Yes could be very snowy if this came off~! - Of course this may not happen. I also find it a little strange how Thursday night/Friday's snowfall event for Devon seems to have evaporated on the 18z GFS Op run but then as you stated above - the GFS suddenly picked up on this snow even for the SW on Sunday:

Can anyone help me with an explanation?

First, this is the best snow since 2010 for me: Since Sunday night, it has not stopped snowing - even if like fine white sand or glassy diamonds at times, consistently -1 degrees, like some winter wonderland, dry fluffy snow heaven, (if useless for snow balls). I cannot remember how long its been the last time I have seen fluffy snow on snow during the winter proper, and such long lasting and stable a Scandi high. Its amazing. Probably 1991.

However, its a very odd phenomenon but the snow settles on the gravel and paving at night, remaining -1 degrees by day and night, and yet melts off the paving during the day, except for a leaf on the path, where it remains on the leaf like tuft, but on the adjacent paving, its wet, even though the  air is below freezing. It’s very odd, but is the stone absorbing light reflected off the snow and absorbing some of its energy? It’s an odd thing to observe. 

Heavy snow again now from this streamer - I dont need to look at the radar to see if its rain or snow - its like being on a skiing trip - just fanatastic...

Ground temperature. In air (or suspended into the air such as on grass, on a spikey mat, on gravel, on a deck etc) it sticks as the air is sub zero. On flat paving in contact with the ground it slowly melts as the sub layer is just above at the moment and well above the deeper you go. This isn’t Siberia - yet! If it’s heavy it will accumulate as the melting itself will draw a bit of heat from the ground and lower its temperature, if only temporarily. There’s other processes to consider but this is probably the main one.

The message this morning is a prolonged cold spell is less likely.

Edit: Of course, you don’t have to check the NWP to know that. Just look at how busy (or quiet) it is on this thread.

enjoy retirement!!

I’m not saying the milder air isn’t going to succeed because this morning it is the form horse but I just don’t see it happening as swiftly as the models are saying..

General expectation of a battle between mild Atlantic and cold Continent this week. Yesterday most soufces were backing the cold to continue; this morning there's a flip to mild. And overnight flips don't give much confidence in the outcome.

Jetstream well S of UK at the moment; it gets closer at the weekend and then enters a week or so (to Wed 17th where there's a strong jet across the Atlantic only to fragment as it approaches the UK with occasional bits flicking north across Britain. Traditional strong throughflow rebuilds, still only  over the S of England, and is in place by Mon 22nd.

GFS builds on this concept, keeping LP on the Atlantic and not progressing, as yesterday, but unlike yesterday the HP over Scandi is much weaker and winds are now mainly from the S, not E or SE, from  Thu 11th through to Mon 22nd when the Atlantic fires up again and pushes a couple of (smallish) depressions across Scotland

GEFS temps rising to seasonal norm by Sun 14th and quite good agreement for a slow rise (in Scotland just levels off) through to Mon 22nd before dropping back slightly. More runs with some rain from the 14th, but still not large amounts. 

ECM agrees with GFS and goes on suggest a positively balmy S-ly on Fri 19th with air starting from somewhere S of the western Sahara. First plume of the year? Snowmelt problems for Scotland?

At least a few GFS Perts showing a return to cold, I.e. 6-7, so not total abandonment just yet, but it is looking bleak - esp after the BBC mention yesterday of a prolonged cold spell - always the kiss of death: LOL

Despite what some say about the BBC having more access to data ,they always seem to be behind the curve of the models we see , certainly in the areas most amateurs focus on ,,extreme cold or snow and storms in summer. 

I can recall many times thinking why are they saying that, when many amateurs are suggesting a change 24hrs earlier. 

Yep looks like it , still cold until probably the 15th though. And a very good chance of an easterly later in the month.

GFS and ECM broadly singing from the same hymn sheet this morning - temps starting to recover from or just after 15th February. Minority options for prolonging the cold in the ens suites still though. Probably worth just mentioning that at the surface the ECM Op doesn't really warm up much for MBY - I assume despite the wind flow tilting southerly or even SSW the continent to my West is cold enough to maintain low level cold.

GEM op similar, no idea about the ens. 

I've not been religiously checking the models but as others have mentioned my impression is today is noticeably more in favour of mild air and a retreating/sinking block and I'd wait and see if that theme stays for a few runs before calling it. If we do keep it cold IMBY until and including the 15th, that'll be 10 days for this cold spell which is very good going!

GEM has been getting better and better recently,  hopefully it will be analysed much more here.

On Twitter yesterday I mentioned the possibility of overcast skies radiating heat back down to the ground as an additional factor. 

GEM OP didn't though (from Rob's comments) stick with the cold, it reduced duration etc. down here very much like the GFS op. It also didn't pick it up first as the ECM extended suite and GFS both had the cold spell from a long way out. ECM though stuck to it one blip aside, GFS suite as we know had a full on toddler tantrum and had to sit on the naughty step for a while. 

All that said GEM is undoubtedly a model worth looking at is it does verify well but I think the stats often show that it's at it's best at T120 versus GFS/ECM and often the worst at T240. (Could be wrong though). 

And more specifically different materials conduct that heat better or worse. A concrete path will be fairly thick and concrete conducts heat quite well so the bottom of the concrete will be able to carry heat from the warmer soil quite a few inches below the surface (depending on the thickness of the concrete and sub base etc).

Tarmac seems to be a better insulator so often you will see the snow settling on tarmac but melting on adjacent concrete paving slabs etc. 

I may be wrong but that is my best understanding of why it happens. I know that certain surfaces around my garden always melt the snow even if it’s very cold. The concrete path around the house is one of them.