Bob Blog 4 April 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 04 April


Sea Surface temperature anomalies as at


Early April (below)

The eastern equatorial Pacific is now near normal so La Nina has faded. Also, the warm blob south of Tahiti and east of NZ is easing. 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 Aleutian Low and North Atlantic low have weakened. The Siberian HIGH continues to weaken. The subtropical ridges have intensified. The doldrums low has shifted north over Philippines/Australia/Indian Ocean, retreated westwards across the Pacific, and retreated eastwards across Indian Ocean. The Antarctic High has strengthened. The seasons they are a changing

Pressure anomolies for past few weeks (below)


Arctic has changed from above normal to below normal pressure. the South Indian ocean has changed from a mixed pattern to lower than normal pressures. The lower-than-normal pressures over Australia and the Tasman sea and along 30S across the South Pacific Ocean have weakened.

Zooming into the NZ area, (note that isobars colours have changed) the 1015 isobar has expanded northwards. The Antarctic High has strengthened. This maybe in response to a switch of SAM from negative to positive and enhances westerly winds in the polar vortex.

Last few weeks (below)



A month ago (above)

Madden-Julian Oscillation

The MJO is a pulse of upward motion that travels eastwards around the tropics from Indian Ocean to Pacific Ocean.

It is now active over northern Australian as can be seen as the blue area of the graphic from


Or in the satellite imagery


The Tropics

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


It is another quiet weekend with no active cyclones. However, there are areas of medium to high potential for tropical cyclone formation around norther Australia. A reminder that, even though we have switched from Summertime back to standard time, and the tropics have been quiet for past few weeks, the month of April is still part of the Southern Hemisphere Cyclone Season.


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 is expected to be a well-defined and rather intense zone from Papua New Guinea to Solomons to northern Vanuatu to north of Fiji to Samoa.

Low is Coral Sea is expected to travel SSE into central Tasman Sea.

Low is expected to form south of Niue on local Sunday and travel off to south.

Low south of Gambier Islands is expected to travel slowly E/SE.


Rain Accumulation next five days from

Subtropical ridge (STR)

Easter egg of a HIGH over northern Tasman Sea should fade by Tuesday.

HIGH is moving over Tasmania and expected to skirt around South Island on local Wednesday then travel east along 40 South.

Aussie/Tasman/NZ troughs

Low in central Tasman sea by Wednesday expected to cross central NZ on Sat 10 April, followed by another low over NZ on Sunday/Monday then strong southerly next week, avoid.

Panama to Galapagos/Marquesas

Moderate northerly winds at Panama but for light variable winds from Tues 6 to Tue 13 April. Light winds around Galapagos.

Good current to south-southwest between Panama and Galapagos. Path to take to Marquesas this week is much the same north of south of Galapagos.


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