then SE winds.


Accumulated rainfall for next week from

Subtropical ridge (STR)

After several weeks, the STR is finally shifting south onto NZ. remains north of NZ this week, High is expected to travel east along 35S across Tasman sea on local Tuesday and northern NZ on Wednesday, then fades off to the NE.

Another HIGH is expected to travel east along 45S across the Tasman Sea on Thursday and Friday and then across NZ on Sat/Sun.

Tasman Sea /NZ/Aus

SW flow easing on Monday/Tuesday. Between the Highs, a trough is expected to cross NZ on Thursday and fade over the North island on Friday.

A low is expected to form off Sydney on Thursday and travel to the NE then E and may reach norther NZ(GFS model) or Southern NZ (EC model mid-next-week.

OK to head for NZ so long as you can reach it before possible strong winds early next week.

From Noumea to Aus may encounter a passing trough followed by a period of up to strong southerly winds south of Brisbane .

Tahiti to Tonga

The burst of large SW swells that affect this route south of 20S today/tomorrow should ease by late Monday local time.

Voyage departing this week may encounter a passing trough, but this isn’t expected to be big deal.


If you would like more detail for your voyage, then check to see what I offer.

Or Facebook at /

Weathergram with graphics is at (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at

Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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