What is a "snow window"?
Solution 1:
Maybe the driver meant to say "snow windrows"?
As per the comments, we may never know...
Solution 2:
I've never come across this usage, and I don't believe it has any meaningful currency. But my guess is the driver was conflating two different idiomatic usages...
"windows of opportunity" (51,900 instances in Google Books)
"pockets of opportunity" (4,110 instances)
"pockets of snow" (7,810 instances)
That's to say, what he meant was some sections/pockets of the route were/might be snow-bound.