Right now Northstar is scheduled to close around 4/27, Alpine on 5/4 and Squaw on 5/11. I suspect those are subject to change, but probably fairly firm. The resorts aren't selling many daily tickets these days so they lose thousands every day they are open.
Its kind of a bummer. I was going to buy a Squaw spring pass if they were staying open until Memorial day. But since they close right after Alpine, no dice.