"Alan (North West Surrey)" wrote in message
...
Will, wouldn’t cold air from the west have to be cold thru the depth, since
there won’t be a surface inversion layer? I would guess an 850mb temperature
of -6c would produce at best a sleety mix. You would need to go down to -8c
before getting proper snow. Are we likely to see such cold air from the
current set up?
====================

It's not cold air from the west per se that will produce snow. It's actually
the warm occluded Atlantic air rising over a cold wedge. 850s could well be
around 0C and it will snow heavily if the surface air wet-bulb is close to
or below zero. Generally the boundary layer temperature structure approaches
isothermal in these situations at around 0 to -1C. In those situations
snowflakes will be large, wet and sticky, giving rapid accumulation. Of
course, it all eases off and turns to rain quickly once the surface wet-bulb
rises above zero when the front comes through and surface Atlantic air
arrives. Forecasting that timing and location is the tricky bit not whether
it will rain or snow. Trough disruptions means that fronts will tend to
"slide away SE" as they approach the block.

PS as a general rule of thumb for lowland UK I take -5C at 850 as a good
snow line. -3C for my altitude at 1000 feet.
The best indicator though is wet-bulb freezing level. At sea-level you need
that at around 200 metres for frontal snow. Obviously showers are different
as they are normally falling in drier airmasses and the wet-bulb freezing
level can descend quickly in heavy precipitation and lightish winds.

HTH

Will
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