Bob Blog 28 July



Compiled Sun 28 July 2019

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.



It is now mid-winter and we are sometime open to blasts from the chilly South. A good parameter to watch, to forecast these blasts is he Southern Annular Mode (SAM), or its proxy called the Antarctic Oscillation (AAO),

SAM describes the north to south movement of the westerly wind belt that circles Antarctica, so it can be used as a predictor for cold air reaching our mid-latitudes.

When SAM is positive, the belt of strong westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over mid latitudes, limiting the northern extent of cold fronts.

A negative SAM event indicates an expansion of the belt of strong westerly winds into the mid-latitudes. This weather pattern results in low pressure systems over mid-latitudes such as New Zealand, with more (and stronger) storms.

We can see the latest forecasts for the AAO (a good proxy for SAM) at


This shows a sudden drop from Positive mode to negative mode in early August, perhaps more extensive than the southerly event of mid-July, especially for Southern Australia and New Zealand. This does have an impact into the tropics, weakening the trade winds, bringing periods of SW swell, discouraging squash zone, and possibly encouraging passing troughs.


The latest cyclone activity report is at and TCFP tropical Cyclone Formation Potential at

TC ERICK is expected to pass by south of Hawaii (just)—but may prove troublesome with its swell.

There is more potential than last week, with probable areas in the NW and NE Pacific, and now also in the Atlantic.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Samoa area.

There is a trough passing over Tonga /Niue the next few days, then travelling off to the southeast so not reaching Palmerston Island. Wind change with trof at Niue is likely to be from NE to SE by Wednesday, so should be OK in its anchorage. Best not to arrive in Niue during this trough because of changing winds.

This is expected to be followed by a ridge between 30 and 25S, typical of a negative phase of the SAM parameter.

Accumulated rainfall for next week from


Subtropical ridge (STR)

The High that has been just east of NZ for the past week is expected this week to travel east along 40S.

The next High is expected this week to shift from Aussie Bight to Aussie interior, poking a ridge across the Tasman Sea along 35 to 25S.

Tasman Sea /NZ/Aus

In NZ, may as well stay put this week. Weak trough crossing the north next few days. Deep Low south of the Tasman Sea 986 hPa is expected by Wednesday to trigger a Low 980hPa with strong winds and swells over 7m in the Tasman Sea, followed by strong SW flow from Thursday 1 to Tuesday 6 Aug.

Noumea to Australia voyages OK for a few days thanks to a ridge forming across Tasman Sea along 35 to 25S, but swells from the SW are expected to be over 3m from Wednesday or Thursday to Saturday.

Tahiti to Tonga

Looking OK to depart from Tahiti on local Mon to Wed, with trade winds weak at times, but after that there may be a 3m southerly swell for  few days. Also need to avoid the passing trough that is expected to reach Tonga around Wed 7 Aug.


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

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Weathergram archive (with translator) is at

Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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