A supercell developed just east-southeast of Leeds and tracked across Tadcaster, Easingwold and Helmsley. Hail up to 3 cm in diameter was encountered during this chase.
A squall-line has developed just ahead of a cold front over the Pennines and tracked across Yorkshire and Lincolnshire. I decided to chase this front with my high-speed camera and managed to capture a few lightning strikes at 1000-2000 fps.
20. April 2012 was characterized by steep lapse rates and enough moisture for 300-400 J/Kg CAPE to be materialized. Most of the UK was under the influence of a mid-level trough with a broad surface low which was centered over the east coast of the UK in Lincolnshire, leading to weak flow and wind shear. Such weak flow conditions are favorable for vortex stretching, if vorticity can develop by some mechanisms, which can lead to spout-type tornadoes and funnel clouds (these are very weak tornadoes and funnels not associated with supercells). One mechanism that can develop horizontal vorticity are sea-breezes. These form when solar radiation heats up land that warms much faster than surrounding seas and oceans. The temperature contrast then forms the sea-breeze – a mini-cold front which then spreads inland and forms horizontal vorticity on its leading edge. The same front can trigger shower and thunderstorm activity the updrafts of which can tilt and stretch this vorticity to form funnel clouds and tornadoes.
Because of the conditions described above, I decided to chase storms along the east-coast sea-breeze in east Yorkshire and Lincolnshire. I took the M62 to near Hull and then went over the Humber Bridge to intercept some showers that developed into thunderstorms and produced a funnel cloud. A more detailed description of this event can be found here in Czech only!
-If CAPE is less than 100 J/Kg thunderstorms are very unlikely (unless they are supported by fast-moving cold fronts mainly in the winter half of the year)
-If CAPE reaches 100 – 300 J/Kg then there is a good chance of at least some lightning if convective showers develop, but the electric activity is generally short lived and sporadic
–If CAPE reaches 1000 – 2500 J/Kg then unless there is a strong convective inhibition thunderstorms are very likely. These thunderstorms can be long-lived and can organize into Mesoscale Convective Systems. Thunderstorms can be accompanied by hail larger than 1 cm in diameter and by frequent lightning. Values of 2000 J/Kg or more are rare in the UK but when such values are present they are nearly always accompanied by strong thunderstorms or large Mesoscale Convective Systems.
-Values of CAPE over 2500 J/Kg are very rare in the UK and do not occur every year. However, such values occur much more often in other countries in central and western Europe (e.g. France, Germany). If thunderstorms form in such a strong CAPE environment they will nearly always be accompanied by frequent lightning and very often by at least some hail. The strength of the thunderstorms then very strongly depends on the wind shear.
-Values over 4000 J/Kg are very rare anywhere in Europe, but occur at least once a year in the US. Thunderstorms are always very strong in such extreme CAPE conditions and can produce hail over 5 cm in diameter if mesocyclones/supercells occur.
-Values over 5000 J/Kg are rare even in the US and there is only a handful of cases of such a strong CAPE recorded in Europe. In the US, the highest values of CAPE ever recorded were approaching 10.000 J/Kg and it’s likely that such extreme values also occur over northern India south of the Andes.As already mentioned the intensity and degree of organization of storms also strongly depends on values of wind shear (how wind speed and direction changes with height). If there was a profile with 4000 J/Kg CAPE but zero wind shear (no wind between the ground and 6 km above the ground) then thunderstorms can be very strong but they will likely not persist for long or they will be disorganized and will weaken with time. During the initiation stage such storms may produce hail of 3-4 cm in diameter, but if there is no wind shear the hail and precipitation would fall into the updraft of the storm, cutting it off eventually.