Three inches of snow is/are expected? [closed]

"Three inches of snow is expected" is current usage and "are expected" sounds unusual to my ears.

Taken with a gram of salt, Google ngram viewer displays "inches of snow is" as having a much higher count than "inches of snow are". Strangely enough, a search for "inches of snow has/have", shows the plural form having the higher count. A Google search, however, produces "has fallen" rather than "have fallen". I would conclude that current usage prefers "three inches of snow is expected" as well as "three inches of snow has/have fallen".

parameters: books from 1960 to 2010, smoothing of 10.