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

27 December 2015

Bob Blog 27 Dec 2015



Issued 27 December 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.


Tropical Cyclones

There is a MJO (Madden Julian Oscillation) episode travelling into the Pacific Ocean – this increases the activity in the South Pacific Convergence Zone SPCZ and thus the chance of tropical cyclone formation during the next week or so.


See and read at this web site  more about the MJO—the main oscillation worth watching during the cyclone season, and note how strong and steady the parameter is at present in its march around its phase diagram.


Also, there are some equatorial westerly winds over Kiribati, not good for the local fishing fleet as these winds blow into the harbours that have been built for shelter from the normal SE wind. When these westerly winds blow steadily they increase the amount of spin available on the SPCZ.


Interestingly, there is a burst of stronger than normal NE trade winds in the northern Pacific: so this means there is not much of a gap between winds from opposing directions near the equator. In fact the computer models are analysing this as a cyclone near the equator at 177E --- I can't remember seeing such a weather ever before,  and because of the division by sine(latitude) in the wind equation there is not expected to be any curvature in the flow at the equator.


Check the MSL analysis as shown on showing near-equatorial westerly winds over Kiribati and strong NE trade winds in the northern hemisphere, meeting in and twirling around a cyclone near the equator.


There are NO tropical cyclones around at present, however this chance of cyclogenesis is HIGH around the Pacific this week and there are two areas of interest at present---


One is 91P near 12.6°S 176.8°W and it has 7 analogues that went all over the place (go to  for the images).


Another, 92P, is closer to the Coral Sea and only has 4 analogues, and one of those went round the Coral Sea in a wide circle.


A third is further west near the active monsoonal trough over Australia near 16.0°S 134.6°E—this system is near Gulf of Carpentaria and had an opportunity to turn into a cyclone during the last week, but is still smouldering away. It has 5 analogues.


And there is another possible spot further west to the NW of Australia at 12.8°S 92.2°E


AT present there is no watch being kept on that pseudo-low near EQ 177E, however the GFS model indicates that this system may deepen into gales near Majuro by 31 Dec—take care.


SO this coming week may get busy in the tropics. A good period to stay put "on holiday" and see in the New Year.


Here for your singing enjoyment in Thursday night this week are the words of one of the lesser known verses of "Olde Lang Syne"


As written by Robbie Burns:


We twa hae paidl'd in the burn,

frae morning sun till dine;

But seas between us braid hae roar'd

sin' auld lang syne.




We two have paddled in the stream,

from morning sun till dine†;

But seas between us broad have roared

since auld lang syne.


With this goes my greetings to you for the transition from 2015 to 2016.


Weekly rain maps for the tropics over the past two weeks,  as seen at,

show intense rain last week over northern Australia (from 99P) and eastern USA.



SPCZ=South Pacific Convergence zone.

The SPCZ is expected to increase in activity from Solomons to Samoa/Tonga.

There is an increasing chance of cyclogenesis this and next week. Take care.


STR= Sub-tropical Ridge

The "Sydney-Hobart-2015-race-trough" roared thru as expected last night, and although not really a sustained gale it managed to cause enough damage to take out several of the yachts including WILD OATS. It is being followed quickly by a HIGH that is expected to  travel slowly across central Tasman Sea this week and glide across central NZ from Thursday 31 Dec to Sat 2 Jan, with a zone of enhanced easterly winds on its northern side affecting northern NZ.


For NZ and Tasman Sea

A deep low with pressure below 990hPa near 40S 170W on Monday has come from the tropics and is moving slowly south. It is blocking a HIGH over NZ for Monday and part of Tuesday, and is expected to delay and weaken the eastwards progression of the "Sydney-Hobart 2015 race trough" (which is now in the central Tasman Sea forming a secondary low). The remains of this trough are now expected to cross southern and central NZ late Tuesday and during Wednesday.



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