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

13 July 2014

BobBlog issued 13 July

WEATHERGRAM
YOTREPS
Issued 13 July 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.

Scott Donaldson (http://doubleditch.co.nz/Tracker) considered conditions to
be getting too unsafe on Friday, after facing gale winds from E then NE then
N since last Monday, with 3 rolls and spending most of time on his side
early Friday and with forecast for another boost of wind to come with a
passing front on Friday evening and with communication battery
failing. SO he abandoned Double ditch after about 85 days at sea.

The Low that got stuck off northern NZ early last week had its normal exit
to the southeast blocked by a belligerent high. This blocking pattern
happens when the steering winds aloft weaken away over a high, as happened
in this case with the High to southeast of the South island.

There is a blocking index which measures this and its latest readings can
be found at http://www.cpc.ncep.noaa.
gov/products/precip/CWlink/blocking/real_time_sh/real_time_index_nrm.shtml
This shows there has marked blocking just east of NZ since the beginning of
last week.

All the wind and the lower than normal pressures in that system that stayed
near Northland for a few days generated a storm surge of around up to 0.4m,
but this happened at a time of neap tides, and that was a mitigating effect.
There is however now a perigean moon (the full moon and the perigee of the
moon are within a day of each other tonight) so if this event occurred in
the king tides over the coming few days rather than last week then its
impact would have been more severe.


TROPICAL TOPICS
Typhoon RAMMASUN is winding up and set to make landfall over Philippines on
local Wednesday 16 July. Also, the weekly rain maps for the past fortnight
show heaps of convection over SE Asia weather and along the ITCZ, and not
much at all in the South Pacific.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
A weak convergence zone is expected to stay put over Solomons towards
Tuvalu.

STR= Sub-tropical Ridge
The STR is behaving normally over Australia and in the eastern South
pacific, but is weak to-non-existent across the Tasman Sea and South
Pacific area. In this area the highs centres are found to be south of 45S
and slow-moving. One is expected to travel from SE Australia across the
south Tasman Sea on Tuesday and Wednesday and then sneak around the south
coast of NZ.

Departing from NZ to the tropics
Low is expected to form in the cold air belonging to a cold front in the
Tasman Sea tonight and then move northeast across northern NZ and deepen on
Monday (a sign that it has SW winds aloft, so is to west of the upper-
trough-apex.
This Low is expected to take a clockwise path thru 30S 180 and then to the
ESE. Anyone departing Northland on Monday should be able to keep well enough
away from is centre and take a good enough ride northwards with the
southerly winds in the wake of this low for a few days.
On Thursday and Friday another dose of cold air and SW winds is expected to
flood into the Tasman Sea from the southern ocean. A similar process should
recur, allowing a low to form in this cold air, this time it is likely to
form over Saturday and Sunday 19/20 July around the North Island. Westerly
winds are likely to be found as far north at 20S, and there are likely to be
strong to gale west to SW winds as far north at 25S along with large swells.
Avoid.

Departing westwards from Tahiti:
This should be a good week for such a voyageno troughs are expected.
However it will be a good idea to deviate to north of the direct path ,
because the contrary westerly winds may stretch as far as 20S at times and
the trade winds may retreat to north of 15S.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
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