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

11 November 2018

Bob Blog 11 Nov



Compiled Sun 11 Nov 2018


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.



I once send a wedding telegram along the lines of “Beware: whatever promotes upward motion can lead to frontal development”. I was reminded of this last Thursday when the front crossing the South Island turned into a monster, dropping around 700mm of rain into Cropp River (Southern Alps , between Mt Cook and Hokitika). The torrential rain mainly fell over the Southern Alps. Rivers flooded, and  the main highways were closed by slips and a washed out bridge.


Meteorologist know that the most important equation they need to understand is the omega equation. Basically, if you tap your barometer and it dips the you know you have falling pressure, and this is linked to rising motion/increased wind/ heavier rain. The equation explains how: it relates omega (change of air pressure over time) with changes in vorticity advection and thermal advection (I won’t explain these in much detail tonight, except that CVA (cyclonic vorticity advection) and WA (warm advection) result in negative omega (upward motion/ thicker clouds), and AVA (anticyclonic vorticity advection)/CA (cold advection) result in positive omega (sinking motion/clearing sky).


The recent weather event over New Zealand shows what happens when a well-defined upper trough with a strong jetstream travels eastwards faster than the associated front near the surface.


1. On Wednesday night, the jetstream was west of the surface front

as seen in an Isobar map 1am Thursday courtesy of MetService with red arrow showing jetstream.


2. Then by 1am Friday the jetstream moved on top of the front.

This was the day the front dumped its heaviest rain. There was also a “tropical pipeline” or moist conveyor belt feeding the front with moisture from the heat trough over Australia. Our own version of the pineapple express.


3.  And  by 1am Saturday the upper trough buckled, so that the jetstream stopped advancing over the South Island, and the front travelled off to northeast of the jetstream and decayed to a trickle.


Between 1 and 2, the CVA and WA reached high levels over the front, turning it into a monster (being over the Southern Alps also helped promote its upward motion). Then between 2 and 3 CVA and WA dropped off and the front faded.


Our global weather models have the ability to watch all these atmospheric equations with precision and can handle the forecasting of these events well in advance. Interestingly, if we add an extra 0.5 C to the air temperatures and re-run the global model, we can end up with as much as an extra 10% in the resulting rainfall.


Note that the subtropical ridge over the South Pacific was sufficiently north of the North Island to allow this front to spread over all of NZ. This is typical of EL NINO and is likely to become a trend over the next few months (but not the coming week). One positive response to the incoming El Nino is the hydro lake levels in NZ. Lake Pukaki was dropping in past few months but is now rising nicely: see



Latest cyclone activity as seen at shows that it is busy at present in the Indian Ocean.

ALCIDE is expected to fade or divert to the north of Madagascar.

GAJA is forecast to travel across southern India.


Looking at the weekly rain maps, last week’s shows high intensity rainfall in the Indian Ocean, and an increase in activity over the Solomon island to Samoa/Niue. and a relaxing of convection in the Intertropical convergence zone across the Pacific.




SPCZ=South Pacific Convergence zone.

The SPCZ is active from Solomon Islands to Tuvalu and is expected to produce a trough that should visit Fiji on Wednesday, Tonga on Thursday, Niue on Friday UTC and then go off to the SE.


Subtropical ridge (STR)

Unlike last week, the STR for the coming week is expected to be around New Zealand until Thursday. Another High is expected to travel across the South Tasman Sea from Saturday 17 to Mon 19 Nov.


Tropics to New Zealand

There is expected to be a squash zone of enhanced SE winds between Fiji/Tonga and NZ especially near 25 to 30S from Tuesday to Thursday. Further south, it looks Ok for arriving in NZ over next ten days. Trough from the south Tasman Sea is expected to spread onto South Island on Thu and then fade over the North Island on Friday.


Between Tropics and Australia.

Looks mostly OK, but maybe a trough offshore Coffs on Friday/Saturday with southerly winds near the coast.


From Tahiti to Tonga

Trough over Tonga on Thu 15 UTC and Niue/ Southern Cooks on Friday 16 UTC then travelling off to the SE. North side of this trough may reach Tahiti around Sun 18 November. Trough is preceded by NW winds and followed by a period of W /SW winds.



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