Snow, Moisture and... WarmthAdded : Monday at 13:45 It always tickles us when we hear "confident" forecasts for inches of snow for locations at sea level based on numerical weather output. You see, any meteorologist (someone who actually has to do this for a living) will tell you that snow forecasting for a maritime island such as ours is pretty much always borderline due to so many variables.
For this discussion we are going to take a look at this frontal system coming in from the Northwest over the next 24-36 hours.
Here is the front at the moment :-
A front is only a front when it has a temperature contrast either side. It's this contrast which allows vertical movement of the air as nature fights to restore the balance either side of it.
If you look at the upper air temperatures you can see this contrast up pretty well :-
If you check out the frontal system at 4pm this afternoon as it sits off the coast of Scotland you can see it falling as rain :-
But, this evening as the frontal system pushes inland it hits the colder air and turns to snow :-
Now, going back to that temperature contrast, as the frontal system pushes Southeast we find colder air coming in behind the frontal system and this reduces the temperature gradient across the front :-
And sure enough, if you look at the forecast for the front for tomorrow morning you can see it still falling as snow, but the front is now less intense and narrower too :-
This is pretty much basic physics and why temperatures are usually the number one reason for busted snow forecasts. One thing though which the model then picks up on is as the front pushes Southeast and reaches warmer water around the Wash :-
Notice how the frontal system peps up once again, this is due to low level moisture and warmth from the North Sea which slightly increases the temperature contrast and as a result the frontal system reintensifies for a time.
So, if you get a cm or two of snow across East Anglia/Southeast England on Tuesday night and into Wednesday you can thank the temperature contrast from milder water from the Wash as the front moves across.
METEOROLOGIST : MARSH |