Good post from Ian Fergusson (BBC Weather forecaster) regarding todays situation. It is a bit technical, but it explains why conditions are so marginal, why snow is very tricky to predict and the process of evaporative cooling that may come into play. Hi folks, not wishing to be a pedant, but I think some folk might be confusing/misinterpreting the nature of evaporative cooling effect that is critical today. Its not a process of dragging-down parcels of cold air etc from aloft (e.g. as we see in convective downdrafts of thunderstorms etc). The process today is one of extracting latent heat as a result of snow melting (as it falls)... this in turn lowers the Wet Bulb Freezing Level (WBFL) as a result of the evaporative cooling, allowing snow to penetrate progressively lower than might otherwise have been the case. Now, in todays set-up, the reason why this process is so important centres upon a cold pool of air - formed by the evaporative cooling effect - which will form just ahead of the low centre, aided by upper forcing giving higher PPN rates (and thus higher rate of evaporative cooling). This cold pool, sitting at the boundary layer, then gets entrained within the flow to the north of the low centre and as it advects to wrap-around westward later today/this evening above the low, it will create conditions where snowfall could turn heavy and readily get down to lower levels. However, the amount of settling is hard to judge: in many areas, snow may readily melt on paved surfaces, but either way, the ice hazard that will follow by early Saturday is a key point of concern.
Posted on: Fri, 26 Dec 2014 15:19:44 +0000
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