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

11 June 2017

Bob Blog 11 June

Compiled Sun 11 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 this week's blog I'd like to let you in on a not so well-known trick with, the website that gives an interactive view of data from the weather GFS mode, the oceanic OSCAR, RTGSST, and WAVEWATCHIII models and the chemical GEOS-5, and CAMS models.

When the word "earth" is clicked on, a menu appears, the first line gives the chosen timestamp for the displayed map, and the fifth line is labelled Control and allows the user to change the timestamp.

So, to go forward an hour, click the > symbol (or the k key), or to go forward a day, click the >> symbol (or do a shift k)

Also, to go back an hour, click the < symbol (or the j key), or to go back a day, click the << symbol (or do a shift j)

The "not so well-known trick" is that the model data goes forward in time for around 33 days for wind data from the GFS model. This is interesting, but keep in mind that weather is mix of pattern and chaos, and a computer model extrapolates the numbers of a captured pattern, and ignores the chaos. The normal rule of computer modelling applies, namely that its mimicry of the real-world fades to below useful after around 5 days. Still, it's interesting to watch what features may pop up next month.

Going the opposite way, into the past, is very useful indeed. I recommend that those of you who have recently finished a voyage use this feature. Looks like you will have to do it one day at a time, but from what I can see the data available goes back beyond a month. You can use this site to review the ACTUAL weather during your voyage, and zoom in and see just where wind or waves came from that ruined your sleep at such-and-such a stage of the trip. My thanks to the crew of SV LEGACY who alerted me to this trick.

The Tropics
Tropical Cyclone 04W MERBOK is forming in the China Sea and heading north, and expected to make landfall as a gale to east of Hong Kong on Monday night local.

The last week's rain map from shows a well-established monsoon over India and a build-up of convection around Malaysia to Philippines, also across Papua New Guinea to Solomons. This seems to be related to an MJO that is moving east. However the parameters we follow seem to show this MJO is weakening during the coming week.

The ITCZ west of Mexico also intensified last week.

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 Thursday/Friday UTC then may go west again.

Subtropical ridge (STR)
High that is in the central Tasman Sea tonight is expected to travel to north of NZ and fade there on Tuesday.
New High is expected to form in central Tasman sea around Wednesday and then move slowly east across central NZ on Saturday UTC.

Australia to New Caledonia:
Not this week. Too much easterly wind on north side of the Tasman Highs. Also there is expected to be a trough off the Queensland coast from Monday night to Wednesday local.

Departing northern NZ to the north for the tropics:
Weather pattern is Ok for going north this week, but try and minimize time spent motoring across the light winds of the subtropical ridge.
Also avoid departing Tuesday/Wednesday due to strong winds/swell from a passing front.

New Zealand to the east (Tahiti)
On Wednesday, a deep low is expected to travel to northeast across the sea east of South Island, with Southerly storm and swell up to 11m offshore.
Wait for that to go first, may be Ok for a departure Thursday or Friday.

French Polynesia to the west:
The SPCZ is lingering as a band across the route.
IF you depart this week aiming for Niue/Tonga/Rarotonga, then will need to sail thru the SPCZ at some stage. It should be east of Niue from end of Sunday local date, and east of Palmerston from around end of Monday local date.
A voyage to Suwarrow should stay north of SPCZ in light to moderate East to NE winds.
See my website for more information
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