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

04 September 2011

BOBGRAM issued 4 Sep 2011

WEATHERGRAM
YOTREPS
Issued 4 September 2011
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.

We are now moving into the month where daylight increases fastest in the Southern hemisphere ---It breaks out onto the Antarctic circle at around the equinox – that's the Antarctic dawn, so its their coldest time of the year. Since the westerly winds of the roaring 40s get their energy from the temperature difference between the tropics and the polar regions, these winds are normally at their strongest and furthest north at this time of the year--- some call these equinoctial gales—I like to think of them as the gales of the Antarctic dawn.

This year things are not quite normal. The tropical ocean temperatures are near normal so the ENSO is in neutral and not having much impact at this stage. However the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.38 on 28 Aug and 0.46 on 2 Sep. This has been producing some lingering La Nina weather patterns in the atmosphere--- and one which has been happening in the South Pacific has taken the current high of the subtropical ridge south of "normal", weakening the roaring 40s.
HOWEVER, it now looks as though the South West Pacific may be in for a period of roaring 40s weather next week and possibly the following week as well.

4 Cyclones are raging in the Northern hemisphere… In NW Pacific TALAS is weakening over Japan and a new one (currently called TC SIXTEEN) is waiting in the winds. Around USA, Southern Louisiana seems to be coping OK with LEE whilst KATIA awaits offshore (KATIA is the replacement name for the now retired name KATRINA).

In our tropics, the South Pacific Convergence Zone got weak over the past week. There are clusters of convection north of Solomons, just east of Samoa and around Northern cooks but northing organised. The main convection that went from southeast of New Caledonia to southeast of Fiji (where it is tonight) is fed by a jet stream rather than convergence--- and will bring squally rain to southern Tonga until late Monday UTC (avoid). This system should continue heading east southeast and bring variable winds of a trough over Southern Cooks on Tue 6 Sep UTC.

SUBTROPICAL RIDGE STR
The current HIGH making its way eastwards across NZ is somewhat south of the normal STR September position, so kiwis should enjoy its light winds whilst they can. Meanwhile it is likely to produce strong trade winds near 20S all the way from about 160W to 180 until Wed 7Sep. Fiji Met Service have a gale warning is place on this squash zone.

Next high is expected to move east off Australia next week , around Tue 13 Sep , and along 25 to 30S into the North Tasman Sea This is the more normal latitude for the STR in September and more indicative of spring and more encouraging to the roaring 4os pattern.

TASMAN SEA/NZ AREA
Between every two highs there is a trough, and in fact there will be a run of troughs across NZ after the High moves off on Thu 8Sep. Firstly a small low may form between New Caledonia and Northland on Friday 9 Sep and then move southeast brushing past NZ's northeastern areas on Sat 10 Sep. This just marks where a trailing upper trough gets moistened by an upper jet - not expected to bring much.

The next trough should be more substantial and should build into a well-wound up low off Sydney by late Friday 89 Sep, and followed by gale force south to southwest winds in the Tasman Sea for Sat/Sun /Mon 10/11/12 Sep, along with strong wet mild nortehrlies over NZ on Sun 11 Sep. Avoid.

Considering the location of the next high being to north of most of NZ, it seems that roaring 40s or disturbed westerlies are likely over NZ during the week Mon-Fri 12 to 16th Sep--- This may change , so read in again next week.

SAILING TO/FROM NORTHERN NZ.
The passing high makes this an OK week to depart or arrive, but NE winds ahead of approaching trough rules out a good departure from Thu to Fri 8 0r 9 Sep.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

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