Bob Blog 3 Jan 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 Sun 27 Dec 2020


Sea Surface temperature anomalies as at

Early January (below)


La NINA (the cool eastern equatorial Pacific) continues and there are warmer-than-normal patches in the Southwest Pacific, notably in the Coral Sea. This has the makings of an active cyclone season, especially near the dateline.

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)


Pressure anomolies for past few weeks (below)


The sub-tropical ridge in the southern hemisphere has strengthened and shifted south. Not sure why, but the Arctic area has flipped from below normal to above normal pressure. Interesting.

Zooming into the NZ area, and comparing monthly anomalies from last month with now, the sub-tropical ridge has gone south – the 1010 isobar over NZ has drifted from 40S to 47S and over Vanuatu area it as drifted from 10S to 18S.

Last few weeks (below)



Previous month ago (above)

The Tropics

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

TC DANILO is in the Indian Ocean.

There are some zones of high potential for tropical cyclone formation around eastern Indonesia/Papua New Guinea this week, and a moderate zone near New Caledonia/Fiji. .



Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue), STR(Subtropical Ridge), and SPCZ (South Pacific Convergence Zone).


Predict wind CAPE mid-week gives two possibilities for SPCZ


SPCZ=South Pacific Convergence zone.

SPCZ stretches from Coral Sea to Vanuatu to between Fiji /Tonga, with another convergence zone north and east of Samoa. SPCZ is active and a Heavy Rain warning is in force for Fiji with a flood warning for low-lying areas.

A trough is affecting the Fiji area for next few days and should spread onto Tonga by mid-week. Another trough is expected to affect New Caledonia for next few days. If you are in these areas , then may as well stay put this week.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH 1018 in mid Tasman Sea is expected to spread onto northern NZ on Tuesday and Wednesday.

NZ/Tasman troughs

Low 1008 over South Island had a cut-off low aloft but this is now fading and the surface low should finally move off to southeast on Monday.

Low 1004 is expected to form offshore New South Wales on Monday and then travel southeast to south of South Island 998hPa late on Wednesday.

Another low 1004 should form in trough of first low in around Lord Howe Island on Wednesday and then travel southeast onto central NZ over the weekend.


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