Snowfall occurs when the column of air from cloud to surface is mostly below freezing, or any above-freezing layers are too shallow to melt the snow completely. It can also snow at 1–4°C when either evaporative cooling (in dry air) lowers the temperature as snowflakes fall or when they are heavy enough to survive melting.
Weather apps routinely forecast snowfall in winter, but the models that they use do not have high enough resolution to take these factors into consideration and make a judgement, leading to inaccurate results. Predicting snowfall requires taking into account a range of factors and local context to be able to make an accurate prediction, requiring both expertise and experience.
For the Vale of Glamorgan, I have spent years studying snowfall in winter. Each map below is a significant snowfall event (i.e., around 10 cm or more), and each time shows a north-easterly flow. For the last ten years, every single significant snowfall event has been the consequence of a system moving in from the northeast. The mode wind direction is 55 degrees (ENE).
Snow showers can move in from the northwest or southeast after a very cold period of time, though accumulation is uncommon due to higher temperatures. A northerly system can bring snow, though this is even more uncommon as it requires more energy due to the rain shadow of the Brecon Beacons National Park. The southwest never brings snow, and this will not change unless the North Atlantic Drift slows and/or begins to collapse due to climate change.
Leave a Reply