Bob Blog 25 Oct

WEATHERGRAM

YOTREPS

Issued 25 October 2015

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.

Tropical Note: A weekly treasure:

The Bureau of Meteorology Australia weekly tropical note at

http://www.bom.gov.au/climate/tropical-note/ is updated every Tuesday and carries comments on the trends in the MJO oscillation.

Even if you are limited to only email via satphone or shortwave, you can read this note by sending an email to query@sailmail.com, no subject needed, saying

SEND http://www.bom.gov.au/climate/tropical-note/

MJO stands for the Madden-Julian Oscillation, an increase in tropical convection that tends to travel eastwards from the Indian Ocean to the western parts of the South Pacific. It helps trigger the formation of Tropical cyclones. The MJO has been weak/ undetectable for many weeks, but is now starting to appear in the Indian Ocean. It usually takes around a month for it to track from there to South Pacific, so having an MJO in the Indian Ocean reduces the risk of a tropical cyclone forming in the South Pacific.

TROPICAL TOPICS

PATRICIA has come and gone, and was category 5 for a while off the west Mexican coast. OLAF is still in the mid North Pacific since last week. And the many “Invest” areas in the Indian Ocean is associated with the MJO.

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From http://tropic.ssec.wisc.edu/

The Weekly rain maps show increasing convection over western part of Indian Ocean , and a weakening in the South Pacific.

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Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing wind, isobars, Sig wave height green lines, swell direction orange arrows, current in small arrows, SPCZ and STR.

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to have a reasonably quiet week near Tuvalu and Tokelau, with scattered convection over Southern cooks/French Polynesia.

STR= Sub-tropical Ridge

The STR is weakening in the Tasman Sea area but is expected to stay strong to east of 180 this week. The squash zone of enhanced SE winds that is tonight between Niue and Austral Islands is expected to weaken from tomorrow.

High in northern Tasman Sea tonight is expected to cross northern NZ on over Tasmania tonight is expected to travel NE across Tasman Sea and cross northern NZ on Tuesday and then weaken.

Next HIGH is further south and is expected to travel around south end of Tasmania on Friday, and then slowly across northern NZ on Sun–Wed 1 to 4 Nov.

Over NZ/Tasman:

ACTIVE FRONT from Sydney to South Island on Monday and from off Coffs to central NZ on Tuesday is expected to deepen into a LOW between Lord Howe Island and North Island on Wednesday and this low is then expected to travel east across the North Island on Thursday with strong to gale SW winds and large swells on its western side. Avoid.

Travelling Tahiti to Marquesas.

It seems that several yachts are thinking of spending the cyclone season in Marquesas rather than Tahiti—seems sensible since this season is likely to be like 82/83 (14 cyclones, 10 in the Cook Islands/French Polynesia/Pitcairn area) or 97/98 (16 cyclones, 8 in French Polynesia).

The good news this week is that weather pattern is looking Ok for voyages from Tahiti and Marquesas for departures on local Monday or Tuesday, maybe Wednesday, with light winds between Tahiti and Tuamotu for motoring, and then easterly winds between Tuamotu and Marquesas. This is a brief window, associated with a weak trough on the weak SPCZ over the Tahiti area next few days, and these windows are rare so go for it by Tuesday.

Travelling Tahiti to Tonga:

The SPCZ should stay north over most of this route and is weak over Tahiti, and the squash zone around Cooks Islands/Tahiti tonight should ease by local Tuesday, then there is a reasonable opportunity for departure. There are likely to be a few convergence zones along this route with some light winds.

The Island Cruising Association is holding the All Points rally to Opua and the Downunder rally from Noumea to Newcastle.

Between Tropics and NZ:

For the All Points Rally, ending in Opua:

A departure early this week should be Ok. It is likely to have a period of SW winds between 25 and 30 S with 2 to 3 metre swells on Thursday and Friday thanks to that NZ Low.

If you are heading for NZ from New Caledonia/Fiji or Tonga, then the main deciding pointers are the fronts crossing northern NZ. After that Low over NZ on Thursday 29 Oct, there is a slow-moving HIGH from Sun 1 to Wed 4 Nov and then a strong NW wind is expected (at this stage) over northern NZ around Thu 5 Nov, followed by a few days of westerly winds. Since there is nothing significant happening in-between the islands and NZ if you arrange a departure this week that avoids arriving in NZ on Thu 5 Nov, then that should work—but seek updates as well as things keep changing.

Between Tropics and Australia

For the Downunder Rally (Noumea to Newcastle)

A departure early this week should be Ok. It is likely to reach just moderate southerly winds and 2 m swells near 26S from that “Wed 28 Low” between Lord Howe and NZ.

Newcastle is expected to have its next dose of strong winds on Monday 2 Nov, and since this voyage may usually take around 6-7 days, depart on Monday this week to try and arrive before these strong winds, or wait for an opportunity later this week, maybe Thursday onwards.

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

See my website http://www.metbob.com for information on tailor-made voyage forecasts– Feedback to bob@metbob.com. Tell anyone you like that to subscribe they should email me.

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