Convective Outlook: Tue 13 Sep 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Tue 13 Sep 2016 - 05:59 UTC Wed 14 Sep 2016

ISSUED 09:21 UTC Tue 13 Sep 2016

ISSUED BY: Dan

UPDATE 09:21 UTC As expected, elevated thunderstorms are developing on the western edge of the warm plume, and the zone of potential will continue to shift north through the day. Still scope for severe thunderstorms late afternoon and through the evening hours over the highlighted areas IF storms can become rooted within the boundary layer. Large hail and heavy rain the primary threats, but gusty winds and perhaps a funnel/tornado not impossible. SLGT has been tweaked in places based on latest guidance and trends, including an extension into SE Scotland (as alluded to in forecast issued last night, below). Another round of elevated thunderstorms will be possible in Cornwall/S Devon tonight as the flow continues to back

As the sharp Atlantic upper trough disrupts over Biscay during Tuesday, backed mean flow will continue to advect a very warm, moist airmass northwards across the British Isles. The passage of various shortwaves running northwards in this flow may serve as the forcing mechanism to destabilise the mid-levels, east of the main frontal boundary, with scope for outbreaks of primarily elevated convection likely at times - beginning across SW Eng / S Wales on Tuesday morning, with this zone shifting north through the day. This may be accompanied by further medium-level destabilisation farther east towards Dorset etc, but some uncertainty as to how far east elevated convection may occur.
There is also some reasonable agreement for another round of elevated thunderstorm activity over the Channel Islands / English Channel on Tuesday night, advecting into Cornwall and perhaps Devon.

Notably steep mid-level lapse rates and MLCAPE 800-1,200 Jkg-1 suggests any storms that do develop on Tuesday, especially farther east, will produce quite frequent lightning. 

However, given rather weak forcing, the biggest uncertainty is coverage, and this is the primary reason for refraining from issuing any higher threat levels - it is quite likely that some or many areas within the SLGT could remain void of lightning. It is difficult to portray the balance between the likelihood of a shower producing lightning (which is high-very high) vs the likelihood of a shower occurring in any one location (low-moderate). 

If any convection can become rooted within the boundary layer, then heavy rain and large hail up to 3.0-4.0cm in diameter would be possible - a SVR has been issued to highlight the area at risk of this. There are some suggestions for the potential (at least) of surface-based convection during the evening hours over N Wales / NW England, particularly where elements of orographic forcing from upslope flow over the Pennines may a play a role etc. Forecast soundings suggest a fairly substantial warm nose (750-900mb) will need to be eroded to achieve this.

Given the large amount of uncertainty, this forecast may be updated as confidence improves - and it is possible that the SLGT may need extending into parts of S Scotland, for example.