Bob Blog 19 Sept

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 19 Sept 2021


Can you tell the difference between a sucker cloud and a puffer cloud?

Clouds that grow upwards into the sky are called cumulus clouds. They start off as fluffy white ones. Type A


Once they get sufficiently tall they produce a shower of rain. Type B.


(photos from

If conditions are strongly unstable this shower may become heavy, perhaps with hail, or thundery or squally.

These type A clouds can be called suckers because that have a zone of calm on their leading edge. As a sucker approaches the wind speed gradually drops away. When you see these clouds, try and sail around and between them rather than under them.

Here is a bird’s eye view of the cloud with its surface wind flow,  red arrow is direction of cloud movement:


Note that as you look into the surface wind, the cloud will creep to your right (in the southern hemisphere). This is because it is propelled by the stronger upper winds. Friction causes the surface wind to “leak to low pressure” and be to be to the left of the winds aloft. When sailing upwind, put the wind to starboard (go left) to avoid the approaching calm, and aim for the LEFT edge of the cloud to catch the extra winds on its rear.

The type B clouds can be called puffers. Once the turrets grow to be taller than their base, they can start producing rain in the colder upper region, this rain falls and drags with it a downdraft that fans out from the back end of the cloud like a waterfall.


Wind speed in knots around a puffer.

As the downdraft ahead of a puffer reaches you, surface winds suddenly increases. There is a direction-shift around the shoulders of the cloud, and a huge calm zone that trails behind it. When sailing upwind into an approaching puffer, remember to do the “squall checklist” — put things away-reef the rig— check the compass—and watch the wind on the water ahead. When the first puff of the downdraft appears, put the wind on starboard (go left) so that we are sailing away from the puffer. We may get lifted for a few minutes as we go around the shoulder of the cloud but keep aiming to the left so we avoid being caught in the trailing calm zone.


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


Cyclone PETER is expected to peel off to the northeast and may swipe past Bermuda.


Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue),

STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (in pink)


CAPE mid-week as seen by ECMWF and GFS from


SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomona to Tuvalu to Samoa to Southern Cooks, in its normal position.

Trough over Niue to Southern Cooks tonight expected to get onto French Polynesia by mid-week and form a low that travels off to the south by end of the week.

Trough reaching New Caledonia by mid-week is expected to reach Tonga/Niue area by end of week.


clip_image017Rain Accumulation next five days from


HIGH1 above 1024 to north of NZ is moving ESE along 30 to 35S followed by H2 travelling along 30S. This maintains useful trade winds this week north of 15S.

LOW1 is expected to form near Lord Howe by Tuesday and deepen to 995 as it crosses central NZ on Wed/Thursday/by Wednesday in mid Tasman. Avoid. Associated trough may form new low east of North Island and south of Niue by end of week.


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