There is potential for some significant amounts of snow over parts of the UK later this weekend. However, the detail of where and when this snow will fall is still uncertain.
So what is happening in the atmosphere? A frontal system that’s been working its way across the Atlantic during the last couple of days will push rather erratically eastwards across the UK this weekend, bringing some persistent and, at times, heavy rain to parts of the country. We also currently have some cold air sitting over the near continent and, as the rain advances eastwards, an area of low pressure will form over southern parts of the UK, causing winds to turn easterly and bring this colder air in from the continent.
How and where this area of low pressure forms will also be critical in determining where and when the heaviest of the snow falls, and exactly where the dividing line between rain, sleet and snow is.
The most likely scenario at present shows the heaviest snow across the Midlands, Wales and northern England with over 10 cm of snow currently forecast.
However, different computer models are currently showing this process taking place in different ways and in different locations, and it is uncertainty within the atmosphere that is making the Chief Forecasters job so difficult. To help us provide the best advice possible to you, the Met Office uses both deterministic forecast and ensemble forecast models.
A deterministic forecast uses current observations of the atmosphere to create a single forecast. In the case of this weekend’s weather it indicated the most likely place to see snow. However, over the last few days the most likely part of the UK has changed each day which in itself generates lower confidence in the detail of the forecast. To help increase confidence, we use an ensemble model which generates a group of forecasts each with slightly different starting conditions. The Met Office ensemble model has 24 solutions and the output of this model shows the spread in probability of where in the UK could see snow. This highest probability is the point where the majority of the 24 solutions are in agreement. This can be combined with the deterministic forecast to help make a final decision on forecasts and warnings.
Met Office forecasters will continue to look at the available evidence with a view to providing the most accurate, timely, warnings and forecasts.