BOBGRAM issued 1 Sept 2013

WEATHERGRAM
YOTREPS
Issued 01 Sep 2013
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.

1st September=start of southern Spring? Not necessarily so: the timing of the seasons is done by various conventions – there is no "official" definition.
Some people, such as climatologists who like to follow monthly means, use 1 Sep as the start of spring—very practical. Others use the four 'corners' of the earth's orbit to mark that start of the seasons, and wait for the equinox on 22 September—very logical. Others again use signs from the garden such as daffodils and can thus compare one year with another ---very interesting. I tend to be one of the logical ones.

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 maintained a value over 1 for most of June, hinting at a La Nina. Since then it has relaxed to around -0.07 (1 Sep).

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 near average so far this year, slightly on the cool side.

GLOBAL TROPICAL TOPICS
The Baja California coast remains busy with cyclones, as KIKO reaches Hurricane force off there and travels northwards. The NW Pacific is quietening down after a few busy weeks. The Atlantic has got through half its cyclone season with NO HURRICANE yet (first such year since 2001), but there are two possible areas of formation there at present.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ currently extends from Solomons to Fiji/Tonga and has increased in intensity during the last week. It is expected to stay in much the same position during this week, but to weaken late in the week. A tropical trough is expected to move southeast along the convergence zone and cross Fiji on Tuesday and Tonga on Wednesday. This trough is expected to weaken in the tropics and continue eastwards, getting to Tahiti by Sunday, and is likely to be followed by a W/SW flow to south of 20S. There are also a few minor convergence zones near Suwarrow and Northern Cooks areas .

Sub-tropical Ridge STR
High over central NZ is expected to fade away on Monday, and High in Tasman Sea is then expected to expand so that the squash zone of enhanced SE trade wind on the north side of this High over Coral Sea and New Caledonia is expected to reach its peak by Wednesday. Also on Wednesday a High centre is expected to break free from Antarctica in the area south of NZ, this should allow the High in the Tasman Sea to extend south, and these two highs may combine to shovel a chilled southerly flow onto NZ. That's a MID WEEK POLAR OUTBREAK.
Later in the week the Tasman High should weaken and shift to the north Tasman Sea.

Roaring 40s and New Zealand
By Thursday, the trough that accompanies the mid-week polar outbreak over NZ is expected to combine with that tropical trough crossing Tonga on Wednesday so that they form a deepening complex are of low pressure east of the North Island on Thursday. The gale SW winds around this low are likely to produce high seas between NZ and Fiji/Tonga on Thursday and Friday. Avoid.
This complex area of low pressure is expected to move quickly off to the east, followed by a disturbed westerly flow over NZ on Sat/Sun 7/8 September.

Route Briefings
Tahiti to Tonga:
Avoid the squalls that are likely to accompany the trough that is expected to cross Tonga on Wednesday (afternoon local) and Southern Cooks by local Friday. This trough is also likely to be followed by a period of W or SW winds, mainly south of 20S, and these are counter progressive.

Between NZ and Fiji/Tonga
Best to wait until after the expected gale SW and high seas between NZ and the tropics on Thursday and Friday 5/6 Sept.

See my yotpak at http://lnk.ie/IFX6/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/IFX7/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/IFX8/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/IFX9/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
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