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

19 September 2010

BOBGRAM7 issued 19 sep 2010

Issued 19 September 2010
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.

Tonight's write up is a bit later than normal as I'm just now back in Auckland after attending NZ Coastguard national Conference... they are a great bunch indeed!

SOI (90 day) is now 1.85, up 0.11 from last week, so La Nina is just getting stronger.

South Pacific Convergence zone is beginning to return to normal now, showing up and bubbly convection over Papua New Guinea and Coral Sea, and also in an extending line from NW of Samoa across to French Polynesia. It is expected to hold this position this week.

There is a conveyer belt/upper trough over inland Queenstown and this is expected to move out to the Tasman Sea on Mon 20 Sep. It should then fade, but its jetstream is likely to feed some more convection in the Coral Sea, worth avoiding.

It is interesting to note that the parameter I'm using to measure the strength of the Polar Vortex went negative on Thu 16 Sep-That's when the large High I mentioned last week came into play and intensified over the entire Australian Bight. This High also had a Southern Ocean Low to help form an eggbeater onto Tasmania and into the Tasman Sea and onto NZ.

Tasmania took the brunt last Thursday /Friday, then, during the weekend, squally showers brought power cuts to Wellington and Auckland, snow that collapsed roofs in Invercargill, and slips /flooding that caused clusters of road closures. As is the nature of the beast with squalls, not everywhere got hit, and the damaged areas were rather random. Swells in mid Tasman got to an estimated 8 metres last Thursday and peaked on west and south coasts of NZ on Friday night

It's not over yet, there's another couple of shovel-full of cold air for NZ, especially on Tuesday 21 and Wednesday 22 Sep. Another 8 metre burst should roll across southern NZ seas on wed 20 to 22 Sep and reach Chathams around Fri 24 Sep, but the swells in the Tasman Sea are expected to ease to more normal levels by Wed 22 Sep.

That high in the Aussie Bight has not ridged all the way to Antarctic ice-shelf so this isn't a classic polar blast, really it's a wintry entrée to the equinox. Around the equinox, the air along the Antarctic fringe is just starting to get some sunlight after 6 dark months--- it is at the coldest it ever gets , and the sun tends to stir this cold air so that, when encouraged by surrounding pressure systems such as a passing Southern Ocean Low, it gets dislodged northwards. In this case, the shovelling is continuing to be done by that Aussie Bight High.

However, that Aussie Bight high is too far south for the STR at this time of the year--- even in a La Nina year (which twigs the STR to the south), the correct latitide for the STR over Australia around the equinox is about 30-35S , sort of along the South Coast. I think this high got knocked south by that upper trough/conveyer belt that has been over North and Central Aussie over the last few days. SO I agree with the models, and they all take this Aussie Bight high and, slowly, shift it north to 30/35S by Thu 23/Fri 24 Sep so that some high cells ooze across the North Tasman Sea from Thu 23 Sep and over North Island on the 25/26 weekend.

Naturally the strong and sometimes squally westerly winds of the roaring 40s and equinoctial gales will continue unabated to south of this STR line, and reasonably useful trade winds are continuing to north of the STR line.

Taking into account the expecting easing of well in the North Tasman sea by Wed and then the high that is expected to cross the North Tasman and Northland area next weekend, it looks to be a reasonable time to sail to NZ this week.

An added advantage during the next week or so is the full moon. This peaks on Thu 23 sep - and that's the date of this years Sep equinox.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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