Bob Blog 15 Sep 2019



Compiled Sun 15 September 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.


We have had a taste of winter in New Zealand late August/early September, followed by a few weeks with disturbed lows and fronts, but this week is looking more on the HIGH side.

A good parameter to watch, to forecast southerly blasts is the 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


Showing a swing to positive SAM and likely HIGHS in the subtropics.



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

For those who want to see the President of US comment on DORIAN possibly hitting Alabama


At present there are two Cyclones HUMBERTO and KIKO and several areas of potential activity in North Atlantic and North Pacific. ‘



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 is expected to stretch from Solomon Islands across northern Vanuatu to a weaker Convergence zone around Tuvalu and Tokelau.

Trough over Tonga on local Monday connected to a Low near 30S This Low is expected to travel east along 30S to 160W by local Wednesday. and then go south. Associated trough is expected to visit Niue on local Monday. Palmerston Island area on local Tuesday and affect Tahiti area with variable winds on local Tuesday night /Wednesday.


Accumulated rainfall for next week from

Subtropical ridge (STR)

The STR is expected to return to 40S thus week, but not until mid-week.

On Mon and Tuesday, a High is expected to travel east along 30S across northern NZ then fade.

Then from late Tuesday to Sunday another HIGH is expected to form near 40S in the Tasman Sea and take it’s time travelling onto South Island before going north to 30S and to East of NZ.

There may well be a squash zone in the tropics on the north side of this High NEXT WEEK>

Tasman Sea /NZ/Aus

Trough (between the HIGHS) over northern NZ on Tuesday, followed by a burst of a long-period SW swell from Tasman to north of NZ on Friday. Should be Ok for trips from Fiji/Tonga to NZ this week.

SE to East wins from Noumea to Australia. No good going other way.

Tahiti to Tonga

Try and time your voyage taking into account the passing trough. It misses Suwarrow.


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

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