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

04 May 2014

Bob's Blog issued 4 May 2014

WEATHERGRAM
YOTREPS
Issued 04 May 2014
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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

SOI. The Southern Oscillation Index SOI sums up the weather pattern over
the South Pacific as one number. It is based on the standardized difference
in the barometer readings between Tahiti and Darwin.
SOI (30 day running mean) was in the pink (below minus 10 in the
farmonlineweather.com graph for much of April but is now blue again.

TROPICAL TOPICS
May 1st marked the nominal end to the Southern Hemisphere cyclone season,
and the beginning of a period where cruising sailors in NZ examine the
weather maps closely searching for a good weather pattern for departure to
the tropics. In my blog last week I picked Wednesday 30 April as being the
best day last week for a departure (to Fiji/Tonga, not so good for getting
to New Caledonia or Vanuatu.

SO here are a few web sites to help examine the possible weather over the
next week or more. Possibly the most reliable weather model around at
present is the ECMWF. Especially in the tropics. This can be found at
http://bit.ly/1iM8kil

Weatheronline is the only website I have come across where I can quickly
compare the ECMWF with the world #2 model UKMO and also with the more
erratic GFS and several other models at http://bit.ly/1fHqGAt. This is a
good site to check when the GFS model (as seen at
http://www.metvuw.com/forecast/forecast.php?type=rain&region=swp&noofdays=7
)
is having one of its flights of fancies.

The PWG regional model (predictwind.com) starts with the GFS,
and the MetService regional model ( http://bit.ly/7daywx ) usually starts
with the UKMO.
A nice interface to loop the GFS ensemble model is offered by
Tropicaltidbits.com at http://bit.ly/1mrigQP

WEATHER ZONES
Panama to Galapagos: After 1 May the NE wind bursts that are good for this
voyage become further apart. Also the SW winds on the route are likely to
become more common. Tropical Lows are now likely to form on the northern
side of the ITCZ over the sea, but these are unlikely to affect
this passage. SO it may be best, if still in Panama or Las Perlas, to
motor off during the next few days. You are likely to experience light
winds for the first half of the voyage and then Southerly winds, which may
be used to sail the second half of the voyage to Galapagos.

Galapagos to Marquesas
There is a convergence zone within 100 miles of a line from 2N 97W to 2N
105W at present. If departing in next few days go SW to 4South and then go
direct to divert around these nasty squalls.

SPCZ= South Pacific Convergence Zone
This is about to spend a week or so more active to east of 180 than in the
west. It is active from Solomons to Tokelau and then more active in a line
stretching across the Tuamotu group.
If you are heading for Tahiti this week then you need to take this into
account.
Also note that there is a subtropical Low near 30S this week, then travel
from 170W 50 150W and then going SE. If you are on a voyage to Tahiti this
week the GO CLOCKWISE around this low.

STR= Sub-tropical Ridge
The STR has been weak over the Tasman Sea/NZ area last week and this is
likely to be the case this week as well. There is a HIGH in the Australian
Bight that is likely to cross Tasmania on Thursday and then follow the STR
across northern NZ early next week.

Departing from NZ to the tropics
A front is expected to cross Northland on Monday night followed by moderate
westerly flow on TUESDAY. This makes TUESDAY an OK day for departing off to
the north.
A squally trough should cross Northland on Wednesday and Thursday and then a
strong SW change on Friday, with swells from the Southern Ocean reaching as
far as 28S. Also on Friday a tropical trough is likely to travel across the
New Caledonia region, but this is expected to weaken as it travels further
east over the weekend. These features may bug a Tuesday departure but not
by much.

On SATURDAY there is an easing SW flow over Northland followed by that HIGH
crossing the Tasman Sea at around 1024 hPa intensity. This should offer a
good voyage off to the north as well. Still too far way to fully comment of
this voyage, but its start does look good at this stage.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe. Weathergram text only (and translator) is at
weathergram.blogspot.co.nz My website is at metbob.com - To unsubscribe,
send a reply email saying unsubscribe. Feedback to bob@metbob.com

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