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

07 August 2016

Bob Blog 7 August 2016

WEATHERGRAM
YOTREPS
Issued 7 August 2016

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.
Weather patterns for the past month:

Averaged weather maps for July may be seen at
www.esrl.noaa.gov/psd/map/images/fnl/slp_30.fnl.anim.html
and pressure anomaly map for last month from
www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

These maps show that during July the weather pattern over the South Pacific
changed from a HIGH to NE of NZ to a stronger than normal disturbed westerly
flow. And there are a pair of amplified HIGHS either side of North America,
aiding bushfires.
And the Indian Ocean weather patterns are completely different from those in the
South Pacific.

The Tropics
TC HOWARD was named on Monday, peaked of Tuesday and its remnants got close to
Hawaii by Sunday (local).
see
www.staradvertiser.com/breaking-news/rain-expected-as-tropical-storm-howard-move
s-closer-to-hawaii/

TC IVETTE has now been named today near 137W and may affect Hawaii area from
local Thursday, but, like HOWARD, is expected to weaken as it approaches.
There is another tropical low near the Mexico coast approaching Baja California.

In the NW Pacific, OMAIS is aiming to travel north along the East coast of
Japan.
And another system is likely to form near 20N 160E on Monday.

The rain maps for the past weeks from
trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif shows a build-up
of convective activity over NW Pacific, and a drop in activity in the South
Pacific.


WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is weak this week and expected to remain draped over North Coral Sea and
from Tuvalu to Tokelau to Northern Cooks. There is a passing trough tonight over
Fiji (bringing rain, welcomed by some) and this should travel off to the
southeast by mid-week.

STR= Sub-tropical Ridge
Next HIGH is travelling along 50S on Mon/Tuesday towards South island on
Wednesday and then to east northeast of NZ crossing Chatham Islands on Thursday.
Since this week's HIGH is coming from so far south it has brought chilled air to
the South Island - at around the time of the year we get our coldest
temperatures.
The following HIGH next week is expected to take a more regular course along
35S.

Voyage Outlooks:

Tahiti to the west
Maybe some showers around Tahiti on local Sunday/Monday, then looks to be more
settled for the remainder of the week for departure. Wind flow is looking good
for getting west this week, but a passing trough is likely to being variable
winds south of 20S especially after Wednesday.

Between NZ and the tropics
Meteorologists has a phrase to describe this pattern, one that seems to pop up
mid-winter --- "LOW INDEX" the means that the zonal index, as measured by the
baro pressure at Auckland minus that an Invercargill (usually a positive number)
is taking on negative values, with Highs over the South Island and lows over the
North Island.

This changes the strategy for departing from northern NZ. The best looking
opportunity is today or Monday. Monday is a day of light variable winds
in-between the Low that moved off northern NZ last night and another low that is
expected to do so on Tuesday/Wednesday. So a departure on Monday takes a voyage
deliberately across the centre of a low-where the winds are light and variable,
but somewhat unsteady, and where there may be rain clouds and perhaps a squall.

If you wait until Tuesday or later then the outlook is for onshore winds and
swell-not such a good idea for departure.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
See my yotpak at boatbooks.co.nz/weather.html for terms used.
See my website www.metbob.com for information on tailor-made voyage forecasts-
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