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

03 December 2017

Bob Blog 3 Dec 2017b



Compiled Sun 3 December 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.


The November weather.

Sea Surface temperature anomalies may be seen at

The SST anomaly image is that of a typical LA NINA, with cooler than normal sea along the eastern Pacific equator linked to stronger than normal trade winds that spread further away from the equator, so that the subtropical ridge (in both hemispheres) is further than normal away from the equator.

There appears to be a warmer than normal zone between Tasmania and the South Island—it may take a few weeks, but this can activate troughs to produce more rain.


To see how the annual weather cycle and the seasons are working out, take a quick look at the average isobar maps from

During November the 1015hP isobar has spread from middle NZ to south of NZ 

—that’s a large jump for the subtropical ridge.

This is a strong regional change and, examining the details, there have been some similarly significant pattern changes in the northern hemisphere too. However, there hasn’t been as strong a change over South America or South Africa. So, the impact of this LA NINA has its peculiarities


Last 30 days of rainfall, and its anomaly, may be seen at

We can see one of the typical impacts of this La Nina by looking at the Intertropical convergence zone ITCZ on the monthly total rainfall total and its anomaly maps. This shows the latitude of the ITCZ has been knocked to the north across the Pacific, further away from the equator, and its normal latitude is now a yellow zone of dry anomalies. But this impact is regional,  and a reversal of it is seen in the Atlantic Ocean.



TC OCKHI is currently in the North Indian Ocean heading toward NW India. It is expected to weaken before it makes landfall around Wednesday. See

The South Pacific Cyclone season has opened with TD 01S DAHLIA off to the NW of Australia .

This is just a tropical depression (doesn’t have a ring of gales around its centre) and is likely to fade in a few days, but does get the season-count rolling.

Models are picking that there may be another tropical depression forming in the Coral Sea this week, but it should fade as it moves towards New Caledonia.

There are some bursts or equatorial westerly winds across the Timor Sea and to north of Papua New Guinea.


Looking at the weekly rain maps from last week and the week before,  at,   we can see an increase in convection in the equatorial Indian Ocean. This cluster of extra convection is expected to make its way eastwards and start appearing in the Coral Sea area around mid-December. This is called an MJO oscillation



SPCZ=South Pacific Convergence zone.

The SPCZ is hovering from Coral Sea to northern Vanuatu to Tonga to Southern cooks, with a reasonably clear gap over Fiji. Small lows are expected to form along 25South, to south of the SPCZ during mid-week. These are formed by a passing upper trough.


Subtropical ridge (STR)

HIGH H1 is expected to linger along 30 to 40S to east of NZ for the whole week.

Then another HIGH H2 is expected to cross Tasmania on Tuesday and southern NZ on Wednesday and then build to east of South Island from Thursday.


Between Tropics and NZ

A Low is expected to form between Fiji and NZ near 28S on Wednesday and fade away to the east on Thursday. This  feature is the  surface reflection of a passing upper trough  and may be reasonably easily avoided.

As H2 travels to east of NZ there should be a NE flow over northern NZ until Mon 11 Dec, providing a reasonable opportunity for getting to NZ.

After that the models differ wildly, so may be better to wait for a more solid favourable forecast.


Between tropics and Australia

Looks ok: The trough that is along and offshore the east coast of Australia is the one that flooded parts of Victoria over the weekend. It should fade and go off to the south by Thursday.


“Sailing is just the bottom line, like adding up the score in bridge.

My real interest is in the tremendous game of life.”

- Dennis Conner


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