BobBlog 17 August

WEATHERGRAM
YOTREPS
Issued 17 August 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

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one number. It is
based on the standardised difference in the barometer readings between
Tahiti and Darwin.
It has been negative since July and has been hovering around -5 (Australian
units) for the past month - enough to be called a weak El Nino.

The Ocean: The warmer the sea the quicker it evaporates, tossing water
vapour into the air, where is rises and cools into cloud. The equatorial
Pacific region hosts the warmest sea on the planet. Thus its sea surface
temperatures SST may be thought of as a factor in the running of planetary
weather engine. An index for this is NINO3.4 and its abnormalities tend to
influence changes in clouds along the equator and thus tweak the latitude
zones of weather around the planet.
This parameter has been warmer than normal since March, but in the past week
has returned to normal. This mean that there WAS an oceanic signal towards
El Nino over past few months, and now its gone.
There WAS a lot of extra heat stored in the sea over the past few months.
At present this is presenting itself mainly as zones of warmer than normal
sea in the North Pacific  right in the middle of the North Pacific cyclone
season.

GLOBAL TROPICAL TOPICS
Cyclone KARINA is located between Mexico and Hawaii along with two other
tropical depressions. These are expected to travel west then north and
weaken, but another cyclone is expected to form in the area later this week.
Busy.
There is also a tropical depression in the North Atlantic. Its future is
uncertain at this stage but it may head towards Puerto Rico later this week.

Taken as a whole, the area between Hawaii and Mexico has shown strong
convection in the past week. Also convection has dropped over the Asian area
in the wake of last weeks cyclone HALONG in early August.
The weekly rain maps for the past fortnight show an enhancement in
convection between Hawaii and Mexico.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is in much the same position as last week, and starting to
intensify between Papua New Guinea and Vanuatu.
The north end of a passing trough is expected to travel east across central
and southern parts of French Polynesia on Wednesday UTC/local Tuesday.

STR= Sub-tropical Ridge
High over 1020 (not very intense) over North Island on Monday is expected to
travel east along 30/35S this week  no squash zone on its northern side.
High over 1025 (moderate intensity) over Tasmania on Wednesday is expected
to intensify and spread along 45S and onto southern NZ this weekend 23/24
August.

Departing westwards from Tahiti:
It may be a good idea to wait for that convergence zone to pass by on local
Tuesday. Also note that the next trough is likely to reach Niue area by
local Thursday, followed by W/SW winds for a few days

Between NZ to the tropics
An intense front is expected to cross North Island on local Tuesday night,
followed by NW winds on Wednesday, and a trough on Wednesday night. On
Thursday this trough is expected to deepen into a low east of the North
Island bringing a southerly gale to central NZ, probably not as cold/snowy
as last weeks was. It should be OK to depart from Northland to tropics on
Thursday and Friday, but not before.

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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