I'm thinking that the singular verbs "is" and "has" are correct below because we're referring to a singular lump of precipitation. Although the subject "inches" is plural, I think the singular verbs are correct below. We wouldn't say, "Fifty dollars are a lot of money."
I think this is basically the same concept.

Good for the singular verbs below?

One to 3 inches of snow "is" expected. (Not: ... are expected.)

Ten inches of snow "has" fallen.
(Not: ... have fallen.)