Bob Blog 24 June



Compiled Sun 24 June 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.

Cyclone Slowdown.

In Nature Magazine this month there was an article about a gradual slowdown in the speed of movement of tropical cyclones. James Kossin ( studied data from 1949 to 2016 and showed tropical steering fields have reduced by 10% globally and by as much as 15 to 20% regionally (as much as 30% over land in NW Pacific). The slower a cyclone moves the greater the amount of rain it can deliver to the places it visits. Indeed the peak rain rates of storms have been measured to have increased by 30$ over the past 60 years, see


As seen at

Marine Barometer.

I’ve been trying out a few smart apps on my mobile phone (iPhone 6S) to use its inbuild gps and motion/pressure sensors to measure air pressure (a barometer). Some can mimic a barograph, but need continual access to your location, and that drains the battery.

The one app I like the best, and I think will appeal to mariners, is the Marine Barometer (for iPhone or android) from Starpath (go to Apple store or Google Store and search for Marine Barometer). In particular it appeals to me for it has an averaging function over 14 seconds, so it’ll work OK on a yacht going up and down in a sea way (sea waves have a period of 7 or less seconds and swells have a period of 7 to 14 seconds). Also, it simply allows you to store your readings and thereby keep a log. See



All quiet at present, but there is a tropical depression off the Mexican west coast.

If we compare the past week’s rain map with the previous week we can see a general relaxation everywhere.



– see



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, sea (magenta). STR, and SPCZ. Pink area = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from northern Coral Sea /Solomon Islands to northern Vanuatu, and be weak over Fiji, and sporadic over Tonga. A passing convergence zone is expected from Niue area to south of Southern Cooks.


Accumulated rainfall for next week from

Subtropical ridge (STR)

A High is expected to travel east along 35S this week, entering Tasman Sea from New South Wales on Tuesday and Wednesday, and then onto central NZ on Thu/Fri/Sat, and then off to NE of NZ on Sunday/Monday. There is likely to be a squash zone of strong SE winds on the nort side of this high in the Coral Sea.

Around Tasman Sea, NZ to tropics /FP

TROUGH is expected to cross Northern NZ on Monday followed by strong SW flow on Tuesday with large swells to 25S. Conditions are looking OK for departure from northern NZ to the topics from Wednesday but may encounter strong SE winds north of 25S next week. 

Australia to tropics

Strong SE winds are forecast in the Coral sea this week, and there seem to be too many SE/E wind for a good trip form Brisbane to Noumea this week.

Tahiti to Tonga

A convergence zone is expected over Niue are on Tue UTC and Raro area on Wed UTC. Otherwise it looks like a good enough week—and be aware there is the possibility of a squash zone next week, so this week may be better.

Galapagos to Marquess

Se to E winds, mostly less than 20 knots. To use available currents, go to 5S 110W and then 5S 120W and then direct.


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