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

09 April 2017

Bob's Blog 9 April 2017

WEATHERGRAM
YOTREPS
Compiled Sun 09 April 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.

TC COOK
There are five features of interest around Australia at present: TC ERNIE to NW of Australia, TC COOK neat Port Vila, TD14(19F) near Niue, a tropical depression near Darwin, and another tropical depression just north of the equator near Micronesia.
The latest satellite imagery (as seen on Google Earth) shows heaps of convective activity around northern Australia and extending to Vanuatu. And provisional track maps from ruc.noaa.gov/tracks/ show a possibly interesting week ahead of us.

The MJO is weak at present, so is not adding to this convection.

TC COOK is, at this stage, worthy of comparison with TC BOLA (a very wet and slow-moving cyclone off eastern NZ in early March 1988), for three possible similarities.
1) A close analysis of BOLA showed it made two loops around Vanuatu, making a slow-moving feature.
TC COOK is in a slow-moving environment when over Vanuatu so it may also do some loops.

2) BOLA's path off to the southeast was blocked by a large HIGH located to east of New Zealand, so that it was knocked slowly to south and southwest lingering near northern NZ. TC COOK may also be blocked by a HIGH and knocked southwest when it becomes extra-tropical and loses its tropical characteristics.

3) BOLA had a tropical depression accomplice to its east that went south over the Niue area as BOLA came south. This helped form converging "rivers of moist air" that hit the Gisborne hills in 1988. TC COOK has a similar accomplice, and the converging rivers of moist air, according to today's data, may be in eastern Bay of Plenty on Friday 14 April. Mind you, there is still plenty of time for this scenario to change, but its worth watching.

Rain from last week, compared with previous week, as seen at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif, shows increasing intensity with the tropical lows around ERNIE, north of Darwin, about Vanuatu and from Samoa to Niue. The track of the remains of DEBBIE show up across central New Zealand. The ITCZ is slowly returning to normal and the convergence zone between Tuvalu and Galapagos is still there, but now with gaps. Equatorial America rains seem to be easing.


WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is breaking into two this week with its southern half travelling south along e with TC COOK and TD 19F. The northern and NW parts of the convergence zone should stay near 5 to 10S and in the western Coral Sea. Not a good week to go sailing around South Pacific.

Subtropical ridge (STR)
HIGH to east of the South Island is expected to expand over 1030hPa as it travels slowly east along 40S from Monday to Wednesday. The High that is expected to slide form Aussie Bight to southeast over Tasmania by Tuesday is then expected to squeeze around southern NZ by Thursday and intensify as it travels northeast to east of New Zealand on Friday. Another High is expected to do this on Sunday/ Monday.
SO, Highs are dominating eastern NZ this week and mostly blocking Lows from the Tasman, but there is a gap to let lows thru on Friday/Saturday.

Tasman Sea/ NZ troughs.
The thundery front and low crossing New South wales tonight is expected to form a Low in Tasman Sea on Monday and this low will have to wait until Friday to cross over Southern NZ.
It may leave behind a low in the Tasman Sea that may fade there on Monday.
SO, this isn't really a good week for trans-Tasman travel.

Mexico to Marquesas:
Should be a good week to consider departure. No Tehuantepec NE blast of wind expected this week. And a moderate northerly is likely off shore most days this week.
As for waypoints: may need to throw one in to even-out possible direct downwind sailing. Useful NE trade winds to around 8N and then light winds to around 6S. ITCZ is near 10 to 5N, may be an idea to find a gap or go across this directly north to south to minimize squalls.
And there is still another convergence zone near 5S, but that may ease over next few weeks. South of that zone there should be useful easterly trade winds for sailing to Marquesas.

Panama to Galapagos
There should be some useful northerly winds for departure from Panama all this week and over Easter, then the prospect is for light winds. Main convection is likely between 6N and 3 to 2N.
May be some good tail currents and it is worth diverting to 5N 82W for these.

Galapagos to Marquesas
The currents seem to have reverted to a more normal west going set now. However, there are expected to be light winds as far as 5S and a convergence zone of squalls between 2 S and 8S worth avoiding by going south of the rhumb line. Maybe go to waypoints 5S 95W then 9S 123W then direct.
The pattern is changing and may well be different in a week or so, so get updates, maybe be able to cut corners next week.

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