Bob McDavitt's ideas for sailing weather around the South pacific

05 August 2018

Bob Blog 5 Aug 2018



Compiled Sun 05 AUGUST  2018

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.


Sea Surface temperature anomalies as at beginning of August  may be seen

The eastern equatorial Pacific around Galapagos is the focal region for ENSO and is now on a slowly warming trend. Temperatures around Australia and into the Tasman Sea are becoming below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America and the Kuroshio current off Japan still stands out as warmer than normal.  These may help steer tropical features to the northeast.

Warm anomalies continue off west side of Mexico indicating a busy cyclone season for next few months.


To see how the annual weather cycle and the seasons are working out, take  a quick look at the average isobar maps from showing average isobars for past 30 days and their anomaly for JULY.

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust. The North Atlantic and Northeast Pacific HIGHS are looking stronger than this time last month. The July lows in the Southern Ocean have been favouring three positions: south of Australia, SW of South America, and SE of South America.

Lower than normal pressures between Europe and Philippines show a more active monsoon. 

Zooming into the NZ area, the 1015hP (between blue and white) isobar has stayed put in the tropics and to east of NZ.  There has been a radical change south of Australia and in the Tasman with a DROP in pressure allowing the 1015 to shift from Invercargill to Auckland.  This explains the increase in westerly winds onto western NZ, and can be taken as an early start to SPRING weather patterns.


The last 30 days of rainfall, and its anomaly, are seen at TRMM at

The rain map shows extra convergence in the ITCZ from Bangladesh to Philippines , and across  the North Pacific Ocean, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere a large dry region stretches from Australia to northern NZ, and the South pacific Convergence zone has been weak around Vanuatu and : Fiji.  It seems that the SPCZ has shifted northwards (an El Nino trait).


In summary, it seems that, although the atmospheric and oceanic parameters are “neutral” at present, as seen in my last week’s blog, there are early signs of an El NINO weather pattern.



There is a regular procession of tropical features off the western Mexico coast. 

The Atlantic remains somewhat quiet, and SHANSHAN is heading for Japan. As seen at


Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific and to central America. 




SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Vanuatu to the Tuvalu/Tokelau then Southern Cooks area.  

A passing trough is likely over Southern Cooks on Mon 6 Aug UTC (squally) and Wed 8 Aug (not so squally), These troughs should weaken east of the Cooks and not affect Society Group (apart from some northerly winds).

Another passing trough is expected to develop around Fiji/Tonga around 11 Aug UTC This may turn into a  Low near 22Z next week, Avoid.


Subtropical ridge (STR)

Weak HIGH is expected to appear in north Tasman Sea from Mon to Wed 6-8Aug, and another should cross northern Tasman Sea /North Island from Thu 08 to sat 11 Aug UTC.

A polar HIGH is expected travel east along 50 to 45S this week, east on NZ, there is likely to be a squash zone of gale easterly winds east of NZ between around 30 and 40S. Avoid.


Around Tasman Sea, NZ to tropics.

Passing troughs on Monday and Wednesday. Best date to depart NZ for the tropics this week is Thursday (local). 


Tahiti to Tonga

With the passing troughs, perhaps the best on offer this week is a Friday local departure to Palmerston or Aitutaki. Then again, if you don’t mind encountering a trough over the open sea, can depart any day, but get some waypoints to reduce the squalls. .


If you would like more detail for your voyage, then check to see what I offer.

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