Regarding temp you will soon know when to give up when yer wfp gear will start to freez up whilst out and about in sub zero temps. Even if you have spent time defrosting it beforhand ground surfaces like concreate paths etc can get considrably colder in sub zero temps than the air temp. I'm afraid to be honest wfp method is prity darn usless below 1c as you will have a constant battle on your hands to keep working as you also have to allow for wind chill factor too.
To try to keep working you could use warm water either from an emesion heater fitted to the van tank warming the water up overnight or using a hot water heater on demand but still the runoff water will start feezing up on paths which is not ideal either. Not to mention most ppl will think yer bonker when the water starts feezing up on the glass. You can't beat mother nature but you can work along side it as simply stay in bed longer and start work when the temp starts to rise.
Snow we don't normally get a lot of but when we do if the roads are bad especially the side roads where most will want to get too is it worth the risk to drive about let alone try to clean some windows?
Some call it a slush fund for a good reason