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

14 January 2018

Bob blog 14 Jan 2018



Compiled Sun 14 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.


Addendum to last week:

Regarding the details of the Niwa SEA LEVEL site--- the bottom graph compares the total SS in blue with the IB component in red, and last week it showed that the IB component made up most of the SS. In other words, the main component to the observed risen sea-level was the IB or inverse barometer effect, not the spring (or king) tide NOR the storm surge.  The incoming low may have brought an associated “dome of raised sea”, and as the Low travelled, so too did the dome, but the sea itself did not travel (just rose and fell as low went by).  The storm tide of piled-up and pushed-around sea water, and the king tide, provided lesser contributors.


In the past few days a HIGH has been crossing southern NZ with offshore winds across Foveaux Strait (Dog Island) and this produced a NEGATIVE storm surge (offshore wind)  and a negative Barometric effect lowering the sea level. The NIWA graph show that while these drops lowered the level of the sea (especially notable on the beach and over the sandbanks) to a level lower than expected by the normal “quoted tide”, they did NOT lower it to what it was at the spring tide earlier in the week.  As seen at


Weather trend over the last month.

Sea Surface temperature anomalies may be seen at

The SST anomaly image shows a typical LA NINA with cooler than normal seas straddling the eastern equatorial Pacific.  Also note the warmer-than-normal zone across the Tasman Sea – this can activate any Tasman Low to produce more than normal wind/rain.  We saw an example of this on Thu/Fri 4/5 Jan and may see another (lesser) example on Tue/Wed 16/17 Jan 

To see how the annual weather cycle and the seasons are working out, take a quick look at the average isobar maps from showing the Average isobars for past 30 days and their anomaly for DECEMBER.

Note the strong three-wave pattern in the northern hemisphere with the deepest lows preferring to form near Japan, eastern North America and western Europe— this is linked to the air aloft and its jetstreams.

During December the subtropical ridge over the South Pacific shrunk, and is now focused across NZ and to west of the Andes. The 1015hP isobar shifted has spread from the Southern Ocean to the southern South Island, and from near Brisbane/Noumea to near Norfolk Island. It looks like the subtropical ridge may have given NZ an early start to summer, and now that it is fading we can expect an early start to Autumn=mist/low clouds. 


The last 30 days of rainfall, and its anomaly, is seen at TRMM at

This shows that both the ITCZ and the SPCZ have an average position last month further AWAY FROM the equator than normal.  This is typical of La NIA. Interestingly in the Atlantic, the ITCZ is closer to the equator than normal—this shows that there is more happening than can be explained by just examining LA NINA.



TC JOYCE went inland to NW Australia on Friday and is now expected to get out to sea again by Tuesday and then travel south so that its rain visits SW Australia.

TC BERGUITTA is in the Indian Ocean to east of Madagascar nd expected to travel SW and then S , avoiding making landfall.


Looking at the weekly rain maps from last week and the week before, we can see that the main convective rain over e past week has been in the Indian Ocean and with TC JOYCE (before landfall). The cluster of extra convection in the Indian Ocean is expected to make its way eastwards and start appearing in the Coral Sea area around late January/early February.  This is called an MJO oscillation




SPCZ=South Pacific Convergence zone.

The SPCZ is hovering from Coral Sea to northern Vanuatu to Fiji/Tonga then to Southern Cooks, and is of weak to average intensity this week.  It is expected to become more active NEXT week, especially in the Coral Sea.

Tropical accumulated rainfall for next week may be seen at


Subtropical ridge (STR)

HIGH to southeast of NZ s expected to travel northeast to 40S 170W and then east.

Next High is expected to struggle and finally get into west-central Tasman after Thursday this week, and another into southern Tasman Sea (SE of Tasmania) by Sunday 21 Jan.  These Highs should travel east – to north and south of NZ- early next week.


Around Tasman Sea

Trough is slowly crossing.  Low in this trough to SE of Lord Howe on Monday is expected to deepen until Wednesday and travel SSE then escape its jet and weaken and travel E/NE to Northland on Saturday 20 Jan.   Worth avoiding mid-Tasman mid-week.  And its fronts are expected to cross NZ on Tuesday night/Wednesday (occlusion/avoid) and then on Fri/Sat (cold/ho-hum).


“It is better to be approximately right than precisely wrong.”

Warren Buffett, October 1994 (When he was richest person in the world).


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1 comment:

Leone Downeyar said...
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