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

21 December 2014

Bob Blog

WEATHERGRAM
YOTREPS
Issued 21 December 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

The Ocean: extra heat has been stored in the Pacific Ocean for a while now
and all the Oceanic indices have crossed the threshold into El Nino
territory.

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 standardized difference in the barometer readings between
Tahiti and Darwin. It has been negative since July and dived below -10
(Australian units) for much of September, and again for a week in November,
but has since been relaxing again--- the atmospheric El Nino is stuttering

In an El Nino event the latitudes of the normal weather zones are drawn
closer to the equator, causing the South Pacific Convergence zone to shift
further north and east of its normal position.

TROPICAL TOPICS
Weekly rain maps over the past fortnight show an increase in convection over
the past week east of 180 and another increase to NE of Madagascar in the
western South Indian Ocean.
A MJO (Madden Julian Oscillation of enhanced convection) is now fading near
and just east of 180 after it made its way eastwards across the Coral Sea
last week.
This has allowed a zone of near equatorial westerly winds to extend along
5 to 15S as far east as Samoa. These near equatorial westerly wind tend to
upset the wets facing harbours in Tuvalu and Tokelau.
These west or NW winds also accentuate the convection in the South Pacific
Convergence zone, and help tropical depressions to form.
One is forming now east of Samoa and north of Niue:
It has strong NW winds on its NE side and strong SE winds on its SW side The
latest thinking is that this system may soon move SE to the area between
Rarotonga and Tahiti by Christmas Day:

Tropical Disturbance Summary For area Equator to 25S, 160E to 120W ISSUED
FROM RSMC NADI Dec 20/2347 UTC 2014 UTC.
TROPICAL DISTURBANCE 03F CENTER {[1004HPA]} ANALYSED NEAR 13.2S 170.0W AT
202100UTC. TD03F SLOW MOVING. POSITION POOR BASED ON MTSAT VIS/IR IMAGERY
AND PERIPHERAL OBSERVATIONS. SST AROUND 30 DEGRESS CELCIUS.
ORGANISATION IS POOR. CONVECTION IS PERSISTANT IN THE NORTHERN SEMICIRCLE
AND DETACHED FROM SUPPOSED LLCC IN THE PAST 06 HOURS. THE SYSTEM LIES IN A
DIFFULENT REGION, EAST OF A SHORTWAVE UPPER TROUGH IN A LOW SHEARED
ENVIRONMENT. CYCLONIC CIRCULATION EXTENDS UPTO 500HPA.
GLOBAL MODELS HAVE PICKED THE SYSTEM AND MOVE IT EAST-SOUTHEASTWARD WITH
LITTLE INTENSIFICATION.
POTENTION FOR TD03F TO DEVELOP INTO A TROPICAL CYCLONE IN THE NEXT 24 TO 48
HOURS IS LOW.
NO OTHER SIGNIFICANT TROPICAL DISTURBANCE ANALYSED OR FORECAST IN THE AREA.
Fiji Met Service tropical disturbance discussion above is as at
http://www.met.gov.fj/aifs_prods/20036.txt
If you are in this area then please remain vigilant and keep up to date with
these latest TC bulletins.

MJO cycles are usually paced about 6 weeks apart and are known to increase
the risk of tropical cyclone formation as they pass. This MJO is this week
bothering places east of 180. My take on this is that for the next few weeks
the Southwest pacific will be between MJO events and the risk of enhanced
convection/tropical disturbances will drop to LESS THAN NORMAL by New Year
and stay that way for a few weeks. Thats good news.

WEATHER ZONES
Note that there are expected to be STRONG NE winds near and north of the
equator, making a E swell there over 2.5 significant metres.

SPCZ=South Pacific Convergence zone.
SPCZ is strong east of the dateline with strong NW winds on its northern
side, strong SE winds on its southern side and gale force squalls in-
between. Avoid.

STR= Sub-tropical Ridge
Finally, after several weeks, a High is expected to get east of central NZ
on Monday. This HIGH should expand to over 1030 hPa and move slowly NE
across Chatham Islands on Tuesday and NE to 35S 165W by Sun 28 Dec.

Over NZ
Next main trough is expected to reach SW of South Island on Boxing Day and
spread NE (and weaken) to northern NZ by Sat/Sun 3/4 January.
Nice Holiday weather over northern NZ (albeit NE winds from 23 to 30 Dec),
but try and avoid sailing during the weekend of 3 and 4 Jan.

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
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