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

04 February 2018

Bob Blog 4 Feb 2018



Compiled Sun 4 Feb 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.


Weather trend over the last month.

Sea Surface temperature anomalies may be seen at

The SST anomaly image still shows cooler than normal seas along the equatorial east pacific, but these are less than last month. Maybe indicating this La Niña period has peaked as far as the ocean is concerned.

The Tasman sea warm zone has been there since November and is affecting the local weather, supporting warmer than normal air temperature, and feeding moist air to the low-pressure systems crossing the Tasman Sea.


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


There was a strong three-wave pattern in the northern hemisphere last month with lows preferring to form near Japan, eastern North America and western Europe— this is no longer the case with a four or five wave pattern, things are more changeable.  In the southern Hemisphere only the zone from Indian Ocean, cross Australia, to the Tasman Sea area has been favourable for low development.


The subtropical ridge over the South Pacific is still in place but weaker than last month.  The 1015hP (blue) isobar has shifted to east of New Zealand, so that the prevailing winds over the North Island are from the north. As long as this continues, air temperatures over New Zealand are likely to remain above normal.


The last 30 days of rainfall, and its anomaly, as seen at TRMM at, shows that, in the Pacific, both the ITCZ and the SPCZ have an average position last month further AWAY FROM the equator than normal. This is typical of La NINA. 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 FEHI travelled across the Tasman sea and its remains reached NZ area last Thursday on 1st February bringing wind and rain to many areas. It arrived during the spring tide after a near-perigean full moon, the second full moon in January (thus a ‘blue’ moon) and, coincidently, the day after a lunar eclipse (‘blood’ moon). The inverse barometric effect from the lower that normal pressures, and the storm surge from strong northerly winds can be seen at the NIWA tide gauge at Porirua (around 12 n miles NW of Wellington) at

This storm surge brought some damage to Nelson wharf area, especially the Boat Shed Café, see


The central part of FEHI took a while to unravel. It was still intact when it visited Auckland on Friday bringing a few hours of squally downpours. This can be seem on Himawari satellite imagery at or MetService radar imagery at .


There is an MJO period of expected extra convection traveling across the Pacific Ocean this week and maybe next week, raising the potential for Tropical Low development to HIGH.  There is also in place a zone of near equatorial westerly winds and four incipient lows, any of these may develop into a tropical cyclone over the next few days. Once formed, these systems will likely travel south. Near-equatorial westerly winds can be seen on , and this is another herald for cyclones.


In the India Ocean TC CEBILE is still traveling off to the south and fading.


Looking at the weekly rain maps from last week and the week before, we can see that the main convective rain over the past week has shifted from the equatorial Indian Ocean to the northern Australia and Coral Sea region. See



SPCZ=South Pacific Convergence zone.

The SPCZ is active across the whole region mainly from Solomons to Northern Vanuatu to Fiji to Samoa to Cooks and to parts of French Polynesia.  Any of the tropical lows currently on the SPCZ may develop to a cyclone this week. The models seem to prefer the low near Fiji.


Subtropical ridge (STR)

HIGH travelling east of Tasmania on Monday is expected to travel east across the south Tasman Sea on Tuesday and Wednesday, across southern NZ on Thursday and then further east to east of NZ along around 35 to 40S.


Around Tasman Sea

East to SE winds at first this week north of 30S, but a trough may approach from the north after Wednesday turning winds E then NE near New Zealand.


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