Don't bother. Use met office !
Just say - I was riding in the -15 in Jan 2010 (and that's standing still) and I was, and it was 'one of those moments' - especially as the studded tyres didn't arrive till the next day, but there was no need - 1 foot of fresh snow - MTB party time.
No need - every car driver will know it was cold - as they couldn't de-ice their windows.
That was a seriously surreal 3 days. Day 1, I got 'good luck' from peds as I cycled down the pavement (only till I could get to the Trans Pennine - the cars were sliding all over) to "OMG a bike" - a quick smile and wave to some folk walking....
I must say the Jan 10/Dec 10 Winters were a blast - something I've not ever encountered on a bike. I've had very cold winters commuting 20 miles, but it was dry, so no road ice, just frost. Recent wet/icy roads have been lethal. I love my Snow Studs though.... and the availability of a 90's bomb proof MTB.