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

16 December 2012

BOBGRAM issued 16 Dec 2012

WEATHERGRAM

YOTREPS

Issued 16 Dec 2012

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.

The Ocean Along the Pacific equator Sea Surface temperatures SST have recently been near normal with some below-normal SST near Galapagos- this situation is considered to support ENSO neutral conditions and computer models are picking this to continue into the new year. There is a zone with SST more than 1 above normal from Samoa to Tahiti—This can possibly encourage the formation of the South Pacific Convergence zone SPCZ during the next few months and bears close watching.

The Atmosphere: Things are starting to change. The Southern Oscillation Index or SOI (30 day running mean) has been hugging the plus 0.5 value since the beginning of September, but in recent days the 30-day average has dropped to minus 0.5, and this is in response to lower than normal isobars over Tahiti, so it is a definite shift towards an El Nino-like episode. Computer models are picking a neutral SOI for the cyclone season.

This edition of my weathergram is all about EVAN. TC EVAN started as a depression L that drifted east towards Samoa. It started intensifying faster than the computer models were expecting and dealt Apia a severe blow. Then it did a loop and went west and WSW. At present it is category 4 near the north end of Futuna and curving to the SW towards Yasawa. The latest forecast track map from Fiji Met Service shows past track, expected future track and possible variations (black line) and three rings at 12 hour time steps;
http://www.met.gov.fj/aifs_prods/65660.gif . The Fiji Met track map web page is very busy so if you need the latest track in a hurry try this URL to a gif image – it doesn't have the text of the full site so can only be used as an unofficial substitute. Remember to subtract 13 hours to convert the times shown here to UTC.

Note how small this system is, but it is extremely violent near centre during next 48 hours and then should ease to Cat 2 as it encounters cooler seas.

The low is expected to slowly continue south this week and become extra-topical. It is expected to bring cyclonic easterly winds and swell to northern NZ mainly around Sun /Mon 23/24 Dec.



http//bit.ly/7daywx gives the MetService model. The map for 1pm sun 23 Dec= 23 0000UTC issued this evening shows Low to north of northern NZ with just strong winds left, and these are spread out over a wide area.

Next week?

At this stage GFS model takes it across the North Island on Mon 24 Dec and EC model takes it west and weakens into a trough in the Tasman Sea (see http://bit.ly/ecoz). I think that by Boxing Day the winds may well be OK for Kiwi holiday sailing plans.]

Sydney/Hobart Race -Still too far away to be certain. In the map for next Sunday above you can see a heat trough deepening over the Aussie interior, and the possibility of a High forming over the Aussie Bight. One scenario is that this trough may break loose into the Tasman Sea on Boxing day and the High may then move towards the south of Tasmania—that would make a squash zone of enhanced S/SE winds mid-way between Sydney and Hobart in a word, challenging.

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

Weathergram with graphics is http://metbob.wordpress.com/2012/11/04/weathergram-4/

Weathergram text only http://weathergram.blogspot.co.nz

Website http://www.metbob.com

Feedback to bob@metbob.com

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