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

14 October 2012

BOBGRAM issued 14 October 2012

WEATHERGRAM
YOTREPS
Issued 14 Oct 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: Equatorial sea surface temperatures SST of out widest ocean act something like a thermostat for the weather engine—when they are sufficiently warmer than normal it is called an El Nino episode. Well they have been above average across much of the tropical Pacific over the last few months but are now relaxing. An El Nino may still occur over the next few months but it now doesn't look as though it will be a big one.
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index is a good site for latest on El Nino and SST.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is relaxing after a bounce back from its low of -1.01 back on 25 Aug. It has been hovering near zero for the past few weeks and was plus 0.2 on 11 October. It's now in neutral mode, and may swing back into El Nino mode over the next few weeks.
These indicators show that, at this stage, the coming cyclone season may have an el nino touch to it but not much of a touch. NIWA are likely to add more info about this in the next few weeks.

Cyclones: Still busy in the northern Hemisphere with PRAPIROON and a tropical Low southeast of Japan, RAFAEL in the Caribbean and PAUL southwest of Mexico. Southern Indian Ocean has woken up with ANAIS heading for Reunion Island making an interesting early start for the southern Hemisphere cyclone season.

http://tropic.ssec.wisc.edu/# is a good site for latest on cyclones.

The South Pacific Convergence Zone SPCZ has shifted in the last week to the north of its normal range and stretches from Solomons to Tuvalu/ Tokelau/ Northern cooks and occasionally around Marquesas.
During the coming week it is likely to drift south, reaching Vanuatu to Fiji by the end of Tuesday and staying there until Friday. It is likely to arrive over the Tonga region the Tonga region around Friday 19Oct and linger there until it develops a Tropical Low that should then move off to the southeast early next week. AVOID planning any departing voyages when the SPCZ is visiting. It brings squally showers and these can be troublesome.

SUBTROPICAL RIDGE STR
STR is well defined at present. High that is crossing the Tasman Sea tonight is expected to move along the 25 to 30S latitude band by Thu 18 Oct. It has squash zone of enhanced wind both north and south of it. It followed by another using the 30 to 35 South latitude zone from Thursday 19 Oct to Sun 21 October. This high is expected to be gentler than its predecessor and only have a squash zone in the Coral zone.

NZ/Tasman Sea
The two Lows that crossed NZ on 8-9 Oct and 12-14 Oct deepened at the same time, bringing lots of wind and rain and consequent damage. This week we have a windy front crossing NZ on Thursday 18 and another on Monday 22 Oct- Note , that Monday is Labour day a public holiday in NZ commemorating NZ as the first [lace in the world to legislate the 8 hour working day (yeah, right). Next front over NZ is likely to be Thursday 25 Oct. These fronts are MOT expected to bring deepening Lows like last week, but it will be easier to arrive in NZ between them rather than during them.
Between the passing Highs, NZ continues to be subjected to a disturbed westerly flow, with LOWS forming and deepening in the mid-Tasman Seas on Monday and Tuesday 8-9 Oct and again on Friday to Sunday 12-14 Oct.
SAILING TO/FROM NORTHERN NZ.
We will soon be approaching the interesting part of the migration season which I call Analysis Paralysis of the Minerva Yacht Club. Cruising sailors are world renown for their ability to gather data from almost everywhere even in the remote Minerva, and then sharing their ideas with their neighbours in a great "show and tell" even if its just over the radio. The result is a committee of motion but no commitment to moving. The problem is that if you over examine the data something somewhere will always say "don't go".
So let me simplify things so you can work out when to go or no.
1: Avoid departing when the SPCZ is overhead or nearby.
2. Respect squash zones—they are not much of a problem this week except in the Coral Sea. Sometimes they can't be fully avoided.
3. Try to time your voyage (depending on the speed of your vessel) to AVOID arriving in NZ on the same day as a front. See dates given above.
4, - a corollary of 3… Fronts at this time of the year are usually less than a week apart and the voyage to NZ usually take more than a week, so front's can't be avoided altogether: better to arrange to encounter them near 30S in the STR where they are usually at their weakest. If things change then 30S is the place with deep water and open ocean, good for hoving to .
I hope these 4 tips help you decide.

And there's safety in numbers--If you are planning to sail from Tonga/Fiji/Vanuatu/ New Caledonia to NZ in October/ November, and looking to buddy with someone, then you may be interested in checking out the ICA's "All Points Rally" to Opua, see http://www.islandcruising.co.nz/ .

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See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at http://metbob.wordpress.com/
Weathergram text only http://weathergram.blogspot.co.nz
Website http://www.metbob.com
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