To understand this, lets look at a simple bit of science and weather.
Snow forms at certain times of year when water vapor (millions and millions of teeny water drops), is lifted into the sky to areas of sub-freezing temperatures. The vapor firstly condenses, and then freezes. Importantly, the frozen water droplets have to freeze slowly to make very unique crystallized shapes i.e snow flakes.
But this can only happen at cloud level, and not on the way down to the surface or at the surface. If the unfrozen water droplets freeze too fast, and drop quickly you get hail or sleet! You also need the high level air to be thin (like on top of a mountain).
So, you need 4 things for snow to form
1) Thin air 2) Sub-freezing temperatures at cloud level. 3) An abundance of water vapor. 4) A mechanism of “transfer and lift”. Transfer and lift is cold air hitting warm air and, the warm air being forced on top of the cold into a wedge shape. If parts of Wales cannot provide any one of these, snow will not form. That’s the end of our little science lesson.
So how does that fit it in with forecasting?
Have you ever noticed that if you make a Chilli con-carne, yours always tastes different to someone else’s? Same basic ingredients, yet different taste. It’s because your chilli powder may be just a tad different to theirs, they may also cook it longer and at a hotter temperature than you. Just a pinch too much chilli and it doesn’t taste right.
This is rather like snow, the ingredients have to be EXACTLY right to create snow – and yes you do get dry snow and wet snow and small flakes or large flakes, depending on the exact mix. This makes forecasting snow quite hard.
But here in the UK, especially on the west coast it gets harder to predict where the snow will fall or even if it will. The problem is, we are an island surrounded by rather warm water – especially so on the west coast which carries the warm Gulf Stream.
To the East of Britain we have a massive land block, to the south we have Europe, and to the West we have Ireland. These coupled with warm water on the west, colder on the east, and even colder to the north, means that all the air really gets mixed up as it hit Britain – and depending on which is the stronger force at any one time, determines who wins. 100 miles either way makes a LOT of difference to the mix (remember our Chilli?).
Phew, so after all that you still have to consider the actual temperature of the land onto which it is falling and the air just above it, (how dry or wet it is will also determine if it lays) because that could also make the difference between snow and sleet.
Here on the West Coast so close to the warm sea at low land height, it becomes rarer to get snow and SO much harder to predict it. This is why you often get contradicting forecasts and last minute warnings, or no snow at all. On top of all that, there are three main weather computer’s we watch, and often they are at odds with each other due to the complexity of the detail.
If you want to accurately forecast snow there is one assured way to get it right. It’s called Lamp Post Watching. Yep you got it. Open your curtains at night, look at the nearest street light and away you go! Thats what all us weather geeks do!
Hopefully you will now have some understanding of why accurately forecasting snow is so hard.