BOBGRAM issued 17 Nov 2013

WEATHERGRAM
YOTREPS
Issued 17 November 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.
It reached a high of 0.52 on 22 September, and since then has dropped. On 17 November it has a 30-day running mean of -0.3, and is trending downwards.

TROPICAL TOPICS
Things seem to have gone quiet at present – no tropical cyclones today.
An interesting comparison of the geographical extent of TC HIAYAN with that of TC KATRINA can be seen at http://lnk.ie/KWJT/e=bobmcd1.bobgram@blogger.com/http://tropic.ssec.wisc.edu/temp/Geographical-Coverage-Hiayan-vs-Katrina.jpg

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has retreated to it more northern position in the past week and is now expected to go through a quiet period for the next week or more.
A Low is expected to form in a trough that is crossing New Caledonia on Monday. This Low should then deepen between Fiji and NZ on Tuesday and Wednesday. On Thursday its associated trough should cross Fiji and on Friday it should cross Tonga. Following this trough there may be a few days of light variable winds over New Caledonia until Friday and over Fiji until Saturday and Tonga until Sunday 24 November.

Sub-tropical Ridge STR
The STR went to the southern limit if its travel last week and is expected to stay well south of normal this week. This is a pattern sometimes called 'low index' and gives plenty of room for lows to form and expand between the SPCZ and the STR. This is exactly what is expected to happen this week with a low deepening between Fiji and NZ.

Tasman Sea/NZ.
High over the South Island is expected to travel off the east on Monday/Tuesday maintaining a NE flow over NZ. Low between NZ and Fiji should reach peak intensity near 28-29S 175-179E on Wednesday and then weaken as it moves off to the southeast.
Trough in the south Tasman Sea is expected to cross the South Island on Thursday and then fade away as it travels off to the east in Friday. This is likely to be followed by light winds for a few days.

Briefings
Between tropics and NZ
The Low between Fiji and New Zealand and the trough and light winds that follow it basically closes down the idea of getting a cruisey sail to NZ this week. Some may want to get going in the light winds that follow, but others will prefer to wait for some steady and useful breezes to return – maybe this weekend or early next week.

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