Convective Outlook: Sat 27 Aug 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 27 Aug 2016 - 05:59 UTC Sun 28 Aug 2016

ISSUED 12:25 UTC Sat 27 Aug 2016

ISSUED BY: Dan

UPDATE 12:25 UTC Scattered thunderstorms, initially elevated, are developing along the frontal boundary, but the environment remains conducive to severe thunderstorms through this afternoon and evening, perhaps supercells capable of producing large hail and perhaps a (noteworthy) tornado within the SVR area

This particular plume destabilisation event carries an unusual amount of uncertainty, even at this short range. Below is the most likely forecast evolution based on guidance at the time of issuance - it is certainly possible some aspect will change with time, and we may not be able to update as this is a voluntary service.

Elevated convection is expected to be ongoing with isolated to widely-scattered thunderstorms (albeit with not much precipitation reaching the surface initially) across parts of the south Midlands, East Anglia and southern England at the beginning of this forecast period, through isentropic upglide as the warm front lifts slowly northwards across England and Wales. This process is expected to continue during Saturday, with thunderstorm coverage (or at least showery precipitation) expanding through the day, mostly across the southeastern half of England - although even here some areas are likely to remain void of precipitation/thunderstorms (taking a theme amongst most NWP output would suggest parts of E Kent up into Suffolk and E Essex are least likely to see thunderstorm activity - but models should never be relied upon to that level of precision, hence are still included in the SLGT). PWAT values in the high 30s or low 40s mm brings the risk of localised flash flooding issues. MDT has been issued where the best overlap between numerous models suggest thunderstorms may develop - although this zone could be shifted to the NW.

Along and south of the SW-NE orientated warm front, environmental conditions will be favourable for severe convection should any storms become rooted in the boundary layer. If this were to occur, then large hail to 3.0cm in diameter and possibly a tornado would be possible, especially close to the frontal boundary where low-level convergence will be maximized. This places the risk of severe convection greatest in a corridor from SE Wales / W Country / Cen S England NE-wards across the S + E Midlands to East Anglia. Extensive cloud may inhibit this potential to a certain extent.

Through the evening hours, the approaching Atlantic upper trough will serve to increase convection coverage across N Wales initially, this also extending across northern England. Given the two zones with greatest potential (the MDT and this second zone through the evening hours), this then may leave a corridor of relative low activity from C + E Wales through the N + W Midlands - not to be taken literally, but emphasizing that some parts of the SLGT are likely to remain lightning-free.