Thanks Will for that explanation. Apologizes I misunderstood, I thought you were also referring to the cold Canadian air pushing through with the rain perhaps turning back to snow in this air mass.

As a rule of thumb if there's a cold continental undercut I use a zero degrees 850mb temperature as guidance, anything higher and you may actually get freezing rain, if the undercut is cold enough. If there's ocean air at the surface, like we have today, I'm more conservative and use -9c as guidance as this should give a surface temperature of zero allowing 'proper' snow. I found anything higher usually ends in disappointment and a unpleasant slushy mess.

Oh well at least it isn't mild!


On Monday, January 14, 2013 10:01:43 AM UTC, wrote:
"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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