Forecasting snow is notoriously difficult which is why there are varying predictions about how much, if any, snow we will see this weekend. The majority of Wales is covered by a two-day warning for snow and ice starting on Saturday and running until just before midnight on Sunday.
The Met Office warning says outbreaks of rain spreading progressively northeastwards later on Saturday and overnight into Sunday will likely be preceded by a spell of snow on its northern flank. It points out the uncertainty about when it will turn back into rain although it does say: “Some significant accumulations of snow are possible across parts of Wales, the Midlands, and northern England in particular, at least for a time.”
This could be 5cm or more accumulating fairly widely with perhaps as much as 20-30 cm over high ground of mid and north Wales. Get the latest Welsh headlines delivered free to your email inbox.
But different organisations have different forecasts for how much snow will fall in Wales and for how long it will last. The Met Office is predicting that there will be snow falling from around 6pm on Saturday while BBC Weather says it will fall mainly as rain in the majority of Wales. Here is what they say.
Met Office
Met Office weather maps show the first snow forecast for around 6pm on Saturday. The Wales forecast says: “Largely dry on Saturday but rain, preceded by snow, likely arriving later. Disruptive snow possible but probably turning back to rain Sunday.”
This is the weather map for 9pm:
More snow for northern parts at midnight:
By 6am on Sunday it looks like the snow will turn to rain for most:
More rain for midday on Sunday:
BBC Weather
The BBC forecast for Wales says: “Saturday will be cloudy and windy with a band of rain pushing in from the southwest which will fall as sleet and snow on its leading edge. Windy overnight and into Sunday with spells of heavy and persistent rain. Monday will be a cold day with a few showers in the morning. Drier in the afternoon with plenty of winter sunshine.”
This is what BBC Wales forecaster Derek Brockway has shared.
This is the BBC Weather map for 6pm on Saturday:
The BBC weather maps show that it has mostly turned to rain by midnight:
BBC forecaster Chris Foulkes said at lunchtime on Thursday: “In the second half of the weekend there is a system that could bring some severe disruption. The snow is not going to get everyone because there are some big temperature contrasts between the north and the south.” He shared a map of where it is likely to fall as rain or snow in the UK:
(Image: BBC Weather)
He said: “”In the north, the snow is likely to get down to sea level but in the milder air in the south you would have to up a mountain that is 2,4000m high and we don’t have any that high so it is going to fall . The zone of snow is likely to slow down on Sunday but there cold be some freezing rain for Wales that would make it very slippy.”
The forecaster also shared a graphic of where the disruption is mostly likely to be with between 20 and 40cm of snow in some places:
(Image: BBC Weather)
WX Charts
The charts from forecasting agency WX appear to show the snowy weather starting earlier on Saturday. This is the chart from noon on Saturday (the purple colours indicate snow):
They also show that it turns quickly to rain for more southern parts of Wales. This is the map for 6pm:
The Met Office has explained why it is so difficult to forecast snow. They say: “As it’s so cold high up in the atmosphere most precipitation either starts off as snow or supercooled raindrops. As it falls to earth it moves through warmer air most of the time and melts.
“Depending on the temperature of the air near the ground we either see rain or sleet or hail. However the freezing level (usually the boundary at which precipitation will fall as snow rather than rain) doesn’t just stay the same every day or even within a day. Sometimes it can change hour by hour, across the country, or even a few miles down the road.”
They say there are several things forecasters look at to predict how low down the precipitation will fall as snow:
- Where the air has come from: If the air has come from a slightly different direction, say for example if it has spent a slightly longer time travelling over mild water, then the air can be ever so slightly warmer and therefore the freezing level would be slightly higher.
- Very heavy precipitation: Even if the freezing level is quite high up in the air it turns out that really intense precipitation can drag the “freezing level” nearer to the ground and keep any precipitation falling as snow lower down too. Prolonged heavy rainfall can therefore turn to snow.
- When warm air meets cold air: This is the most difficult situation to predict snow. Weather fronts mark the boundary between cold air and warm air so when a weather system moves in there will be warm air and moisture wrapped up in it. While we need the moisture for snow to form the warm air makes it very tricky to forecast if it will be turning to rain. As the warm air bumps into the cold air it slides up over the top (warm air rises). The precipitation falls into the colder air lower down but in time the air mixes together and makes the cold air slightly warmer and the warm air slightly colder and so then rain becomes more likely. You’ll often find that there’s a fine line between who sees snow and who sees rain. Sometimes a fraction of a degree is the difference between rain and snow. That’s what makes forecasting snow difficult..