Bobgram

WEATHERGRAM

YOTREPS

Compiled Sun 23 June 2019

 

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.

 

Solstice was Friday 21 June 1554UTC, but for the next six weeks, as the days’ get longer in NZ the cold gets stronger.

 

Tahiti to Tonga

Now that many yachts are about to travel west from Tahiti to Tonga, it is time to review the weather, in general terms, for this route.

Getting from the Tahiti area to Tonga/Fiji means crossing the South Pacific Convergence zone, SPCZ

 

1. This zone may be weak or may contain squally showers. In the models it is portrayed as a zone of light winds. To see a forecast for this zone, use windy.com and rain accumulation 5 days.

This zone may linger in the north (it does this in an El Nino, and we are nearly in an El Nino), or, it usually hovers between Samoa and Southern cooks. To avoid it choose a different latitude or aim for gaps in the zone.

2. Another thing to avoid is a SQUASH ZONE, when a large High travels east along 30S.

When the central pressure in the High is 1030hpa or above, it gets dirty (above ten thirty is dirty) and squeezes the isobars north of the high in the trade wind zone closer together making a "squash zone" of enhanced winds and rough seas. These usually occur around 20 to 25 South, and may last for several days. They are reasonably easy to forecast, and are worth avoiding. If caught in one, change your latitude rather than your longitude.

3. The third thing to avoid is the passing trough or passing low.

The systems tend to form on the SPCZ and travel southeast along it, propelled by upper NW winds as a steering field. They also tend to have their own mini-squash zones. Isobars give a clue about the intensity of a passing trough: those above 1010hP are usually weak and those below 1007 are worth avoiding.

There are three routes between Tahihi and Tonga, northern, central and southern:

The northern route goes via Suwarrow to Samoa and can be used when the SPCZ is further south

The middle-route offers stopovers such a Palmerston Island and Niue and is a good alternative when the SPCZ is further north than normal.

The southern route, going via Rarotonga and maybe Beveridge reef, is not often used but may help avoid a squash zone or sometimes a passing trough.

Basically, avoid passing troughs and squash zones, and go thru the SPCZ when it is weak. This may mean taking short hops rather than getting direct from Tahiti to Tonga in one go.

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THE TROPICS

TC VAYU brought moderate damage to NW India last week when it made landfall. It was the strongest cyclone in this area for 21 years. ( 8 fatalities and 12 injury reports).

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are areas of potential development near the Philippines and to north of PNG, also off the west coast of central America.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomons to near New Caledonia to south of Tonga this week. A weak passing trough (down to 1016hPa) is expected to travel southeast along this zone to east of New Caledonia and south of Tonga on Mon/Tues, and then another stronger passing trough is likely in the New Caledonian /Fiji this weekend (down to 1004, worth avoiding).

 

Subtropical ridge (STR)

HIGH 1028hpa is expected to travel along 30S to south of Tahiti next few days—with a small squash zone near Southern Cooks.

Large HIGH is expected to slowly travel along 40S this week, reaching 1040 hPa over South Tasman Sea on Thursday, and crossing central NZ this weekend. There are likely to be squash zones between NZ and New Caledonia/Fiji with this High.

 

Tasman Sea /NZ/Aus

Weak trough is expected to linger around North of NZ on Monday/Tuesday with light winds and scattered showers.

After that the winds north of NZ are easterly, increasing to strong in squash zones. Avoid. Should be Ok for getting to Australia , not the other way.

HIGH is expected to be followed by a trough in the Tasman Sea next week.

 

Tahiti to Tonga

Trough now moving off to east of Society Islands. Small squash zone west of Tahiti on local Monday. Weak passing trough over Tonga on Tuesday 25 traveling east to be south of Southern Cooks local Thursday.

Next passing trough expected to reach Tonga this weekend, worth avoiding.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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