Convective Outlook: Sat 21 Apr 2018
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sat 21 Apr 2018 - 05:59 UTC Sun 22 Apr 2018

ISSUED 19:59 UTC Sat 21 Apr 2018

ISSUED BY: Dan

UPDATE 19:59 UTC MDT removed from N England, while MDT over southern England extended NE-wards
Activity over N Wales / Irish Sea and West Country are both further west than earlier model guidance - hence the focus for thunderstorm activity has shifted more to Cumbria, southern Scotland and perhaps the east coast of Scotland. Some thunderstorms are still possible over other parts of northern England later this evening and overnight as better forcing approaches from the west
Farther south, as the mean flow continues to shift from the S to the SW, latest guidance suggests clusters of elevated thunderstorms will continue to drift NE-wards within the MDT area through the remainder of the evening and overnight. Some uncertainty as to how far north lightning activity will persist with each thunderstorm cluster

UPDATE 13:55 UTC MDT shrunk in size over N England, while MDT added to English Channel and adjacent counties of southern England / Channel Islands.
Mid-level cloud and showery bursts of rain continue to drift north across the Midlands this afternoon. While an elevated thunderstorm is possible from this area of mid-level instability, coverage is not anticipated to be widespread enough for a MDT for now.
Surface observations suggest low-level wind convergence is beginning to develop over N Wales, and should continue to become more pronounced this afternoon, while expanding into NW England. This combined with increased forcing aloft should allow a few surface-based thunderstorms to initiate from about 17-18z onwards, initial focus over N Wales and then gradually shifting NE-wards across northern England during the evening hours. Particular focus is given to the western half of this MDT area.
An increase in coverage of elevated thunderstorms seems possible late afternoon and into the evening over the English Channel, drifting NE into portions of southern England. Some uncertainty as to how widespread this activity will be, with some suggestions it may begin to decay as it approaches the M4 corridor, hence limiting the northern extent of the MDT for now. Other elevated thunderstorms may develop elsewhere over the Midlands and East Anglia this evening/overnight in a rather sporadic fashion, perhaps even as far north as E Scotland

... SATURDAY MORNING ...
Elevated showers and a few thunderstorms may develop on Saturday morning over the English Channel and drift north, as high ThetaW airmass advects from France into S / SW England and begins to destabilise. There is some uncertainty as to how much destabilisation may take place at this stage of the day given only subtle forcing, and then the exact track of such storms given large model guidance spread - generally Devon and Dorset, but perhaps as far east as West Sussex and as far west as Cornwall. Any storms that do form will track to the N or NW with time, into S Wales / SW Midlands by late morning/midday, though possibly with a weakening trend. Initially, precipitation may not reach the ground due to very dry air through depth, though subsequent showers will help moisten the column with time.

... SATURDAY AFTERNOON ...
A complex picture, with ongoing elements of elevated convection drifting north associated with plume axis across Wales and the Midlands. Forecast profiles exhibit a capping warm nose around 850-900mb; however, provided there are enough cloud breaks to provide sufficient insolation, then there is scope to erode this cap and allow surface-based thunderstorms to develop. The focus is primarily over N + E Wales, N + NW Midlands into northern England, aided by developing low-level wind convergence and orographic forcing. This very much dependent on how much mid/upper level cloud will be present from earlier elevated convection approaching from the south. Initiation may be quite late in the afternoon and into the evening hours, when better forcing arrives. Uncertainty also exists over the north/south position of areas that may be affected, and the MDT issued may need to be adjusted southwards, depending on trends.
Shear is a little on the weak side, but given the degree of potential instability (800-1,200 J/kg CAPE) hail up to 2.0cm in diameter will be possible from any stronger thunderstorms, along with frequent lightning, gusty winds (40-50mph) and some localised surface water flooding given 25-30mm PWAT.

... SATURDAY EVENING / NIGHT ...
Stronger forcing will arrive during the evening hours as the Atlantic upper trough approaches from the W - this will likely result in an increasing coverage of (mostly elevated) thunderstorms as the ThetaW plume continues to destabilise. This will happen in a rather sporadic fashion along an eastward-advancing N-S orientated line, and hence difficult to pinpoint exact areas where thunderstorms are most likely to occur.
There is some model agreement for thunderstorms to erupt over the English Channel and drift NE across areas from Hampshire eastwards to Kent - given the magnitude of instability, lightning will be frequent with any thunderstorms that do develop, with the potential for some hail from the strongest cells. However, due to the uncertainty in coverage of thunderstorms have refrained from issuing a MDT for now, but one may be introduced if confidence improves a little.
It is possible some thunderstorms could develop as far north as E/SE Scotland, or at least offshore from there over the North Sea.
In all cases, most activity will have cleared to the North Sea by Sunday morning.