The synoptic development of the thunderstorms of Aug 3rd over Southern
England and the Midlands
A complex upper level and surface situation had developed across the UK by the afternoon of
the 3rd August. Thunderstorms had already been affecting the Midlands during
the morning with frequent lightning and some very heavy rain. Residual medium level instability was still present across
much of the north of the UK throughout the remainder of the day.
Thunderstorm activity weakened during the morning as the upper level trough relaxed away to the
NW. Further south across inland southern England, large areas of clear skies led to a rapid rise in temperature. 1000-500mb thicknesses reached 564 DAM in the SE during the day (as sampled by
the Herstmonceux 12Z ascent).
As temperatures rapidly rose, a light southerly sea breeze advected north aided by the slight
SE'ly flow. Marked boundary layer convection started to develop across Hampshire and Wiltshire around lunchtime. A mid morning sounding was requested from Larkhill (on Salisbury Plain) and it
indicated a slight cap or warm nose was present at around 800mbs, however the same sounding indicated that afternoon temperatures
would likely be sufficient to set off deep convection and that once
initiated there was nothing to stop the clouds reaching around 38,000 ft (around the 200mbs level).
Ironically the first showers developed around Larkhill itself and Odiham also by around 12Z
and drifted slowly north. Examination of the water vapour imagery from 12Z indicated a marked upper trough about to move
into southern England from near the English Channel, this associated with a marked shortwave upper trough and a significant
amount of PVA (Positive Vorticity Advection - dynamical forcing that can much assist the development of Intense convective
situations). The first sferics appeared about a half hour later over the same region as clouds rapidly rose and become deep
enough to support a few rumbles of thunder.
During the next 90 minutes the cloud growth was phenomenal. The 12Z Herstmonceux ascent (see the
tephi diagram on the right, click for the full sized version) indicated that the cap at 800mbs had been completely eroded.
The 12Z plot incdicated large amounts of CAPE were already present at 12Z but that storm initation required a trigger for
it to be released. The red line is the parcels trajectory at 12Z, the light blue line is a Normand Point construction based
on an afternoon temperature and dew point to the SW of London. It becomes clear that very significant and large amounts of
CAPE were available for release and that tops could breach the equilibrium level given sufficient forcing.
The forcing was provided quite early on as the afternoon progressed. The combination of marked PVA
from the upper trough and a slight convergence zone initiated rapid and deep cloud growth, Huge towering Cb's were now developing
across the M4 corridor to the SW of London and SE of Bristol -all about to drift slowly north. The 12Z ascent also indicated
the presence of dry mid level air, this assisted in hail formation and also by keeping the storm cells seperate and powerful
and assisting in developing "daughter cells".
A regional radar image from the south east at around 3pm BST (see image, right-click for larger
version), indicated intense thunderstorms had broken out across several areas.
Reports of frequent lightning and hail were observed along with isolated local flooding. During the next 2 hrs heavy or intense
thunderstorms moved slowly north whilst expanding over the Thames Valley. Meteosat 6 Rapid Scan imagery showed the rapid development
of the cells and EIR imagery indicated tops were exploding upwards towards the tropopause. Satellite cloud top temperatures
were at around -60 to -70C indicating that in fact the Cb tops were overshooting the equilibrium level.
The intense storms continued north during the evening as mopping up and a slow journey home followed
for workers in the capital. The cells eventually merged into an MCS (Mesoscale Convective System) over the south Midlands
during the early evening, moving north through the Midlands as the evening wore on whilst slowly weakening.
A classic example of a loaded gun scenario, and early morning slight cap eroded by strong surface
heating and upper level forcing was provided by a trough and marked area of PVA. Certainly an afternoon many in the Thames
Valley will remember for a while.
Report written by P Blight Oct 2004
