Tonights StormsAdded : Friday at 8:46 Quite a complex discussion coming up for a Friday, so put your glasses on, look intrigued and grab a cup of coffee as we enter the realms of inversions.
First of all check out this chart from Arome for this afternoon :-
You can see the warm temperatures across parts of London and other cities with the cooler air across higher ground of Dartmoor. Pretty normal right?
Now check out the chart for midnight tonight :-
Hang on snowflake?! What's going on here? Why is Dartmoor warmer than its surroundings at night? Not just that, but check out the North Downs and South Downs too.
The reason is that an inversion has developed across the UK where the air at higher levels is warmer than the air near the surface. This provides us with blue skies and sunshine as air heated near the surface rises but then finds itself cooler than its surroundings and returns back to the surface. The inversion is also known to meteorologists as the CAP.
Later this evening and we have a cold front approaching from the West :-
Just ahead of the front is a destabilising trough which messes with the CAP and allows the atmosphere to release all that energy stored near the surface.
This is likely to happen across Southwest England and South Wales overnight tonight. These features allow elevated thunderstorms to form very quickly and this mornings soundings are backing up what we are expecting.
Arome too is picking up on this now and shows the storms developing during the early hours of Saturday morning and running Northwards into South Wales :-
Now we get a bit technical... The soundings show us that the mid level CAPE of these systems could release something in the region of 1,000-1,500kj of CAPE with something like 40-50mph deep layer shear. This is enough to tilt the storms and keep them active as they push North.
Add in the vertical velocity of 30-40mph at mid levels and the risk of moderate hail of 2-3cm in diameter is a possibility.
Into Saturday and the risk heads East into the Midlands, but we are still a little unsure as to how strong the CAP is for Central areas, but we will have a better view of this later today when the latest soundings come through and take a look at that later.
The main risk tonight though is some rapidly developing cluster storms for Southwest England and South Wales overnight which will be elevated and bring the risk of sheet lightning and moderate hail stones.
At least now you know why.
METEOROLOGIST : MARSH |