If you grow up with a certain saying or phrase, you tend to assume it's universal, only to realize later that maybe it isn't. This morning I looked out the window here in mid/slightly north New Jersey to see big soft fluffy snow flakes falling to lightly cover the ground, but melting on the pavement. "Onion snow!" I thought to myself, remembering my grandmother saying that these unexpected spring snowfalls portend onion planting season. But no one at the library had heard that expression. None of the reference collection's American slang books and none of the weather books I looked at mentioned the phrase.The Old Farmer's Almanac even failed to enlighten me about the saying. Googling turned up various results that revealed the saying is mostly used in Pennsylvania, which makes sense. That's where I grew up.
Read more about onion snow here and here.