Convective Outlook: Wed 23 May 2018
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 23 May 2018 - 05:59 UTC Thu 24 May 2018

ISSUED 21:10 UTC Tue 22 May 2018

ISSUED BY: Dan

... WEDNESDAY DAYTIME ...

Very little lightning activity is expected during most of daylight hours, though elements of elevated convection / mid-level instability will advect westwards across East Anglia and SE England at times, bringing occasional showers from high cloud bases here and hence a low (non-zero) risk of lightning. Much of any precipitation will tend to evaporate before reaching the ground.
If enough of a sea breeze / orographic forcing can develop, one or two isolated heavy showers may be possible during the second half of the afternoon over southern coastal counties of England, steering winds then pushing them offshore to the English Channel - however, this is likely quite a low risk, with sea breezes struggling to develop due to a strong offshore gradient wind.

... WEDNESDAY NIGHT ...
Advection of warm, moist low-level air from France into SE England during the evening and night hours, combined with subtle cooling / forcing aloft, should gradually increase mid-level instability and hence coverage of elevated convection - initially over SE England, then expanding west or northwestwards through the early hours of Thursday.

The exact timing / overlap is crucial in determining how quickly this destabilisation takes place, and therefore how widespread thunderstorm activity may be during this forecast period (and also the exact areas that may be affected). In general, elevated thunderstorms are more likely to develop / be more widespread later in the night - so maybe fairly isolated / well-scattered over SE England, but perhaps a little more numerous further west over central southern England etc.

A SLGT has been issued where the best multi-model overlap exists, though it should be noted this may need adjusting / expanding northwards (as per ECMWF) or southwards (as per EURO4) depending on trends during Wednesday. One main precluding factor could be rather weak shear, which may result in messy convection where an individual thunderstorm cell may not last a particularly long period of time.