Plant closing and notice period questions - any advice?

My employer announced that the site will close and gave us written notice. There are 10 people counting on me, so I am trying to be careful. I do not know if I should start looking now or wait for severance news. Has anyone been through this?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.