Bob Blog 2 Aug

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 2 Aug 2020


Sea Surface temperature anomalies can be seen at

Early August (below)


Not much change since in the sea surface temperatures during July. But there is still a slight weakening in the warm pool in the Indian Ocean/Tasman Sea/Fiji. However, the possibility of a NEGATIVE Indian Ocean Dipole – bringing above-average rainfall over parts of Southern Australia later this year – has weakened.

ENSO is currently neutral, but, according to BoM (Australia) that cooling sea in eastern equatorial Pacific is significant. ENSO (El Nino/southern Oscillation) is likely (around 50%) to bring a La Nina later this year. La Nina tends to bring the subtropical ridge southwards.

SAM (Southern Annular Mode) is currently positive for a few weeks, but had 3 dives to the negative in July encouraging southerly outbreaks and is likely to go negative next week.

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


Pressure anomolies for past few weeks (below)


The Atlantic high has increased in intensity (as per normal for July). The monsoon has also increased in intensity.

Zooming into the NZ area, and comparing monthly anomalies from last month with now, shows that the sub-tropical ridge has shifted north over NZ (as per a normal July), but strangely has intensified over Australia.

Last month Below




Last few weeks (above)

The 1010 line has shifted north a little in the Tasman Sea, allowing more fronts to spread north over NZ. A 1025hPa line has formed in southern Australia.


The Tropics

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

There are FOUR active storms at present, ISAIAS is about to attack Florida an d there is another cyclone in Eastern Atlantic, Also SINLAKU and HAGUPIT in Asia. There are several areas around with increased potential for cyclone development.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)


SPCZ=South Pacific Convergence zone.

SPCZ is likely to be more widespread this week from Solomons to Tuvalu /Tokelau/, and should be most active over Fiji and Tonga/Niue. . Passing trough over Fiji tonight and Tonga on local Monday/ Tuesday, and Niue Sothern Cooks on local Wed/Thu. Another passing trough over Austral Islands tonight.


Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH 1036hPa well east of South Island at 33s160W with a ridge lingering over central NZ until Wed.

Next High 1020-1018hpa is expected to arrive in central Tasman Sea from Thu and travel past northern NZ on Sat/Sun.

Tasman Sea.

Low over northern NZ tonight moving off to east on Monday.

Another Low expected to form off Tasmania on Tues and go S of NZ on Thu/Fri to 986hpa, preceded by strong NW, accompanied by a front on Fri, and followed by strong SW flow until Sunday.

From Panama: ITCZ 5N to 12N, light winds maybe NW for starters this week, but W/SW from 7S 80W to 90W.


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