On Monday, July 22, 2013 5:16:29 PM UTC+1, General wrote:

Is it that there's a tiny chance of an inundation of truly biblical proportions?


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More or less.

It's for a low probability high impact event. A typical warning for this sort of situation with scattered or isolated but potentially very heavy showers / thunderstorms. A high probability moderate impact event would also generate a yellow warning.

Look for the matrix on either of these pages:

http://www.metoffice.gov.uk/guide/weather/warnings

http://www.metoffice.gov.uk/weather/.../warnings.html

Stephen.