Bob Blog 27 Sep.

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 Sun 27 Sep 2020

CAPE or Convective Available Potential energy, is the integrated (total) amount of work that the upward (positive) buoyancy force would perform on a given mass of air (called an air parcel) if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise and is an indicator of atmospheric instability in any given atmospheric sounding, while negative CAPE will cause the air parcel to sink.

Instability and rising motion in a cloud can cause a thunderstorm, CAPE values over 5Kj/Kg are significant in creating thunderstorms. This means that CAPE can be used as an indicator of the intensity of the South Pacific Convergence Zone. So I’m adding the mid-week CAPE map (from to my illustrated edition of weathergram, as it shows another rview of the SPCZ..

The Tropics

The latest cyclone activity report is at and Tropical Cyclone Potential is from

Just two tropical depressions today. 15W and 14W (ex DOLPHIN at 40N 146E)

Also, some high potential for development off west Mexico coast., near Philippines, an d in the N NE Atlantic, off Africa



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.


Predict wind CAPE mid-week gives two possibilities for SPCZ


SPCZ=South Pacific Convergence zone.

SPCZ is moderate and in its normal position, from Solomon Islands to Samoa. The CAPE map also shows some convection along 15S.

Passing trough over Fiji /Tonga on Wednesday, and Southern Cooks on local Tuesday/Wednesday and Southern Cooks on Thursday.

Accumulated rainfall for next week from


Subtropical ridge (STR)

HIGH 1024hPa over New South Wale on Tuesday and broaden as it crosses Tasman Sea , reaching North Island on Thursday and Friday, leaving a lingering ridge over North Island.

Tasman troughs

Low 966 hPa south of NZ moving south and deepening to below 050 hPa at 55S on Monday, then moving east., followed by vigorous S flow over NZ until Wed with huge swells. .

Front over South Island on Friday with NW winds followed by westerly winds.


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