Run for it!

January 22, 2013 — Leave a comment

The front this evening has come further North than expected so snowfall over Reigate could amount to 3-4cm by end of tomorrow: snow should die out overnight, only light flurries tomorrow likely, if any.

Above is an “ensemble run” of temperatures and precipitation for London until early February (850hPa represents 1500 metres above the surface – so temps are not for ground level). To get an accurate forecast, weather super-computers run complicated models several times, called runs, with slight “perturbations” in the data (labelled P1-P20). Perturbations are deliberate “errors” fed into the initial weather data which are designed to see how far each run agrees with each other. If all the runs (lines shown) are close together then weather forecasters can place more confidence in their predictions. Naturally, things get a lot less certain into the distant future: hence the increasingly squiggly lines towards February: i.e. anything could happen between those lines! The above runs all agree that we are due to warm up over the weekend. However, look closely at the detail from Friday 25 Jan to Sunday 27 Jan and it shows the warming could take a little while and significant precipitation could arrive from the west on Saturday BEFORE our cold dense block of air budges out the way. The incoming warm air is less dense and will tend to ride up and over the cold air mass still sitting on Reigate. Any initial precipitation is therefore likely to fall as snow or freeze-up when it hits the cold layer and turn into icy rain. This can be nasty stuff and coat freezing surfaces with ice too boot. The warmer air will eventually arrive (Sunday) but the transition could potentially be quite messy. Next week looks wet and windy with a return to Atlantic westerlies.

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