"Jon O'Rourke" wrote in message
...

"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.

Don't like to steal Martin's 'thunder' :-) but I was wondering whether
it
was a split cold front with the showers mentioned developing on the
surface
cold front and the main band on the forward upper front. Will be
interesting
to see what's on the 12Z analysis.


.... yes, it certainly is marked. All I can come up with atm is that the
00Z Watnall ascent (can't see the 12Z yet) which was in the warm air at
that time, *would* produce deep moist instability to the surface
temperature/dew point regime just ahead of the surface cold front - i.e.
Air temp ~19, dew point ~16 or 17. The key would be the surface temp
getting high enough as the AS thinned behind the upper cold (as Jon
noted above), allowing surface-based convection to be initiated and
'focussed' ahead of a PVA-maxima swinging around the upper trough (which
seems broad on the low-res charts I have available - so it will almost
certainly contain minor lobes of +ve vorticity) to both help initiation
and force the resultant convective elements to run 'in the flow'. The
thing we have here that _perhaps_ is not always present is the lobe of
notably high surface dew points that has been advected ENE'wards (as
noted in another thread).

That's my starter: now bring on the clever boys and girls!

Martin.