Bob Blog 2 May 2021

Bob McDavitt’s ideas for sailing around the South

Disclaimer: Weather is a mix of pattern and chaos;
these ideas are from the patterned world.

Compiled Sunday 2 May  

One weather app for your smart phone that I didn’t
mention last week, and is worth trying out is – it has been
engineered for Apple devices but can be used on android devices (  It has been optimised to download
weather data on low bandwidth connections., supports iridium Go and RedPort


THE LAST MONTH (April 2021)

Sea Surface temperature anomalies


Compared with a month ago:

La Nina has gone, and eastern equatorial Pacific is now
near normal. , but La Nina is gone. The warm patch near West Australia has
weakened.  Everything else is much
the same as last month.

To see how the annual weather cycle and the seasons are
working out, we can check the average isobar maps for past 30 days and their
anomaly from

Average isobars for past few weeks (below)


The subtropical ridges in both hemispheres have grown
during the past month, especially in southern Indian Ocean and across Australian
Bight.   The doldrums have
shifted both west and east off Indonesia.

Pressure anomolies for past few weeks


Pressure falls over the Arctic as spring arrives there.
Swing from high to low pressures in north Atlantic and from neutral to low
pressures over Tasman Sea / New Zealand area.

Zooming into the NZ area

Last few weeks (below)



A month ago (above)

There is a difference in colouring of the isobars
between these images.  even so the
1015 isobar has shrunk around and started to travel northwards across Australia
as the continent cools inland as the nights get longer.

The seasonal weather pattern is on target for the end
of the cyclone season in the Southern hemisphere.

The Tropics

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

There are some weak zones of potential development east
of Philippines and between Vanuatu and Fiji.



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

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


Predict wind CAPE mid-week gives two possibilities for


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to be weak and mainly north of
Fiji and Samoa, and another branch from south of Fiji to Southern Cooks

A trough is expected to form between Queensland and New
Caledonia this week. This may bring a weather pattern that is Ok for sailing
from Australia eastwards.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH 1031hPa in the Tasman Sea weakening over northern
NZ Mon to Wed.

HIGH in south Tasman Sea by wed developing to 1038hPa over and east of the South
Island northeast-wards from Thurs to Sunday.

These highs are expected to make a squash zone of
enhanced ESE winds on their northern side, mainly around 20S.


Aussie/Tasman/NZ troughs

Low is expected to form off NSW coast by mid-week and
then travel off to the south.

Front with southerly change over Tasmania by Monday
morning should reach southern NZ by late Tuesday and North Island by late


Panama to Galapagos/Marquesas

Light winds for Panama. SW winds from around 7N to 5N
then southerly winds to Galapagos.  Currents help make the trip via north of Galapagos better this week.


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


Published by metbob

Pattern and Chaos

Join the Conversation


  1. Hi Bob. Thanks for mentioning LuckGrib! I found it surprising that there is some web site advertising support for running the app on Android. I have no affiliation with that site. The only supported ways to run the app are by downloading it to your Mac, iPhone or iPad via the Apple App store. Anybody downloading the app via that link for Android should first do a backup, as there is a great chance that the download will corrupt your device…

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