Bob Blog 30 Aug

WEATHERGRAM

YOTREPS

Issued 30 August 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.

WESTWARD HO

Several yachts are currently in Tahiti and waiting for the right conditions to go west.

Over the past week or so an intense trough has been stationed west of Tahiti, blocking their way. Today this zone is weakening away over the Austral Islands — that cloud which looks like a rope is a whole line of squally showers, worth avoiding.

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From http://www.meteo.pf/observation.php?carte=pfvdo

By Monday local this zone should have faded away and over the Society Islands there should be a wind change to southerly winds and clearing clouds. This is a good weather pattern for yachts to go west. Their next challenge is likely to be a weak trough/convergence zone near 165W on Friday 4 Sep local date, and at this stage this looks to be a weak trough.

Going west, there are three popular routes

1. The northern route: via Suwarrow and Samoa

2. The central route via Palmerston/Aitutaki

3. The southern route via Rarotonga/ Niue

1. Since we are having an El Nino, the South Pacific convergence zone SPCZ is spending more time than normal over the northern route, so that may be the squally way to go unless you can pick a period when the SPCZ is weaker than normal.

2. There can be occasional squash zones of enhanced trade winds over the central route. These are produced on the north side of any large HIGHS that travel east along the sub-tropical ridge about 30S. However the next few HIGHS are likely to be weaker than normal, so no squash zones are expected for the next few weeks.

3. The southern route is most at risk from the big swells that roll out of the southern ocean.

This week I think that often the central route may provide the least risk for adverse weather.

To help visualize the position of the SPCZ over the next week or so use the rain accumulation overlay option on windyty.com (found on the right) and select Next 10 days. The day-to-day variations of the SPCZ may be difficult to follow, but the 10-day accumulation shows its trend well.

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From windyty.com

TROPICAL TOPICS

Things are quietening in the Asia and NW Pacific region, but very busy in the Hawaiian area, KILO continues from last week to west of Hawaii, IGNACIA is likely to pass off to the north of Hawaii on local Monday/Tuesday, and JIMENA may follow by the coming weekend. In the Atlantic, ERIKA left 20 people dead in Dominica and has now faded near Cuba before getting to Florida (but its remnants have brought some intense rain to Florida). And there is a developing storm in the eastern Atlantic.

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Map from http://tropic.ssec.wisc.edu/

The weekly rain maps over the past two weeks show the intense rainfall from these tropical cyclones. Rain in the South Pacific has intensified since last week.

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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 south Pacific Convergence zone SPCZ extends from east of Solomon Islands across Rotuma to Samoa and then southeastwards. It has decreased in activity during the past week.

STR= Sub-tropical Ridge

The STR is the zone that divides the trade winds of the tropics from the disturbed westerly winds of the southern ocean. It is weak at present and north of its normal position.

The HIGH that is east of NZ is expected to travel east along 30 to 35S this week. It is NOT expected to have any appreciable squash zone son its northern side (between the High and the Trough on the SPCZ).

Another HIGH is expected to travel along 25S across the Northern Tasman Sea from Wednesday UTC, crossing Minerva reefs on Fri/Sat UTC. This HIGH is also NOT expected to have a squash zone.NZ

A tale of woe/LOWS. LOW1 is expected to form off NW of NZ on Monday and travel slowly across the North Island on Tuesday and Wednesday. LOW2 has been bothering New South Wales since last Thursday and is The LOW that Trough is expected to finally waken into a trough crossing North Island on Wednesday. After a brief ridge on Friday LOW3 (crossing Sydney on Thursday) is expected to cross the North Island on Sat/Sun 5/6 Sep. There is also a possibility of LOW4 over Northern NZ on 8/9 Sep.

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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.

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