Weather Musings....Weekend Chill!

simonjk

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6 Mar 2003
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Hi,

You'll start to hear more over the next few days about the risk of a cold snap coming through this weekend, so I thought you might appreciate a quick synopsis of what is causing this cold weather, and how to spot it on the charts.

Firstly take a look at this chart
http://expert.weatheronline.co.uk/cgi-bi...90d4210cf5d6f0c

This is the 500mb (about 18,000ft) chart for the next few days from Weatheronline. It shows the height of the 500mb surface (solid lines) and the temperature (dashed lines). So why show you an upper air chart, instead of the surface? Well, the reason is that the upper air chart IS the key to the surface chart.

Look at the cvhart (top left) for midday today. Just west of Ireland there is a little dent in the contours. Now, look at the next couple and see how this dent, develops into a closed circulation. This is out upper low developing. Now, the winds flow with the contours in the same way as isobars at the surface, so behind the low the winds turn into the northwest. This brings increasingly cold air, mixing with milder air to the east and causing the upper low to strengthen further.

Can you see that by midnight Friday there is a distinct dip in the temperature contours? This is the upper trough and it is in this zone that the weather will be at it's most unsettled. By midday on Friday the trough lines through the country, and it is as this trough lies through the country that we will see some very, heavy and wintry showers. A distinct risk of thunderstorms.

To see the air becoming more unstable as the troiugh moves in take a look at http://pages.unibas.ch/geo/mcr/3d/meteo/meteogr/nmm22/LARKP.htm
This is the forecast sounding, unfortunately only out to Thursday evening, but what is does show is the temperature line, leaning backwards. The further back this line leans, the more unstable the air. If you watch this over the next day or so, you'll see it leaning further and further back.

Incidentally, Meteolue is a great new site. Lows of information on there and well worth look at. See http://www.meteoblue.ch

Back to the Weatheronline chart. See how by midday Saturday the upper trough is clearing to the east. A ridge is building from the west, and so our disturbed weather is going to be short lived, although intense.

Have your cameras ready on Friday because there may be some spectacular cloud formations around. If you spot anything, email it too me and I'll pop it on Weatherweb.net for all to see.

Hope this has been useful to you.

Best wishes,
Simon

Simon Keeling
 

Cornishman

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Thanks for that, Simon. Our local BBC weatherman David Brain explained it nicely on the lunchtime Spotlight South West News today, too. And I'd only just washed and put away my thermals for the winter!
 
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