Bob Blog 21 July

WEATHERGRAM

YOTREPS

 

Compiled Sun 21 July 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.

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Why do weather models differ so much from each other?

I’ve been asked to try and explain why the output from the EC and GFS often differ as seen at windy.com

Many top nations develop and produce their own global weather model, each with its own equations that take data from the WMO observing network and massage these numbers into forecasts. The EC is still top for accuracy, and the other model mainly accessible is the GFS which was revamped last March.

First problem is that the weather observing network is not 100%. Some data may be incorrect due to measurement error, and the data is spread out and fixed to places on earth so doesn’t gel with the matrix of dots in the computer model. There are huge gaps.

Then there is the matrix of data points. Different countries select different resolutions. Even if the dot separation is small, such as the 9km EC model, the accuracy of the forecast depends on the amount of observations it is based on.

In the tropics, away from good observations, the portrayal of a passing trough may be a zone of averaged out light variable winds, but in the real world there maybe squalls .

From www.researchgate.net/figure/WMO-exchanged-scores-from-global-forecast-centres-RMS-vector-wind-error-over-tropics-at_fig9_312093998

is a graph that is a few year’s old now but gives a comparison of performance of some main weather models including EC from WMO.

Sometimes some models handle the weather pattern better than others. Also, you can usually tell if a particular model goes “off track” as its future scenarios start looking very usual (sometimes ending is unlikely extreme weather).

When the differences are minor, try and work with the average and make allowances that cover both scenarios.

 

THE TROPICS

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 is potential for another tropical system to form in the Indian Ocean or east of the Philippines or west of central America.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Tokelau/Samoa.

There is a trough passing over Niue on local Sunday 1008 hPa and Rarotonga on local Monday followed by a day or so of W/SW winds. It may be squally as it is associated with a compact low further south deepening as it moves off to the SE. Should be weaker over Tahiti by local Wednesday with a period of N winds followed by a SE wind change.

This is expected to be followed by a ridge travelling east along 30S (along with a High along 42S) from local Wednesday onwards.

There may also be a passing trough between Tuamotu Islands and Marquesas.

 

Subtropical ridge (STR)

Weak High up to 1022 hPa in Tasman Sea from Mon to Fri this week, quasi stationary.

Another High, east of NZ, is expected to travel north from 50S to 40S along 165W from Thursday to Saturday

 

Tasman Sea /NZ/Aus

Low is expected to travel from Tasmania on local Tuesday, deepening to 10008hPa by Thursday, then fading over Northern NZ around Saturday 27 July

This means it’s OK to depart NZ for the topics on Monday/Tuesday.

Also it should bring a zone of light winds between Noumea and Brisbane or vice-versa, mainly from S/SW, and OK. SE winds further north for a voyage to Bundaberg. Peak swell around 2.8m on Friday.

 

Tahiti to Tonga

Maybe wait for a passing trough to clear Tahiti around local Wednesday with a switch from N to SE winds.

After that should be SE winds for week or so, OK to go.

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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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