Yes, an upgrade from chilly with rain, to cold with rain.
Originally Posted by: warrenb
Well, I'd say chilly rather than cold - 850s now get to the frozen depths of -3C in the middle of next week here, rather than the -2 of the earlier runs.
But wait, I hear you say, Q is all excited! Well, yes, you *can* get snow all the way up to zero 850s - I've seen it happen twice in the past 21 years - but it's darned rare here at least.
I can only speak about my local area on this, but here it only happens towards the end of a cold spell, when the boundary layer is thoroughly chilled and fronts "ride up" over the cold air. This happened frequently in the 80s and 90s, and gives the classic fine snow turning to think splodgy flakes [hence the old Kentish saying "when the snow is fine, there's more behind"], then it'll fizzle into drizzle as the front clears. The cold spell of 2005 - the last textbook easterly - was symptomatic of this, at the end it snowed right up until 850s reached zero, then turned to light drizzle, leaving behind a slowly thawing mass of snow.Â
I can't remember this sort of thing happening *without* snow already on the ground, though, and the crucial thing to me is all the previous events started off with deep cold air aloft a few or several days prior, which is why I look for -10s before I get excited. I suspect it's due to the nearby sea (half a mile away to the NE, or 33 miles to the south) causing things to mix out a little. Even in the 80s we'd look for daytime highs in Calais of -2 or -3 to predict an ice day here.Â
These current charts? I simply can't see that there's enough time for the boundary layer to cool enough - and become deep enough - to support snow down here. If I were a good way inland (think Reading) I might be more interested, but even then - I'd far rather see some proper deep cold air in place first.Â
The other side of the coin is the complete lack of frosts in the forecast down here. For snow to not instantly melt on impact and to accumulate you need all sorts of things, and a frosty ground helps no end when temperatures are still above freezing.
I'd also mention that the ICON, at least the charts I can see, only has a rain or snow precip type field, it doesn't seem to show sleet in the way ECM does. As such it falls foul of the same thing that GFS has, i.e. a flake of snow in the rain shades the area as having snow.