There are a few more issues to consider here.
Are you a member of a union with a collective bargaining agreement and termination procedures?
Does your employer have a written employee manual that adresses termination?
Is there anything in writing, email or even past conversations that might have changed the at will reality into a contract for employment?
Was your termination really a retaliation for a protected activity?
There are others. The bottom line is that while WA is generally an "at will" state, that term is not as iron clad as it appears at first glance.