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

26 November 2017

Bobgram issued 26 Nov



Compiled Sun 26 November 2017


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.


Pattern and Chaos

At my talk to the All Points Rally last Friday, Peter Mott of Northland Radio commented that two vessels about 100 n miles apart had completely different wind speed—one ESE 18 knots and t’other 34 to 38 knots from SE. The computer models at that time was forecasting much the same conditions for both around 20 to 30 knots from ESE.

A scatterometer satellite (measuring the wind speed over the surface of the sea by using microwaves to measure the sea foam, helps to show what was happening. The computer model averages things out and thus produce a smooth image showing the weather pattern. However, actually, the stronger winds lay across the sea in filaments, with avenues of weaker wind around them.  This can be seen at 

This is a good example for anyone using grib data. Weather is a mix of pattern and chaos. The computers can only give us a pattern, but the real world always has elements of chaos- sailor are on the receiving world of the real world, and thus notice the chaos more than the pattern, The lesson is to take the grid data with a “grain of salt” and allow a +/- of around 10% even when the data is given to the nearest 0.1 of a knot.



As well as running MetBob, I am also President of a club called Cruising and Navigation Association of New Zealand, This is a small club that, as it name implies, aims at organising events for a group of Auckland-based yachts– some interested in cruising around the Hauraki Gulf and some interested in the navigation and meteorology of sailing. We welcome cruising yachts visiting Auckland from overseas. We may be able to help as local liaison and answer questions such as

· Where to shop/bank/get things fixed

· Local knowledge for anchoring

· Visiting Yacht clubs

If you are visiting Auckland and have a query then let me know at or and I’ll see what can be arranged.



There are no named cyclones around at present, but there are two small tropical depressions south of the equator in the Indian Ocean, and that means we may be in for a visit from a MJO.

MJO is the name given to a equatorial weather feature; its full name is Madden Julian Oscillation , named after two meteorologists who wrote a paper describing it back in 1971. See

An MJO occurs when a conglomerate of tropical convection forms in the Indian equatorial Ocean, and then this slowly moves eastward across the Indo-Australian zone into the western equatorial Pacific. It may take 30 to 50 days. The arrival of an MJO and it increased cloud/rain and convection in the equatorial western Pacific may help trigger tropical cyclones, so this is a feature worth watching .

There is a conglomerate of convection forming in the Indian Ocean this week, and it shows itself in the first map below as a zone of blue. The following maps are forecasts of that extra convection showing where it is expected in 5, 10, and 15 days time. It may reach the Papua New Guinea area around mid December. (In these maps OLR = Outgoing radiation). See


Looking at the weekly rain maps from last week and the week before, we can see an increase in convection in the equatorial Indian Ocean. See



SPCZ=South Pacific Convergence zone.

The SPCZ is hovering from Coral Sea to northern Vanuatu to Samoa to Southern cooks. It is expected to shift south onto Fiji/Tonga from Tuesday to Friday, and then move off to the southeast as a trough/Low.


Subtropical ridge (STR)

HIGH H1 to southeast of NZ tonight is expected to slowly travel NE and then east along 40S this week.

Weak trough should cover inland NZ from Tuesday to Thursday (with inland showers).

Then another HIGH H2 is expected to form Se of NZ by Thursday and then slowly follow or merge with H1.


Between Tropics and NZ

With isobars around 1020+ over Northland and around 1010 to 1005 around Fiji, the wind flow between tropics and NZ is mainly easterly (and over 20gust30 knots in places).

Next week, a trough/warm front is likely to make its way across the Tasman Sea – at this stage its arrival in northern NZ is anticipated to be around Sat 9/Sun 10 December, but that may change.



“The Sea, once it casts its spell, holds us in its net of wonder forever.” –

Jacques Yves Cousteau, Oceanographer.



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