Convective Outlook: Thu 23 Jun 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Thu 23 Jun 2016 - 05:59 UTC Fri 24 Jun 2016

ISSUED 16:59 UTC Thu 23 Jun 2016

ISSUED BY: Dan

UPDATE 16:59 UTC Large mass of imported elevated convection, and general slow cloud clearance, has resulted in limited instability being generated, despite surface dewpoints near 20C locally. The threat of severe convective weather is now considered too low to warrant a highlighted area, with the main threat now being rain and the associated risk of flooding. Lightning activity fairly scattered also (rather than widespread) and so the MDT areas have been reduced to SLGT once again

UPDATE 08:40 UTC Early convection beginning to clear eastwards this morning across East Anglia / SE England, leaving a legacy of extensive low cloud, but this should begin to thin and break at times by the afternoon to give some hazy sunshine. Surface dewpoints are already 17-19C - and may climb 1-2C through the day - which combined with any surface heating that can develop could yield 700 - 1,000 Jkg-1 MLCAPE. Isolated to well-scattered thunderstorms should develop late afternoon along areas of low-level convergence, particularly the N-S orientated cold front near A1 corridor. Strong DLS should allow organisation, perhaps into supercells during first 1-2 hours, but eventual growth into a line seems probable as the risk transfers NE across East Anglia / Lincolnshire with time through the evening. Also a risk of further imported thunderstorms from the English Channel, although this signal less obvious than compared with yesterday. SLGT has been expanded in size, with a MDT issued to areas with a better chance of sfc-based convection later today. A SVR has been issued to cater for the risk of hail up to 2.5-3.0cm in diameter (particularly during the first couple of hours of initiation), strong, gusty winds and the ongoing flash flood risk given saturated ground from thunderstorms last night. The tornado threat is fairly low, but not zero. This does not mean everywhere within the SVR will experience severe convective weather, but that the environment is favourable for the strongest cells to pose a risk of some severe weather

Atlantic upper trough will continue to track eastwards through Thursday, while a high WBPT low-level airmass covers the southeastern quarter of the British Isles. There are two main areas of deep convection likely:

(1) As the upper trough advances eastwards, accompanied by cooling mid-levels, strong surface heating will yield 300-400 Jkg-1 MLCAPE across the Republic of Ireland and Northern Ireland. Scattered showers and a few thunderstorms will develop in an environment with 30kts DLS, with some organisation possible. As a result, hail up to 1.5cm in diameter is possible, along with gusty winds.

(2) A surface cold front marks the boundary of this very moist, warm airmass (typically east of a line from The Wash to the Isle of Wight), but with a large amount of uncertainty as to how extensive this air will be across the SE. This seems to be tied to the development of a small surface low during Thursday morning, with a more developmental low (as per 18z E4 and multiple GFS runs) likely to lead to better backing of the flow and advection of this high WBPT airmass across parts of the British Isles, typically yielding afternoon dewpoints of 19-20C - quite exceptional for UK standards. A less developmental low will restrict this advection (as per 00z E4), with dewpoints reduced to nearer 16-17C typically. This will play one of the crucial factors on whether deep convection develops on Thursday afternoon/evening.

One other crucial factor is cloud cover limiting insolation. Either way, it seems likely that elevated convection will be ongoing at the beginning of this forecast period on Thursday morning across eastern parts of East Anglia / SE England (questionable as to whether there will be any lightning activity left with this, though), this gradually clearing eastwards but leaving a legacy of cloud at all levels. It seems reasonable that sufficient breaks should develop to allow strong surface heating with 700-1,000 Jkg-1 MLCAPE, and given rather slack conditions with proximity to small surface low, sea breeze component (and other areas of low-level convergence along / near cold front boundary) will likely provide the focus for isolated to well-scattered thunderstorm development during the late afternoon and through the evening hours (note: potential for late initiation). Within a strongly-sheared environment, initial storm mode may be supercellular, especially where local backing of the low-level winds is a little more pronounced. However, low-level flow is not particularly strong, and given unidirectional winds aloft, it is likely that such storms will tend to line-out with time.

Moistening of the boundary layer through the evening hours, and local backing of the surface winds, may promote the risk of a tornado, but the main hazards will be hail up to 2.5cm in diameter, damaging winds and local flash flooding (given PWAT 35-40mm). This would certainly satisfy our SVR criteria and it is possible an upgrade to highlight this risk may be required nearer the time. Uncertainty over how widespread this activity will be (if any at all) precludes the issuance of a MDT for now. The overall situation then becomes messy through the mid-late evening hours, with elevated convection likely to be imported from northern France / English Channel to mainly affect the extreme east of the SLGT area.