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

15 December 2013

BOBGRAM issued 15 Dec 2013

WEATHERGRAM
YOTREPS
Issued 15 December 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.

SOI 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.
SOI is currently steady after a recent rise. Its 30-day running mean reached 0.75 on 15 Dec.

TROPICAL TOPICS
The Indian Ocean is expected to be a place o0f tropical action this week. There is already a depression near 15S 80E moving west. On Monday another tropical low is expected to form near 8S100E and this one has a good chance of growing into a tropical cyclone.
This activity is related to a pulse of increased tropical convection in the Eastern Indian Ocean, So far, this extra convection seems to be staying put, however it is also likely to start travelling east onto northern Australia by the end of this week or next week and , if so, then it may reach the Coral Sea and tropical dateline areas by the end of December or early January, increasing the chances of development of tropical depressions.


WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is in its normal position roughly between Solomons and Fiji with a side branch between Tuvalu and Northern cooks. It is expected to become active over Fiji and Tonga from Friday 20 to Monday 23 Dec, developing a small depression on Sunday.
A subtropical low is forming south of the Southern Cooks and is expected to travel south over the next few days, stealing away the winds over the Cooks.
The cricket players in Perth are enjoying the inflated temperatures found in a heat trough, and playing for the ashes.

Sub-tropical Ridge STR
The STR is also near its normal position as we approach the longest day (next Saturday). It is strong across a wide longitude belt and there are strong SE winds on its northern side over the Coral Sea and occasionally over New Caledonia. This makes it very hard to sail from Noumea to NZ this week.

Tasman Sea /New Zealand
Over the North Island this is a week of passing anticyclones—but there is cold air aloft so the convergence zones formed between these passing highs are likely to trigger daytime convection. OK for sailing because most of the convection is inland or coastal rather than over the sea.

Over the South Island there are occasional passing troughs, especially on Monday/Tuesday and Thursday, and Lows, especially on Sat/Sun. Because of NZ's terrain these feature bring days of strong wind to the Cook Strait area.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
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