The temp has to go above zero in order for the snow to melt.
The suns rays alone, even when combined with traffic won't melt the snow if it's below zero.
Powder should stay powder, even for weeks down the road as long as it stays below zero.
Regardless of the sun.
I was up there on Tuesday & Thursday night. It rained on Tuesday, killing the 40 cm they had the night before.
Thursday night was bullet proof, the 40 cm ruts were frozen solid.
We got there just before dusk & tried to squeeze a quick run down Moons before it was pitch black.
Bad call, 2 foot unbreakable ruts, down single & double black diamond in the dark
(Canadian dbl black diamond, not Bumfuck Idaho blacks)
(Edit: No offense Bumfuck, I'm sure your parks are awesome. Haha)
It was insanely fast though, you just needed a few hundred feet to stop.