Bob Blog 30 May 2021

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 30 May

New Zealand Storm

It’s awe inspiring (awesome and awful) to a meteorologist:

The Low in the Tasman Sea tonight has a central pressure of 983hPa and a arc of southerly winds on its west side to over 50 knots, producing 10m 12 second southerly swells. The closed isobars are all 1010 or lower and cover an area the size of Australia. The upper low is slightly east of the surface centre and has a jetstream to 167knots at 250hPa.

A hose of onshore easterly winds around 30knots are driving air onto Canterbury where the rising terrain is assisting rainfall to around 145mm/day, producing river flooding and evacuations.


These images have been downloaded from



Isobars and Wind gusts in knots (above)


Waves (above)


Rain accumulation next 24 hours (above)


250hPa surface showing jetstream (above)

Last Tuesday the full moon and perigee were within hours of each other (and moon’s orbit took it into earth’s shadow). The resulting King tides, and storm surge are evident on the Timaru tide gauge :

The storm surge is a combo of higher than normal high tides due to a perigean moon, plus a little lift from lower than normal isobars (each hPa below 1012 can raise the sea 10mm), plus the piling up of the sea by strong on-shore winds.



The Tropics

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


There are no named cyclones tonight, but there is a tropical depression (FOUR) east of Philippines and a zone of high potential of the Mexican west coast.


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

STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE9 (in pink)


Predict wind CAPE mid-week as seen by ECMWF and GFS


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to be rather thin and weak, stretching from Solomon Islands across northern Vanuatu to Fiji/Tonga, much like last week.

A weak passing trough is expected over Tonga on Tuesday and southern Cooks by end of the week.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH up to 1023hPa is expected to move off Australia and into central Tasman Sea by Tuesday and over northern New Zealand on Thursday and then further east along 35S.

A ridge over South Tasman Sea is expected to develop into a High east of the South Island by Saturday and then move east along 45S nest week.

Aussie/Tasman/NZ troughs

The large Low over Tasman/New Zealand tonight is expected to travel over the North Island on Monday and then off to the southeast followed by a SW flow on Tuesday.

Another trough is expected to reach the South Island by Friday, triggering another Low to form in the central Tasman Sea by Saturday. This low should be able to be used as a shepherd for yachts wishing to get from Australia to Fiji.

Panama to Galapagos/Marquesas

Light winds for Panama. SW/S winds from 5North to Galapagos. As for going to Marquesas, currents are better if go via north of Galapagos.


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 64277762212


Published by metbob

Pattern and Chaos

Leave a comment

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.