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>> Simon's Weather Musings <<

Trough means rain for western Europe
The unsettled weather continues

By Simon Keeling in Wombourne 20:00hrs  19/04/2012
Temperature: 7.3C    Weather: Rain

A great example here of why the weather across western Europe is going to be staying so unsettled in the coming 10 days.

This chart is from the GFS ensemble model, and notice that 'u' shape in the colours extending from the UK, through western France and into Iberia? That's a trough in the 500mb flow, and when you see one of these, you know the weather below it will be unsettled.

500mb is at a height of about 18,000ft and you will hear me mention that chart frequenty. It's because it's the best height to give a good overview of the jetstream, and lower level features which have an impact on our weather.

Troughs (or otherwise called waves) move through the atmosphere at differing heights and speeds, and each one has an impact on conditions. They also move at different speeds with shortwave troughs moving quickly, and longer wave troughs moving more slowly. The longer waves tend to be more indentifiable at higher levels (about 300mb), the short waves lower down (around the 700 ot 500mb level). Now, if a short wave trough moves into a long wave trough the tendency is for the trough to deepen and vice versa. This deepens accentuates the trough, slowly it down and making the weather below it more disturbed.

Next week sees the jetstream staying overhead with trough moving through the jet. These help areas of low pressure to form, and as they move through the long wave trough high in the atmosphere they are able to deepen, the surface lows deepen and the weather stays wet.

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