Convective Outlook
VALID 06:00 UTC Tue 28 Jun 2016 - 05:59 UTC Wed 29 Jun 2016
ISSUED 20:25 UTC Mon 27 Jun 2016
ISSUED BY: Dan
A complex situation for Tuesday, with a broad upper vortex situated across the North Atlantic, and various disturbances running around its periphery. One such feature will bring a spell of showery rain on the forward side of an upper shortwave, this generally tracking eastwards across southern half of Britain through the day. A messy mix of dynamic/frontal precipitation with elements of mid-level instability is expected, the latter likely to erupt on the forward side (N Wales - N Midlands - N England - East Anglia), albeit with limited depth and hence questionable as to how much, if any, lightning activity may occur with this activity. A broad LOW threat has been issued to cover this potential.
Once early showery rain has cleared SW England / S Wales, modest heating of a residual SMZ with Tds 12-15C through the afternoon (assuming sufficient cloud breaks) will yield 200-500 Jkg-1 MLCAPE in a highly sheared environment. There is scope, therefore, for some elements of deep convection (heavy showers) to develop, perhaps focussed along distinct lines parallel to the mean flow.
Dry intrusion in the mid/upper levels will limit convective depth somewhat, so given the uncertainty surrounding the true lightning potential we have remained with just a LOW threat level at this stage. However, any convection that does develop will benefit from strong shear, and perhaps also local backing of the surface winds - given relatively low LCLs, there is scope for a tornado to occur. This situation needs monitoring but relies very much on a good overlap of parameters in a relatively short window, hence the uncertainty at this stage. Potential also for some strong wind gusts, this risk extending north into mid-Wales with any stronger convection that manages to develop, especially so on the wrap-around feature.
Elsewhere, a more classic situation of diurnally-driven showers given cold mid-levels and surface heating, yielding 300-600 Jkg-1 MLCAPE, especially across Ireland. Hence a scattering of showers and a few thunderstorms can be expected, with the best lightning potential within the SLGT areas. Hail up to 1.5cm in diameter may accompany the strongest cells. Overall a better overlap of relatively higher instability and better shear suggests lightning will be more widespread in the Ireland SLGT area.
