Having only just got my hands on the Nottingham ascent for 1200, it seems
that the ingredients for the rapid convective development lay in a
combination of potential instability and low level sensible heating and
convergence.
The theta w for the ascents 03354 and 03882 are given below, together with
an interpolated intermediate location. Although the low level high theta w
air was well capped by post frontal subsidence at 03354, the strength of
this capping decreased southwards across the frontal band, although the
potential instability also decreased southwards, it would still have been
significant at the intermediate location. It is likely that very high theta
w air had detrained to the north of the frontal band in the boundary layer,
and insolation through the thinning upper cloud would have increased this
further. Local topographic forcing and flow convergence, possibly at post
frontal discontinuities parallel to the front, acted as 'micro fronts'
providing further positive feedback on convective initiation process.
Seeding from the overhanging upper cloud may have been a modulating
influence on the shower intensity.

Theta w
03354 03882 INTER
P
940 12.5 15.9 19.0 (after heating)
900 11.8 16.2 14.0
850 12.0 16.2 14.1
800 10.1 15.7 12.9
700 10.8 17.7 14.2
600 12.0 17.1 14.5
500 13.7 18.1 15.9
400 15.9 18.4 17.1


--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html



"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
Unusual line of very sharp showers shows up on the 1300
radar stretching across south Oxon, Bucks and Herts.

I've seen this before, though rarely, when vigorous
cumulus penetrates the overhanging altostratus shield
of an ana-coldfront.

Martin ... mechanism please !!

Philip Eden