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

07 January 2018

BobBlog 7 Jan 2018



Compiled Sun 07 January 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.


That low over northern/central NZ last Thursday night to Saturday brought lots of wind damage and some sea-inundation damage. Indeed worth avoiding.

Yes, it did deepen quickly on Thursday, but not quite steeply enough to satisfy the definition of a meteorological bomb. Its central pressure dropped from 1000 to 981 in 24 hours at average latitude 36S, that’s 19 hPa, or 23 hPa when corrected to reference latitude of 45S. SO,it reached a max of 0.95Bergernon, not quite 1B.


LOW was 1000hpa at around 33S on Thursday 1am local, then LOW was 981hpa at 1am Friday near 37S.


The term “bomb cyclone” has been given plenty of mention in the past week because of the intense low over eastern north America directing cold air for the chilled Arctic. A good write up of how the original term was coined is at

The lower than normal air-pressure around northern NZ last Friday lifted the sea level (inverse barometric or IB) at the rate of 1cm for every hPa below the norm of 1012 —so for 995 hPa that’s a lift of 170mm for Whitianga on Friday. The raw tides were more extreme than normal last Thursday/Friday because the full moon occurred close to perigee. The stronger NE winds completed the tri-factor and brought a large storm surge of around 200mm in on top of a larger than normal high tide and the inverse barometer effect.

Adding it all together, there was a 350mm rise to the sea level measured at the Whitianga tide gauge last Friday as seen at the NIWA plot at On their plot the full moon (on Tue 2 Jan) is shown as a yellow circle and the time of perigee is shown as a purple circle.


It should be pointed out that the low wasn’t of tropical origin and went thru its deepening process in the 30 to 35S latitude band. Basically, the surface low managed to have an upper trough move over it so that it found itself between two jetstreams—on the polar side of a jet exit and on the equatorial side of a jet entrance. In such a zone the surface air rises and is carried away by the jetstream. In this case the air was blown off faster that it could rise, and so, as air was removed from the low, its central pressure dropped “explosively” the jetstreams got energized by the release of heat as the clouds around the low turned to rain. This is a classical mid-latitude low—NOT a tropical cyclone. Once in a while (maybe as often as once per month in winter) we see a low develop like this in the Tasman Sea, but few are as serendipitous in the mixing as this one, and we haven’t seen one like this for several months.


Let’s hope the next one is after summer.



The Tropics:

Looking quiet in the South Pacific for the next week or two.

There are two tropical cyclones in the South Indian Ocean at present: IRVING near 15S 85E is travelling SW and expected to turn off to the south before affecting La Reunion.

And TC AVA, as seen at, is further west and fading as it travels south along the east coast of Madagascar


Looking at the weekly rain maps from last week and the week before, as at, we can see pockets of intense convection over Indian Ocean and northern Australia, and an easing of activity in the Coral sea to Fiji area.



SPCZ=South Pacific Convergence zone.

The SPCZ is still active between Samoa and Southern cooks, and rather weak but building over the Coral Sea and Vanuatu/Fiji/


Subtropical ridge (STR)

HIGH1 is expected to travel east from Tasman Sea across northern NZ on Wednesday and Thursday and then off to the east of NZ along around 30 to 35S.

The next high , HIGH2, is expected to spread east across Tasmania on Wednesday and across the South Tasman sea on Thursday and then build to northeast of NZ from Friday, moving along 40S from the weekend into next week.


Low crossing central Tasman Sea.

Trough is forming across the central Tasman Sea this week.

A low should form in the trough around mid-Tasman by Wednesday and then travel southeast towards the South Island on Tuesday, but is likely to stall west of the southern alps and fade away on Friday.Trough should remain in central Tasman Sea and perhaps brew another low next week.


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1 comment:

maha mohamed said...
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