When will snow fall? Why snow forecasts can be unreliable

During the winter season, many of us might be thinking of when snow will fall in your area. But here is why you shouldn't believe all that you read.

Snow is actually very hard to forecast and the bottom line of a snow forecast is that you should never believe one that is for more than three days ahead. Even then, forecasts for one day ahead can be incorrect. Let us explain why.

The temperature of the air is perhaps the most crucial aspect when looking at a snow forecast. Generally, when temperatures are below 2°C, snow is possible with temperatures less than zero bringing more dry and powdery snow due to the lower moisture content.

While this sounds simple, whether the precipitation actually hits the ground as snow depends on the temperature of the air it's falling through.

Most precipitation first falls as snow from clouds, even sometimes in the summer months because it is so much colder higher up in the atmosphere. If the air temperature is low enough throughout the snowflakes fall it is likely to stay as snow.

However, if the snow travels through a warmer patch of air, it will melt and turn into rain or sleet. So while the air on the ground might be cold enough, temperatures further up could be slightly different meaning it won't hit the ground as snow.

Altitude is another important aspect. For every 100 metres in altitude, temperatures generally fall by 1°C. This explains why mountaintops often see more in the way of snow during autumn and winter compared to areas at lower levels.

Thirdly, location is important. If you live in a city or on the coast, it’s likely to be warmer than the surrounding area, so the snow could be just a few miles up the road. The same goes for it generally being colder at higher altitudes.

It isn't just temperature, altitude or location that influences whether it will snow or not, wind is also a factor.

A sudden change in wind direction can mean you will see either heavy snow or rain depending on where the wind has come from. When our winds are from the east during winter, the wind often carries dry and cold air, with snow more likely in the east.

The freezing level can change suddenly if the wind direction changes, altering where the air has come from or how long it has spent over the sea. If precipitation is falling heavily, it can drag down the freezing level down, turning heavy rain to snow.

Additionally, the track of a system can also affect where exactly it snows. With low pressure systems this week carrying milder air with them, a slight shift in the location of the system could alter the precipitation type.

If the temperatures well below freezing, then snow becomes easier to predict. But in the UK and Ireland, our temperatures are usually nearer freezing, and a fraction of a degree is all that separates rain and snow, making it one of the most tricky parts of a forecast.

You can see when we do forecast snow where you are using our WeatherRadar. We show snow using the colour pink, where the darker the shade, the heavier the snow.