Convective Outlook: Fri 25 May 2018
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Fri 25 May 2018 - 05:59 UTC Sat 26 May 2018

ISSUED 07:39 UTC Fri 25 May 2018

ISSUED BY: Dan

UPDATE 07:39 UTC Forecast re-issued due to a technical error

Heavy rain with embedded elevated convective elements will be ongoing at the start of the forecast period over East Anglia / Midlands / Wales / northern England. This will continue to drift north while weakening, with any remaining sporadic lightning over East Anglia / Lincs soon decaying as instability weakens. 

In its wake, extensive low cloud and mist (with coastal fog) will cover much of southern Britain, though this should gradually thin and break with better insolation developing for the afternoon. Sea breezes will then develop provided there is enough surface heating, which will then provide the focus for a few isolated heavy showers / thunderstorms over southern counties of England.  

Over East Anglia, residual high dewpoint air (15-16C) combined with strong insolation and proximity to a shortwave trough should help yield up to 1,000 J/kg CAPE. Much of this will be capped, especially if surface temperatures fail to reach the low 20s Celsius (depending on how quickly the morning cloud clears), but sea breeze convergence may be enough to trigger an isolated intense thunderstorm. Warm noses aloft also raise some concerns over how deep convection may grow. Confidence is not particularly high, but a low-end SLGT has been issued to highlight the area of interest. While shear is weak, it may be sufficient to support some marginal supercell characteristics, capable of producing hail up to 2.0cm and perhaps some localised surface water flooding. Low cloud bases and backed low-level winds, combined with convergence, suggests the potential at least for a weak tornado.

On Friday night, a cluster of thunderstorms will likely develop over northern France eventually drifting northwards across the English Channel. While there is potential for the remnants to bring an increase risk of lightning to southern coastal counties east of the Isle of Wight, there is some uncertainty as to how quickly this activity may decay. A SLGT may be required if confidence increases.