Beware over there the Beeb has forecast frost here so you'd better take
care.

I have no idea how it works but it seems that this sort of weather is
likely to be the sort of weather that causes tornados accross the pond.

Local effects go like this:

1) a deep layer of dry air aloft (represented by the relative humidity
field at 700 mb)

2) a lower layer of moist air where the mixing ratio is high relative
to the dry air aloft (indicated by the moisture ridge at 850 mb)

3) a core of strong winds in the 10,000-20,000-ft layer (displayed
as the axis of maximum winds), and

4) a low level of free convection normally below 650 mb [level of free
convection (LFC)].

The likelihood of severe weather increases as the strength of these
fields increase.
More especially the way they increase in relation to each other puts it
on the button.

More good stuff on he
http://ams.allenpress.com/amsonline/...ume=87&issue=4

Deep stuff but worth the wade through. (Bloody awful discourse, though.)