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

20 September 2015

Bob Blof 20Sep 2015



Issued 20 September 2015

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.


Welcome (on Wednesday) to the vernal equinox (in Southern Hemisphere).

Our Sun, viewed from earth, crosses our equator at 08:21 UTC (8:21pm NZST).

The next full moon (fullest at 2:51UTC on Monday 28th Sep) occurs within a few hours of this month’s lunar perigee (closest at 1:46UTC). This just happens to be the closest that the moon gets to earth for the year at 357,

126 km. Yes, this means the full moon this moon will appear to be bigger than normal. It is a perigean moon, some call it a “super moon”. The moon is also going to cross the ecliptic and thus move into earth’s shadow (lunar eclipse). Mid–eclipse is 2:47UTC. This eclipse is best viewed from the America’s, and they are preparing for a “blood moon”, not visible from South Pacific. The reason I’m relating all this background info is that the corresponding tides are called “king tides” or “perigean tides”

and will have higher and lower extremes than normal, reaching their peak around Wednesday 30 Sep. Take adequate precautions.


Astronomical data on the 28Sepeclipse is available at


NIWA in NZ have now published their September edition of the seasonal Outlook for the South Pacific at




El Nino is expected to intensify and that is likely to weaken the trade winds and pull the South Pacific Convergence zone north and east of its normal position. There are likely to be bursts of equatorial westerly winds around PNG and Solomon Islands.

Drier than normal conditions are likely around New Zealand and in the New Caledonia/Fiji/Tonga area Note that this is a seasonal forecast, and, as we shall see later in this blog , this week bucks this trend.

Also, cooler than normal sea surface temperatures around New Zealand and in the New Caledonia/Fiji/Tonga area may reduce the likelihood of severe tropical cyclones in these areas.


AN El Nino occurs when the Earth’s ocean/atmosphere system stores incoming energy from the sun as heat in the sea. Temperatures in the target zone (measured by the NINO3.4 index) in the eastern equatorial Pacific have now intensified to 2 full degrees above normal as seen at

c=nino34&p=monthly .

They are still intensifying and are expected to peak around the turn of the year.

The graph shows that they still have a while to go to rival the 1997/98 El Nino.



There are more tropical features around than last week including two tropical cyclones.

As seen at

In the Atlantic we have TC IDA

And off to NE of Japan we have TC KROVANH The weekly rain maps over the past two weeks show that rainfall in the South Pacific is intensifying and extending to the southeast.

Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.



SPCZ=South Pacific Convergence zone.

There is a lot of activity between Eq. and 5S from 160E to 180 at present and this moisture is expected to feed west then S then SE along SPCZ that extends from eastern Solomon Islands to Fiji. Another branch of the SPCZ is expected to linger around tokelay and Northern Cooks.

The 10 day rain accumulations at confirms these ideas


STR= Sub-tropical Ridge

This is very weak over the Pacific at present.

The High over Australian Bight on Wednesday is expected to spread eat to South Island for Fri/ Sat 25/26 Sep. The reason it is being diverted so far south is because of the cut-off lows in the Tasman Sea. We meteorologists have a name for this pattern – “low index” meaning that one of the zonal parameters we use to measure the weather (Z1) is taking negative values --- the isobars are higher in reading over the South Island rather than the North Island. These low index patterns occur occasionally, even at this time of the year (the equinox) BUT they are the antithesis of El Nino, so I suspect that next High will take a different route and go NE across the Tasman Sea and return us to a “high index” pattern early in October. Meanwhile the eastern and Northern North Island will welcome the rain this week, for the season forecast is for dry, dry, dry.


Travelling Tahiti to Tonga

The rain accumulation map shows that it should be dry from Tahiti to Southern Cooks. However, Trough over Tonga on Monday UTC and Niue on Tuesday UTC. After that the trough is expected to weaken and there may be a ridge of light winds along the route—OK for motoring, but not so good for sailing.

The outlook shows a better looking pattern next week.



The cut-off Low located east of Auckland at present should dominate

conditions over the North Island until Wednesday local (some call this

the “olde man southerly” pattern). Then a brief ridge for the North

Island on Thursday.

Another Low is expected to form and cut-off just east of New South Wales

on Tuesday. Its deepening should bring a gale to Sydney/Coffs by

Wednesday. Then this low should travel east along 30S affecting northern

NZ from Friday to Sunday 27 Sep.


Between Tropics and NZ:

There are a lot of southerly winds on the western side of these Cut-off

lows until Friday. Things may be OK for a departure after Saturday 26Sep,

not before.


See my yotpak at for terms used.

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