Geostrophic wind forecast

[BlueSkyNick]

OK I know I have asked about this before, but I still don't get it.

What is the relevance of the ECMWF Forecast where it indicates wind strength at 850hpa etc?

What conclusions can we draw about the wind at sea level from this forecast?

More to the point, why is there a yellow blob over Finisterre on the very day I would like to be getting there ?? /forums/images/graemlins/mad.gif

 

[ChrisE]

As I understand it, young man, this is the weather forecast for about 5000' up. The practical info you can take is that the wind strengths are less at sea level and I can't remember whther it is 30% or 50%. I'm sure that someone else will know this.

Regarding big yellow blob, have you cleaned your screen lately?

 

[JasB]

does the yellow blob mean 15-20 m/s?

Finisterre is Fitzroy now...isn't it?

 

[ChrisE]

Yes, at 5000', so prob about 15-20 knots at sea level.

 

[BlueSkyNick]

Thank you so much for your help, old boy.

I am now kicking myself for postponing for a week as the gribs are currently showing predominantly SW'lies for the next couple of days, but veering to N'lies at the weekend. /forums/images/graemlins/mad.gif

 

[simonjk]

Hi,

Good question.

You are correct that the 850hPa level is about 5000ft, 500hPa is around 10000ft. Now, you cannot draw much comparison between the upper winds and the surface, especially at such high level above the surface. 925hpa (3000ft) may be extracted sometimes, although even this is gard.

The yellow blobs on the chart so the zone of strongest winds at 850hPa.

Goestrophic wind is the uncorrected wind result from the geostrphic balance, which is the blance between coriolis and pressure gradient force. Don't worry if this is complicated because for all practical reasons,the gostrphic speed is the speed you measure off the isobars on a chart and then apply to see or land. if you excuse the plug, I've included a section on measuring and using the geostrophic wind in my book (which will be published in the next couple of months and I will tel you about here).

Hope that helps.

 

[BlueSkyNick]

thanks Simon - my interpretation is that this forecast is not a great deal of use to the average yachtie for predicting sea level winds in advance. However, it does give a reasonable high level view (pardon the pun) of the weather systems coming into play.

Will look forward to the book /forums/images/graemlins/wink.gif

 

[simonjk]

Kind of, if you know how to use the upper winds then you can spot area of development for lows and highs etc... However, for most sailors this work will have been done by a forecaster already.

What is useful to sailors is the ability to predict surface wind direction and speeds from surface charts.