18Z cosmo looks the same as the 15Z.

No visible upgrade.

Bit it is a argument sublimation because of low DP and wind speed variants normally associated with cold spells from the east.

when you use your AC on your car it’s is the dryer air and increased fan speed that evaporate the moisture on the screen similar effect to that of ice outside exposed to similar theories 

 Both posters have valid argument imo

Back to the models.

This will be the most well-forecasted end to a cold spell if it occurs as shown  at the moment, if it was the other way around we’d all be urging caution.

saying that the GEFS ensembles has had cross member agreement for many runs now that a gradual warm up will occur and be maintained from the end of the weekend into fantasy island.

before that I’m quite interested if ground temperatures will increase back to near or above average though, especially with clear skies overnight and a good frost.

I really hope these don’t come off though.  If It does I’m looking for the warm up to end after a few days with a pattern shift back to colder, that’s been the case on many occasions this winter. 

As ever more runs are needed 

I haven't worked through the calculations but the effect of sublimation will be very small compared to melting. Where sublimation does come into play is on the surface of snow flakes as they fall through the air. Thin films of water with a large surface are will evaporate and keep the flake solid. Its partly why its snowing everywhere despite temps being sometimes above freezing.

Anyway the dominant factor will absolutely be melting in the strong sun.

To be honest Q if it was a exam I would use  both SD and Brian’s as part of the answer.

Yes basic principle you have used here is correct in that going directly from the ice to vapor and bypassing the liquid phase entirely.

ie Sublimation 

In these situations a better measure is not the 9c difference but the 100 miles difference in the positioning of the warm front.  At 5 days away that's really nothing at all in model terms.

Sublimation?

Evaporation?

Melting due to air temp?

melting due to indirect sunshine?

Who knows?

What I do know is that most of the 2cm of snow that fell overnight and was still there at midday, suddenly disappeared as soon as a heavy snow shower came along, trying to deposit more but ended up melting everything that was on concrete slabs, roads, drives, cars and rooftops.

We since had another shower about 3pm and this has restored the 1-2cm dusting we had this morning.

we had blue sky and sunshine between the showers today but no direct sunlight is possible anywhere in my back garden at this time of year.

If there had been no melting or sublimation we would be somewhere between 5-10cm which is present in certain spots on foliage and raised plastic garden furniture.

Plastic argument is complicated because it has always disappeared from my wheelie bins. But remained on my garden furniture.

Having been in Canada, at latitudes equivalent to the south of France, and seen the effect of the sun when the air temperature was a lot lower than it's been here, I think that's essentially correct.

Speaking topically.....looks like a sudden battleground breakdown seems to have evaporated from the models.

these snow showers are pants

Nope. Not in this case. 

UHI and/or warm earth. Very simple really. 

That might be true for where you are but we haven't had even a minute's sun since this cold spell started. I've got scientific evidence that I'm right for this location.

I tend to use scientific and factual visual evidence - but then that's just the old fashioned scientist in me!

  I think a few days ago I queried this with you but you are absolutely correct, for this time at our location. Soil type may of course be a factor. Saturated clay will be close to the day night average and only change slowly under the surface so given the lack of frosts in the last few weeks it will be a factor.

Where our air is coming from by Sunday.

Isobars can be deceiving. For the vast majority of the UK its Bavaria. 2m temps in south Germany are between -5C and -10C during the day.

SW of Devon and NI its either the Canaries or the Mediteranian. The 18Z icon is a subtle upgrade on the 12Z which is keen to bring the orange source across the UK much quicker.

The red source doesn't really get anywhere mind you even on the 12Z.

If it is sublimation why are puddles forming?

Sublimation is definitely occuring but really it's honestly not that much when the temperature is around 0C.

Again, if the snow is only dissapearing between 11am and 2pm then its clearly not the dominant factor. The dominant factor is without doubt the strong sunshine.

None of you complained about this during the frontal snow events. Weaker penetrating power through clouds. None of you complain about this happening at nighttime.

Absolutely. For example, Epping Forest is entirely London clay. It’s not risen above freezing here for 50 hours. Yet when I went tobogganing earlier there were boggy patches in the valleys, the snow and the the ice weren’t thawing at all but were serving to insulate the warm earth beneath.

 Yes. Conditions in the last couple of days have been very similar. 

I can remember learning about sublimation as a schoolkid in the 80s and there were several opportunities to see it in action. What we had here yesterday was definitely not primarily sublimation. The wet surfaces and puddles told the story. I'm sure that with air temperatures being close to or below 0C an amount of sublimation was happening, but it was not the main cause of snow loss.

Bare soil is darker and radiates heat quicker.  Grass on the otherhand would potentially insulate the soil underneath and radiate slower thus retaining a higher soil temp than bare soil on its own.

 LLTNP 105m asl 

Just to make clear that the snow melt here yesterday took place under solid cloud cover. That was the reason I advanced the idea of heat being radiated back by the clouds.

We are not talking thick covers are we?

The grass melts faster because there are blades of grass sticking up, whereas soil forms a homogenous surface that is white and reflects the sun's rays.

I bet the surviving snow patches are on low lying beds completely protected from the south and the sun.

I've noticed often that, when there's still frost on the grass in the shade on a sunny morning when the air temperature has reason a couple of degrees above freezing, as soon as the thinnest of clouds passes overhead it immediately melts.