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

08 July 2018

Bob Blog 8 July

WEATHERGRAM

YOTREPS

Compiled Sun 08 July 2018

 

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.

 

Acceleration zones

Terrain effects near mountainous islands interrupts the wind flow and divides it into rivers of wind and puddles of calm. Today’s EC (9km resolution) model as seen on windy.com shows this in the trade winds over Fiji, with an onshore seabreeze developing around Momi Bay. The EC model data is running with only a few real world observations but its portrayal closely matches the observed data at Nadi Airport.

When I was a forecaster in Fiji back in the 70s I discovered that whenever the pressure difference across Fiji gets around 2 hPa or higher (isobars drawn 2hPa apart, the winds in the accelerated zones such as Vatu-i-Ra passage, Bligh water, and the SW end of Viti Levu may be funnelled to over 25 knots. Today the EC model shows orange-coloured speeds in these acceleration zones, for 21-averaged knots or more. These zones are strongest over the sea, and it would be good to be able to position some wind stations on nearby land.

The Fiji MetService has several local rules they use for compiling the Fiji Marine forecast. This evening’s edition responds to that timeless “isobar rule”:

Marine Weather Bulletin

Issued from the National Weather Forecasting Centre Nadi

at 7:30pm on Sunday the 8th of July 2018

A STRONG WIND WARNING REMAINS IN FORCE FOR ALL FIJI WATERS.

Situation:

A high pressure system to the south of Fiji continues to direct

strong east to southeast wind flow over the Fiji waters.

 

Forecast to midnight tomorrow for Fiji waters:

East to southeast winds 20 to 25 knots. Rough seas. Moderate southerly swells.

 

Further outlook: Easterly winds 20 to 25 knots. Rough seas. Moderate southerly swells.

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Interestingly, the GFS model, with its 13km resolution, and .25x.25 degree grib distribution does not capture these terrain effects:

 

TROPICS

Tonight, we have Storms THREE and BERYL in the North Atlantic and MARIA in the NW Pacific.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Japan (severe flooding there in the past week). Also, the rain across the South Pacific has become more intense. It remains very intense around central America.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to weaken during the coming week. It is expected to hover from Solomons to Samoa, and the zone over the Solomon Islands is expected to drift slowly south this week.

The trough which visited Samoa over the weekend is expected to travel SE across the Tahiti area on local Monday and fade over the Tuamotu islands on local Tuesday.

The next trough from the west is expected to reach the New Caledonia area on local Friday, Vanuatu on Saturday, and Fiji/Tonga/Samoa early next week.

 

Subtropical ridge (STR)

HIGH is travelling east along 30S to east of NZ this week. It should have a squash zone of enhanced trade winds on its Northern side, reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

Next HIGH is expected to travel across South Tasman Sea on Tue/wed UTC, northern NZ on Thu UTC and then to east of NZ along 35S from Fri UTC, fading near 170W by Sunday UTC.

 

Around Tasman Sea, NZ to tropics.

Large trough is crossing NZ on Monday and Tuesday with winds backing to SW then southerly and bursts of squally showers.

Too unsettled to depart on mon/Tue. OK to get away of Wed, but will need to allow for encounter with northerly winds ahead of the next trough

A Trough is expected to move off Australia and onto the eastern seaboard on local Thursday, and then deepen into a low between Lord Howe and New Caledonia on Friday. It is further is forecast to go southeast towards northern NZ over the next three-four days, turning in to a multi-centred system with centres below 1000hPa.

 

Australia to tropics

Stay put until after that low forms and moves away. Should be OK to go from around Sun 15 July.

 

Tahiti to Tonga

It may be better to stay put next few days – due to squash zone of enhanced trade winds reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

A departure on Thu/Fri UTC may be able to get as far as Palmerston Island before the next trough arrives from the west, maybe Niue.

 

Galapagos to Marquess

SE to ESE winds, mostly less than 20 knots. To use available currents, go to 2S 95W and then 6S 125W and then direct.

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

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

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

Contact is bob@metbob.com or txt 6427 7762212

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