Convective Outlook: Fri 27 Jul 2018
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Fri 27 Jul 2018 - 05:59 UTC Sat 28 Jul 2018

ISSUED 21:39 UTC Fri 27 Jul 2018

ISSUED BY: Dan

UPDATE 21:39 UTC No major changes, SLGT reshaped over SW Britain to cater for slightly quicker motion of secondary cold front compared to expectations 24 hours ago, and to extend the SLGT farther west over Scotland to cater for ongoing thunderstorm cluster in SW Scotland

... FRIDAY MORNING ...
Elevated thunderstorms will likely be ongoing on Friday morning along the ThetaE plume from Cen S England, through eastern England to perhaps the Northern Isles. These should continue to drift north along the same corridor through the day, but probably with some reasonable gaps developing as forcing aloft and instability weakens temporarily in the wake of each thunderstorm cluster.

... FRIDAY AFTERNOON / EVENING ...
Strong surface heating with temperatures in the mid 30s Celsius and dewpoints of 18-20C will create a very unstable environment by the afternoon, with CAPE values in excess of 1,500 J/kg possible. This combined with elements of low-level convergence and stronger upper forcing arriving, as a shortwave drifts northwards on the forward side of an approaching upper trough, should provide the trigger for scattered surface-based thunderstorms, particularly during the late afternoon and into the evening hours. There is also some signal for orographic forcing during the evening hours to allow a few thunderstorms to develop over the northern Pennines, drifting north into southern Scotland, hence the inclusion of a small MDT.

Given the magnitude of instability, strongly-sheared profiles and dry intrusion in the mid levels, a supercell or two will be possible capable of producing large hail over 3cm in diameter along with damaging outflow winds and very frequent lightning. Some surface water flooding could occur in a few places. As such, a SVR has been issued to cover the risk of large hail, damaging winds and torrential downpours.

... FRIDAY NIGHT ...
On Friday evening and night, increased forcing aloft will destabilise the instability plume more widely, with an increasing coverage of elevated thunderstorms likely to erupt over eastern England, gradually clearing to the east - perhaps bending back westwards over parts of northern Scotland and the Northern Isles. A MDT and HIGH have been issued to cover this potential combined with earlier surface-based storms - though should model guidance trend eastwards, the focus will naturally shift eastwards to the North Sea instead, and the HIGH may need to be removed / MDT trimmed.

... SW ENGLAND / S + W WALES / CELTIC SEA / S IRELAND ...
The secondary cold front will be subject to strong forcing as a sharp upper trough over-runs it during Friday night. Several hundred J/kg CAPE will be present, with some lightning activity and gusty winds possible as the cold front surges eastwards. A SLGT has been introduced to cover this potential.