When The Sea Is Cool...Added : Friday at 14:20 You would have seen some impressive showers head in from the South today with some heavy rain across Southern and Southwest England, in face, more thunderstorms are expected tonight. But, one of the things we pay close attention to is the temperature in the English Channel.
The AROME model has handled today well. You can see the rain across Western areas where the main forcing has been within the front :-
AROME also brings some heavier showers North later this evening and overnight :-
For thunderstorms, then we tend to think of warm land or sea in order to generate them. This is normally the case, especially for home grown systems. But, these systems develop in France and continue to cross the English Channel, which is a fair old way for storms to continue to develop... why?
There are two reasons for this. The first thing we look at is the entire temperature profile. So, you might expect surface temperatures of 30c to generate bigger storms than temperatures of say 10c, but it really depends on the temperature of the rest of the atmosphere. If at 10c a parcel of air is still 15c warmer than its surroundings, then it makes no difference how warm it is, but it's the difference which counts.
The other thing is the shear in the atmosphere. If there is lots of shear, then the parcel rising up is forces in another direction, think of it rising diagonally in the atmosphere as opposed to going straight up. This means that the systems can still tap into warmer air in Northern France as they move over the Channel, but tonights ones lack this shear which means that most storms will cut off the warm surface flow and become "elevated" by the time they reach us with hail being a specific risk.
That said, the chance of some decent cloud to cloud lightning displays along the South coast today and now you know why, it's not always a no show due to cold seas, but I certainly wouldn't go for a swim in them at the moment...
METEOROLOGIST : MARSH |