Convective Outlook: Wed 14 Sep 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 14 Sep 2016 - 05:59 UTC Thu 15 Sep 2016

ISSUED 20:00 UTC Wed 14 Sep 2016

ISSUED BY: Dan

UPDATE 20:00 UTC Isolated storms over the Midlands have decayed, but the area of instability will continue to drift NW out over the Irish Sea tonight. Still rather low confidence in anything developing here, and so the SLGT is considered very marginal at the moment. Some high-res models suggested sporadic thundery outbreaks may develop locally over C / W Scotland during the early hours. SLGT introduced to English Channel and Channel Islands as remnant thunderstorms / thundery rain may affect the area later in the night, with some uncertainty as to how far north - several models bring ppn as far north as the M4 corridor

UPDATE 08:20 UTC SLGT shifted a little farther SE and LOW extended into extreme south coast, based on latest guidance. Parts of N/NW England, N + W Midlands and Wales need monitoring for the potential introduction of a SLGT - there could be isolated but explosive thunderstorm development if the cap ~850mb can be eroded late afternoon into the evening

Disrupting upper trough eventually becoming a cut-off upper low over Biscay, causing the flow across the British Isles to become increasingly backed and limiting any further advection of high low-level ThetaW airmass. As the trough continues to disrupt the main upper forcing will tend to slide south across Biscay/France, and consequently any remaining elevated convection at this start of this forecast period, particularly along the eastern periphery of the frontal boundary straddling E + S Scotland to E Ireland, should weaken and dissipate with time through the morning. One main exception will be close to Cornwall / S Devon where some lightning originating from elevated convection will be possible first thing, and this risk may continue for a while longer migrating into S Ireland.
Also worth noting that the GFS (and now with a little more support from some other models) has consistently signalled elements of mid-level convection over parts of SE England late Tuesday night into the first part of Wednesday morning. Whilst AcCas with virga etc will be possible from medium/high cloud, confidence is not high enough to introduce lightning threat levels here for now.

Aside from these risks, much of the day will be dominated by capping preventing any surface-based convection, and with little upper forcing to destabilise the mid-levels either. That said, models should not be taken literally and given an ingredients-based approach one would need to consider that an isolated thunderstorm could develop just about anywhere - perhaps moreso over Wales / W Mids / NW England with hints of low-level wind convergence and some aspects of orographic forcing. IF a storm can develop then it will likely be a prolific lightning producer with perhaps hail up to 2.0cm in diameter, given notably steep mid-level lapse rates and CAPE 1,400-2,000 Jkg-1, but the overall risk in any one location remains quite low. Hence main threats would be large hail and localised flash flooding.
There does seem to be a better consensus for more scattered thunderstorm development to occur over the Irish Sea on Wednesday evening, this then ultimately advecting northwestwards into eastern Northern Ireland and perhaps SW Scotland. Some uncertainty surrounds this potential, but have issued a low-end SLGT, if nothing else to make this area stand out more. Some medium-level convection may also be possible over the English Channel later in the night towards Thursday morning, but staying largely over France.