Hello and welcome to this blog about snow. There is a chance of snow in the forecast for the Western Isles for next week and it’s certainly going to turn cold. There is a lot of uncertainty with next weeks forecast and I will keep you updated. I am getting a lot of questions about when we are going to get snow and if we are going to get a white Christmas.
How does snow form
The first place we start is how does snow form with this video from the Met Office.
In the Western Isles
Here in the Western Isles we don’t often get huge amounts of snow. Most winters we will get 2/3 “snow days’. This is where snow sticks on the ground and lasts more than a couple of hours. Quite often it quickly melts away. Last year 2020 saw a number of snow days.
Years like 2009/10 are quite rare we don’t usually get long spells of snow. Other notable snowy winter where 1995 and 1991 in the last 30 years.
In December on average the UK sees 3.9 snow days. 5.3 days in January and 5.6 days in February. With March seeing 4.2 days according to the Met Office. So you are more likely to see snow in March than December.
So why is so hard to forecast snow?
The Met Office explains all –
Why is snow forecasting in the UK difficult?
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.
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 up.
- 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 (and often frustrating!).
Stornoway covered in snow in 2018 taken by Catherine Kennedy
Avoiding clickbait and crap apps.
So did you see the headline of the blog and click on it without reading the description first? See this how clickbait headlines work and they entice you in. Weather stories and ones about snow are some of the most clickable stories. Hence why the Daily Express churns them out. Yeah I am still awaiting the 300 days of snow they promised a few year back. The articles tend to have grains of truth in them but the are mainly just huge amounts of hype. It is best to avoid anything about snow from newspapers or news sites. They more you click the more you fund there advertisement revenue.
Watch out for the so called crap app. These are mobile phone apps such as XC Weather or the iPhone weather app. These are called crap apps because they just ingest weather data with no human input. So you are seeing raw model data without the human input that understand the differences and the local variations of the forecast.
For example a few years ago 20CM of snow was forecast for several days by XC Weather in Stornoway. It that did happen I would struggle to stand in Stornoway Town Centre. I stood there in the rain with zero snow falling and zero snow had fallen that day. Yet people take this as gospel.
So where are good places to get snow forecasts. Because it’s so difficult to forecast more than a day of two out anything further then that is probably pushing it for accuracy. I will let you know if there is a snow risk on my updates. Met Office warnings, text forecasts and press releases are other good places to look. Netweather has in the past done good blog posts on the subject of snow as well.
A snow covered castle in Stornoway taken by Eric Mcmillan
White Christmas 2021
Will we see a white Christmas this year is a question I get a lot. It is far too early to know yet by a long shot. Our first White Christmas forecast will be issued on the 1st of December so look out for that.
We can’t end a conversation about snow without a time lapse video of snow from the USA
I hope you have enjoyed this blog update any feedback is welcome.