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

27 September 2015

Bobgram issued 27 Nov 2015



Issued 27 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.


Note that NZ has switched to Daylight Time today .


There is a full tide in the middle of the  night when we have the  last quarter moon.  Well, the one after  the equinox (and that’s around 5 Oct this year) normally triggers a spawning of the Palolo coral warm. They drop off their worm-tails or pods; jade (female) or brown (male); and these mix together in the swirling tide, with each having a light sensitive spot that directs it towards the moon as it sinks in the west (or a flashlight). At dawn the pods dissolve, allowing eggs and sperm to get together and form the new generation. The rising is only on the turn of that one tide and only takes place for a few hours. Actual timing varies each year, and is usually in October, but may be followed by a minor event a month later.

A bowl of Palolo may be seen at http://www.ird.


This is worm sex-Pacific style. The pods can accumulate in the sea in massive amounts. Ask the locals about this and if you time it right you may be about to collect some of this rare delicacy, or photo it. They might look yucky, but taste really nice on toast, something like caviar, so I’m told.


El Nino and the SOI

AN El Nino occurs when the Earth’s ocean/atmosphere system stores incoming energy from the sun as heat in the sea. Its atmospheric aspect can be measured by a parameter related to the barometric pressure difference between Tahiti and Darwin, a parameter called the Southern oscillation index SOI. This went into El Nino territory briefly in June and more definitely in August and has been very strongly negative throughout September.

This index usually encourages the fronts and disturbed W/SW winds of the southern ocean to visit the NZ/Tasman Sea area. Yet in the past week we have had the opposite weather pattern with a cut-off low feeding easterly winds onto northern NZ—Heavy rain in places which desperately need it.

Weather is truly a mix of pattern and chaos.



There are three tropical cyclones and a tropical low at present In the Atlantic is the remains of TC IDA Near the Mexican coast is TC MARTY Near Hawaii is TC NIALA And approaching Taiwan is TC DUJUAN The weekly rain maps, as seen athttp://trmm.gsfc.nasa.

gov/trmm_rain/Events/big_global_accumlation.gif, over the past two weeks show that maximum rainfall in the South Pacific has shifted so that it now is situated between NW of Fiji and the equatorial dateline.



SPCZ=South Pacific Convergence zone.  There is a lot of activity between Eq. 180 and the NW of Fiji at present.

This is expected to spread eastwards (westerly winds aloft) and stretch across Tuvalu and Tokelau towards Northern cooks and maybe the Tuamotu Islands. A new branch of the SPCZ is likely to form in the Coral Sea later in the week.


STR= Sub-tropical Ridge

High us building eats of NZ on Monday and Tuesday UTC and expected to travel east along 35S from Wednesday to Friday with a squash zone of strong trade winds over the Fiji /Tonga/Samoa area in parts on Thu 1  to at 03 Oct.

New HIGH is expected to travel from interior of Australia to Tasman Sea on Fri 2 Oct then travel slowly east along 30S until 4 Oct and then along 40S from Mon 4 to Tues 06 October. On the northern side of this HIGH there is likely to be a squash zone of stronger trade winds over the Coral Sea from 30 Sep UTC stretching onto New Caledonia by Friday 2 Oct and Fiji by Sun 4 Oct maybe lasting at least until 10 October.


Travelling Tahiti to Tonga

It is a better looking pattern than last week:

There is a Low well off to the south of Tahiti at present. Its squalls are well off and of no concern but it has “stolen” the winds around Society winds so that departure in net few days are in light winds.

Southerly winds are forecast to return by Tues UTC/Monday local and Se winds from Wed UTC /Tues local. Squash one of strong SE winds and 3

metre+ swells is expected to be strongest over Tahiti area around Sat 3

Oct/Friday local. Remainder of voyage westwards to Tonga basically has SE winds 20 gust 30 knots this week.



NE (onshore) flow onto Northland on Monday to Wednesday as HIGH to east of country builds and finally moves off to the east, Cold front on Thursday is likely to be followed by strong SW flow on Friday 2/Saturday 3 Oct as the front may turn into a Low off the E of North Island.

An intense Low 980 hPa is likely to travel east across South Tasman and South Island along 48S on Sun 4 / Mon 5 Oct, followed by a strong S/SE flow spreading over NZ on Mon 5 Oct and Fiji /Tonga on Wed 7/Thu 8 Oct.


Between Tropics and NZ:

There are no comfortable voyages from Tonga or Fiji this week, unless they can reach Northland by Thursday 1 Oct, before the SW winds of 2 Oct.

Departures from New Caledonia or Australia to NZ may be able to cope with these SW winds, and may find the swell bumpy but OK.


See my yotpak at for terms used.

See my website for information on tailor-made voyage forecasts– Feedback to Tell anyone you like that to subscribe they should email me.

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