Bob Blog 16 Oct



Compiled 16 October2016

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.


Chuck on JACARANDA has replied as follows:

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The Tropics

When referring to the Fiji Day LOW last Monday I noticed that the upper trough responsible was also shifting a “Vorticity max” out of the Tasman Sea and onto Vanuatu

See my Facebook post at


An upper trough penetrates the South Pacific convergence zone over Fiji on 10 Oct, causing winds to spin and spiral downwards forming a surface low over Fiji=The Fiji Day low. The extra upward motion brought heaps of RAIN… and as a sideline, an entity of the west side of the upper trough known as a Vorticity Max is taken by the southwest winds aloft from the Tasman Sea onto New Caledonia (shown here with image from then onto Vanuatu.

Well, A village just north of Port Vila (Tongamea) had a significant HAIL STORM as a consequence:

I received the following photos from my good friend Alex from Honiara Weather Office:



We are having a full moon this weekend, and so next weekend is the start of the last quarter of the waning moon. That’s when the full tide is overnight, and it’s the first of these events after the equinox that triggers the season for PALOLO… when the coral worms spawn. 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 to a flashlight). At dawn the pods dissolve, allowing eggs and sperm to get together and start the new generation. The rising is only on the turn of that one tide and only takes place for a few hours. Samoan Met Office say Oct 22/23 is this year’s best chance



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.


Bureau of Met and NIWA have both issued their cyclone forecast for the coming season in the past few days—I’ll cover that next week for you.

In the NW Pacific tonight we have TC SARIKA and HAIMA


As seen at

And in the Atlantic, after a very destructive path last week from MATTHEW, we have NICOLE keeping well offshore, but making a direct hit over Bermuda as a Cat 3 (Saffir-Simpson scale).


As seen at


Bermuda coastline is in green near centre.


Rain for the past fortnight from



Rain accumulation maps show an increase in convection over Philippines, the track of MATTHEW and NICOLE in the Atlantic, and a burst of rain over Fiji thanks to the Fiji day low, but a removal of convection from Solomons as a consequence.


Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, swell black arrows / Sig wave height purple lines, swell and wind waves, SPCZ and STR.



Tropical accumulated rainfall for next week from

Tropics to NZ/ Australia: – Strategy

This is the time of year that yachts are staging themselves in Tonga (or Fiji or New Caledonia) and waiting for the right weather pattern for sailing to NZ or Australia.

The Island Cruising Association are supporting the All Points Rally, from all major parts of South Pacific to Opua ending in a one-week seminar 16 to 20 November in Opua. Boats joining the rally are assisted with weather info, resources and planning tools to help make the passage to NZ as easy as possible. On arrival participants are welcomed to Opua and entertained with a week of fun and games, entertainment and seminars on a range of topics. This rally is FREE thanks to the assistance of Main Sponsors Bay of Islands Marina and Boatyard and the Opua business community. See

Further west there is the GO WEST Rally. Again from any port (most are leaving from Noumea) this time to BUNDABERG, with welcome week starting 6 November 2016.


In deciding upon a departure date, it’s as simple as 1.2.3:

1.The first factor to consider is the local weather: the South Pacific Convergence Zone SPCZ sometimes brings squalls, but is well to the north this week and still recovering after being vented by the Fiji day Low last Monday. However, we have a passing (mainly upper) trough over Fiji on Monday/Tuesday, and this trough is expected to slowly cross Tonga area on Tuesday/Wednesday. This trough is likely to bring squalls and maybe some strong SE winds, so better to wait for it to go before departing.

Some like to use Minerva reef as an extra staging post since it is 1.5 to 2 days sail south of Tonga, and just out of the tropics (so marine insurance that may be void in the tropics may work in Minerva). But it only shelters you from the waves (so long as they don’t topple over the reef) and not from the wind or rain. It has that passing trough on Tuesday (local) with some rain until Thursday, and is only likely to have fresh easterly winds form this feature.

2.The second factor to watch are the HIGHS that travel along the Subtopic ridge STR, that zone between the trade winds of the tropics and the disturbed westerlies of the roaring 40s. This zone is usually near 30S, and if there is a big high then on its northern side there is usually an accompanying are of enhanced trade winds, what I call a SQUASH ZONE. This week there is a HIGH moving along 30S from N Tasman Sea tonight to east of NZ on Wednesday, and there is a squash zone between that High and the trough over Tonga, mainly on Wednesday/Thursday. The next High should start further south and cross the Tasman Sea from Thursday to north of NZ on Sat 22 Oct, without any squash zone.

3.The third factor is to avoid bursts of strong southerly winds and heavy swells during the trip or upon arrival. These come from the Southern Ocean and on a different pattern to the passing troughs in the tropics. Sometimes a southerly burst inter-reacts with a tropical trough and things get nasty. Not this week. You can use to see expected weather features at your arrival point for the next week, and aim to arrive “in-between active features”, but remember that these outlooks are just ideas, and real world will do its own thing. Over northern NZ there is expected to be a weak passing trough on Thursday, and then northern weakening trough on Sunday 23, and then maybe some strong SW winds from a passing southern Low on Mon 24/Tues 25 Oct. And over the Australian coast there is a passing trough this Monday and another mainly south of 30S, on Friday, weaker further north on sat/Sun. I think you can handle these fronts with suitable waypoints/timing.

Let’s apply this 1,2,3 rule to some popular destinations:

Travel between the Tropics and NZ:

If heading from Fiji or Tonga, wait for that trough to go first. Maybe Wed from Fiji and Thu from Tonga. but that may change. Arrange waypoints so as to encounter that Sun 23 over NZ front near 30S. That depends on boat speed, and configuration, so will vary from vessel to vessel.

Travel between New Caledonia and Brisbane/Bundaberg area:

Trough that was over New Caledonia yesterday has gone east now, but there is a squash zone between it and the Tasman Sea High. That should ease during Monday. Then OK to go anytime, but expect to encounter a weak passing trough around next Sat/Sun as you approach coast. If you arrive over the weekend then there’s a surcharge to pay.

Travel from tropics to New South Wales

No real point in taking the rhumb line for such a voyage as the subtropical ridge should be gone around rather than thru. Instead head for Brisbane, and around 158E turn to your destination. This should also give you the benefit of the assistance of the East Aussie current. However, remember that there is an active trof south of 30S on Friday 21/Sat 22 Oct.


See my yotpak at for terms used.

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