Bob Blog 29 Jan 2023

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 29 Jan 2023

The Auckland deluge, an eggbeater

On Friday 27 January , the day before a long holiday weekend, a deluge of rain moved over Auckland delivering downpours of 100-200mm rain mainly in around 5 hours.


Rain gauge measurements around Auckland for last Friday

These are tropical proportions, and Auckland’s drains are engineered to cope with up to around 30mm per hour, so many didn’t cope.


Here is a graph of the hourly rain amounts for the top five wettest days ever measured at Auckland Airport:


In the past that 30mm/hr threshold has been briefly exceeded , and last Friday it was well and truly exceeded for around 4 to 5 consecutive hours. Even the airport was flooded, and Elton John’s concert cancelled.

Looking at the Aotearoa NZ rain map for Friday shows that Auckland and Coromandel bore the brunt of rain hosed in by a flow from the northeast


The weather pattern for this deluge is a typical example of what I call an eggbeater with two main ingredients.


1: A large HIGH over 1030 east of NZ that is being held in place by light winds aloft.

2: Low pressure areas are being formed in the Tasman sea, but can’t move east because of the blocking high.

These ingredients combine like an old-fashioned manual eggbeater…


One spinning one way, and the other spinning the other way….

To feed tropical air into the hose of northeast winds aimed at Aotearoa NZ.

Similar weather patterns in the past have produced similarly wet days. This one was super charged, and the reason for that may be that the Oceans are now the warmest ever measured ….


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


Cyclone CHENESO brought lots of damage to Madagascar and is now moving off to the south.

There is potential this week for cyclone formation between Indonesia and northern Australia.


Weather Zones Mid-week GFS model showing isobars, winds, waves (purple), rain (red), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) CZ (Convergence Zone) and CAPE (lime)


CAPE maps mid-week GFS from Predictwind, showing chance of lightning.


Rain Accumulation next five days from


The SPCZ is expected to be very active from PNG all the way to French Polynesia.

Low L1 near Norfolk Island is expected to intensify as it travels slowly southwest to west of Northland by mid-week and then fade further south.

High H1 east of Chatham Islands is quasi -stationary and expected to crawl east along 45S.

Low L2 is expected to form near 20S to south of Tahiti early in the week and then travel southwestwards and deepen.

Low L3 in the monsoonal trough over inland Australia is expected to bud off coastal Sydney on Tuesday and then go southeast and fade in the Tasman Sea.

A tropical Low may form near Fiji this weekend. Keep an eye out for this.


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