BobBlog issued 29 June 2014

WEATHERGRAM
YOTREPS
Issued 29 June 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.

Kiwis can brace for a real bit of winter on Wednesday and Thursday this
week. This isn't going to be southerly eggbeater but is expected to be a
polar outbreak, good for snow for the ski-fields, just in time for the
school term 2-week break from 4 July.
The polar vortex is the name given to that westerly spin of wind around
Antarctica, holding the cold in the south. Sometimes the vortex weakens and
that allows high cells to snap free from the ice shelf and travel to the
northeast, dislodging ice-chilled air and shovelling it northwardsthats
what I mean by a polar outbreak, NIWA use a parameter called the Southern
Annular Mode, or SAM, to measure the strength of the polar vortex. When this
dives suddenly negative the chances of a polar outbreak increase. Sometimes,
such as this week, the polar outbreak gets into the Tasman Sea. I don't have
any real time access to the SAM index, but a good proxy for it is the AAO or
Antarctic Oscillation, and its expected future trend is shown at

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.a
ao_index_ensm.html
and this shows a sudden dive into the negative on 1 to 3 July, but not for
long.
SO by noon Wednesday most of the inland South Island is likely be have air
temperatures below zero, and in many places be wet enough to snow


TROPICAL TOPICS
There is a lot of convective activity this week off the west coast of
Mexico.
And there is still less than normal convective activity over India, Reports
are that the monsoon is over 40% deficient over much of central India.
All of this shift of convection is consistent with an observed warming of
the sea in the eastern equatorial Pacific Ocean as measured by the NINO3
index
NINO3.4 index measures the difference from normal of sea temperatures in the
EEP (eastern equatorial Ocean) and this has been on the rise in recent
weeks, but still hasn't swung the SOI index (in the atmosphere) towards El
Nino.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
This continues to intensify in the Coral Sea to Tokelau area and is expected
to drift south towards Vanuatu /Futuna and maybe New Caledonia on 5/6 July.
A weak convergence zone is likely to linger over the west end of French
Polynesia during Monday and Tuesday UTC.

STR= Sub-tropical Ridge
The STR is in its normal latitude position of 30 to 40S this week. One High
cell that was over northern NZ on Sunday is expected to travel east along
30S this week. The next high leaving Australia on Thursday is likely to be
weaker than its predecessor and diverted to 30S to north of NZ by Saturday 5
July.

Departing from NZ to the tropics
Large trough is crossing Tasman Sea and NZ over next few days, the first
front is expected to cross NZ on Monday, preceded by N /NE flow, and
followed by squally NW/W winds. Then the cold SW/S winds from the ice at 60S
are expected to spread across NZ on Wednesday and Thursday.
SO Friday looks to be the best weather pattern for departing Northland this
week.

Departing westwards from Tahiti:
There is a lingering weak convection zone around Papeete on Monday/Tuesday
UTC and another is expected around Sat/Sun 5/6 UTC and these zones weaken
the trades and may provide a few tropical squalls, Between these the weather
pattern is looking good to go.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
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