Weathergram

WEATHERGRAM

YOTREPS

Issued 30 Sep 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: The energy balance of our weather engine is thermostatically controlled by the sea surface temperature along the equator of our widest Ocean- The Pacific. For the past few months these have been between half and one degree ABOVE normal— showing a tendency towards an El Nino period. Over the last few weeks the sun has been directly overhead the equator allowing this target area maximum solar input. Interesting the impact has been a small region of DECREASING anomalies. And in the sub-surface the region of positive anomalies has started to shrink—there is now a region of negative anomalies growing at depth in the mid Pacific. The impact of all this is that the probability of an El Nino episode dominating the weather engine is still high but now LOWERING, even though it remains above 50% for the remainder of 2012.

We are currently in neutral but volatile…by early 2013 neutral conditions (between El Nino and La Nina) are more than 50% likely.

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Consensus probability of El Nino/La Nina/Neutral from http://www.cpc.ncep.noaa.gov/products/analysis_monitoring

/lanina/enso_evolution-status-fcsts-web.ppt

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is stalling after a bounce back from its low of -1.01 back on 25 Aug. It eas been hovering around plus 0.4 in the past week. So the atmosphere is locked into a monthly swing from El Nino to neutral. It’s now in neutral mode, so is likely to swing back into El Nino mode over the next few weeks.

Tropical cyclones: Things are still ticking over with JELAWAT east of the Philippines last week and expected to make land fall over Japan in a few days. NADINE is looping around the mid-North Atlantic.

In the South Pacific, the South Pacific Convergence Zone SPCZ is now activating over the Coral Sea, drying out over Fiji and weakening east of the 180. During the coming week it is expected to weaken in the Coral Sea by Tuesday, stretch eastwards across Vanuatu on Wednesday and across Fiji and Tonga on Thursday and Friday. Avoid the active parts.

SUBTROPICAL RIDGE STR

High HI marking the STR in the east is currently near 43S way to the south of French Polynesia FP and is moving slowly east, cradling a large low that is moving slowly south along around 135W . This low is expected to deepen below 1000hPa on Monday, making for gales all around it and throwing some large swells to the south end of FP.

High H2 marking the STR in the west is taking a different latitudinal track from HI. It is currently in the Australian Bight and is expect to travel NE across eastern Australia on Monday Tuesday Wednesday and then east along 25S from Thursday to Saturday and then along 30S when it clears 180.

The next High H3 is expected to take a more southern latitude across Tasmania on Sat/Sun 6/7 October.

NZ/Tasman Sea

A disturbed southwesterly flow should cover NZ from Monday to Thursday. On Friday, as H2 zips like orange pit eastwards between Fiji and NZ, A strong NW flow should cover the Tasman Sea/NZ area, followed by a front and SW change on Saturday.

At this stage the most likely scenario is that this front may stall over northern NZ and allow a low to deepen there on Sunday and move off on Monday 8 Oct. This scenario may change by then so seek updates.

SAILING TO/FROM NORTHERN NZ.

This next ‘window” of OK conditions for approaching NZ from the north, at this stage, is on Monday/Tue 8/9 October… if you are in a hurry to get here now then aim for those dates.

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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No bonus extras to this week’s blog and next week’s may be delayed a day or so.

Here’s a snap of me talking to a seminar about marine weather at this weekend’s Auckland Boat Show.

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

Weathergram with graphics is at https://metbob.wordpress.com/

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

Website http://www.metbob.com

Feedback to bob@metbob.com

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