Bob Blog 18 June



Compiled Sun 18 June 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.

In last week’s blog I mentioned a tip about, that it can be used like a time-machine by using the >> symbol (or do a shift k) to go forward a day, or, to go back a day, click the << symbol (or do a shift j)


Thanks once more to the crew of SV Legacy, we can extend this tip; here’s a quicker way to dial up the date we wish to visit:

If you click one of the date arrows in the control menu to change the date or time, the URL will show a date like this…


Then you can just CHANGE THAT DATE IN THE URL to suit yourself and hit enter. It can be a little confusing as the date in the URL is UTC and the default display in the “earth” menu is local.


There is data stored at the earth.nullschool website from 1 November 2013.

The year is turning;

Solstice is at 0424UTC on Wednesday June 21st. Enjoy.

For those of you who are celestial navigators, little d is zero at the solstice. Yippee.


The Tropics

The monsoon has slowed down to a crawl over the northwest end of India/Pakistan :


Monsoon progress map from

Tropical Cyclone MERBOK gave Hong Kong a pounding last Tuesday night (local) cancelling 41 flights and delaying more than 300.


Tonight, there are some tropical depressions off the east coast of Central America, and the east coast of South America (north of the equator). These one near Belize is expected to travel north then northeast, and the one near 5N40W is expected to travel off to the WNW towards Honduras.


The last week’s rain map shows a stalled monsoon over India and a build-up of convection around Malaysia/ Indonesia to Papua New Guinea. The track of MERBOK can be seen. The intense rain that visited Tuvalu /Tokelau two weeks ago has faded in the past week. And there is a wet low forming in the Tasman sea.



Rain for the past fortnight from


Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, lightning, Sig wave height purple lines, SPCZ and STR.


SPCZ=South Pacific Convergence zone.

The SPCZ stretches as an almost continuous band from Papua New Guinea/Solomons to around northern Vanuatu/Wallis/Futuna and then between Southern Cooks and Society Islands. It is expected to travel somewhat to north and east this week, visiting Samoa on Saturday/Sunday UTC then may go south again.

A trough is expected to cross the Austral Islands around Thursday 22 June UTC and then form a low to the southeast of French Polynesia that should travel off to the southeast and deepen. Associated front is expected to cross the Society Islands on Thursday UTC and the Tuamotu Islands on Friday UTC.


Tropical accumulated rainfall for next week from

Note how the dry/wet zones at 30South are different from those at 50south—this can be forecast by looking at Rossby wave amplitude and wavelengths/wave number. However, I haven’t found a publicly accessible website with this data for you yet, and will keep looking.

Subtropical ridge (STR)

High that is departing NZ on Monday is expected to travel east along 40s to around 160w and then merge with a 1040 hPa high developing in cold air near 50s 150w for a day or so.

Next high is forming over SE Australia on Monday Tuesday, and may have its easterly progress blocked by that Tasman Low and so is expected to travel slowly north and reach the North Tasman sea by the weekend.

Australia to New Caledonia:

There was a good-enough looking opportunity over the weekend for this voyage, riding in the southerly winds on back end of the Tasman Low.

A departure on Monday may encounter strong southerly winds and rough seas for starters and then have a reasonable voyage. A Tuesday or later departure may encounter easterly head winds this weekend.

Departing northern NZ to the north for the tropics:

Not this week. Low has formed in the Tasman Sea and is expected to travel across NZ on Tuesday to Saturday, after that the prospects are ok for good enough weather for departure from next Sunday 25 June.

New Zealand to the east (Tahiti)

A departure on Monday (depending on where you are) may be in moderate northeast winds or light winds—and may be a possibility. After Monday, the incoming low is expected to bring increasing NE winds and rain, so may as well stay-put for the next opportunity from Sunday 25 June.

French Polynesia to the west:

There is a squash zone of enhanced trade winds from now until around Wednesday 21 UTC between Rarotonga and Niue, as a large HIGH travels east along 40s.

A trough is forecast to be crossing the Austral and Society islands on Thursday UTC and then the Tuamotu Islands on Friday UTC, preceded by weakening NE winds, accompanied by showers, and followed by moderate to fresh south to southeast winds. Stay put during the trough, and jump a ride on those southerlies once the swell is ok.

If you would like more detail for your voyage, then check to see what I offer.


See my website for more information

Feedback to or txt 6427 7762212

Weathergram text only (and translator) is at

Weathergram with graphics is at,

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