The 'loaded gun' in action: a synoptic discussion of the thunderstorms
which developed over England on Wednesday 10th May 2006
Summary
During the afternoon of the 10th May a large multicell cluster of storms developed
over the Chilterns and moved slowly west up the M4 Corridor to south Wales. Some very heavy rain generated some local flooding
in places and lightning damage was reported in both Wiltshire and South Wales.
The Storms developed in an upper and surface
level environment which was only marginally conducive to storm development but where topography and convergence played
a big part.
Discussion
The synoptic situation at 12Z on Wednesday 10th May is shown in Fig 1 (Surface) and Fig 2 (300mbs). It can seen
that a broad high pressure zone covered much of the area to the north and north-east of the UK with a slack easterly flow
covering the UK. Dry subsided air within the anticyclone had been making slow but steady progress south west during the preceding
couple of days from the source of dry air over Denmark and northern Europe, thus making the development of storms rather
uncertain.
Fig 2 shows the situation aloft
where conditions were less stable with some slight vorticity over Southern England and the Benelux countries.
A similar situation was present over Germany on Tuesday 9th - an environment
where high based storms were generated. The same scenario was looking likely
on Wednesday across Southern Britain.
Wednesday dawned with relatively clear skies across England & Wales. The
06Z and 10Z Larkhill Ascents both indicated the presence of a 'warm nose' between 800-900mbs, but steeper lapse rates
above this were associated with the minor upper trough stretching across the south of England. Dew points were running at
7-10C early in the morning with only a slow rise likely during the day as sea breezes developed but were compensated
by drier air continually being advected SW.
The warm nose effectively stops clouds from towering up until later
in the day when a certain trigger temperature is met which allows the convection to rapidly tower upwards and rapidly release
the large build up of Convective Potential Energy (CAPE) which is available aloft. These
late morning ascents indicated a trigger temperature of above 21C was required to release such large amounts of CAPE.
Temperatures steadily rose during the morning and by lunchtime were approaching
20C across inland Southern England. However, because of the lack of any decent amount of ascent across England another ingredient
was needed to initiate convection regardless of the temperature. In the absence of large scale synoptic ascent topographical
assistance is a good trigger mechanism- along with sea breeze convergence.
During the afternoon three areas saw rapid cloud growth: Devon and Cornwall;
South Wales and the Chilterns. In all three cases the trigger was the uplift generated by either convergence or topography
or both. At the same temperatures over and to the west of London reached 21-23C which was enough to break through the warm
nose and send cloud growth rapidly upward into the atmosphere.
