Bob Blog 4 June

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 4 June 23

A review of April weather

Here is a link to a YouTube clip giving an animated loop of the isobars and streamlines in the South Pacific for the last month

May 2023 brought an end to a blocking high east of New Zealand and then a series of lows travelling east across the South Pacific. The Highs were brief, making for very few good opportunities for sailing form NZ/Australia to the tropics. The low on 20May was measured on my baro in Palmerston North as 979h.


Sea Surface temperature anomalies from


Warm sea temperatures continue to spread westwards along the equator from Peru.

Marine heat wave around NZ has shifted towards Chathams. Another marine heat wave covers eastern North Atlantic

Average isobars for past month (below)



Subtropical ridges have shifted northwards. Now the Arctic has high rather than low pressures

Pressure anomolies for past month (below)


The anomaly pattern in the southern hemisphere is almost the reverse of last month’s. In particular, both Arctic and Antarctic have flipped from positive to negative .

Zooming into the NZ area

Last month below



Previous month above

The subtropical ridge has now drifted to a mean position north of NZ. The 1015 isobar on its northern side now stretches from Australia north coast to Tongatapu.  The westerly flow in the 40s and 50s is increasing.


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


Cyclone MAWAR dropped to an estimated central pressure of 897 hPa and Cat5 to Ne pf Philippines sand then faded on its way to southern Japan. There are now tropical depressions I both west and east of the Philippines, where potential is high for development this week.

The MJO, a burst of extra energy in the tropics, has finished crossing the Pacific and a period of relative quiet is expected for next few weeks.


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


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


Rain Accumulation next five days from


The South Pacific Convergence zone is expected to stretch from northern Coral Sea to northern Fiji and northern Tonga areas. Another weaker convergence zone is lingering between Northern Cooks and north of Tuamotu Islands. A trough is expected over Fiji on Tuesday going southeast and developing into a low south of Niue late in the week. Avoid.


Low L1 is the surface reflection of a cold pool aloft and as such is expected to travel only slowly across and to the east of northern NZ this week. Avoid.

High H1 in the south Tasman Se is expected to swing around southern NZ and to east of Chathams. This feeds cold sir into the moisture of L1 , a good recipe for snow making in the squashed isobars between L1 and H1 over central NZ. Avoid.

Low 2 well to south of Tahiti tonight is expected to deepen with the arrival of a cold southerly and then travel off to the east.

The gulf of Panama = Not recommended due convergence zone and SW winds.


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