IFR on the Fly

Let's say your 15 miles from your home airport in VMC, not on a flight
plan. You know the area well. On your way, unforecast conditions
(snow) causes visibility to start dropping below 3 miles. Ceilings are
at 1800. You are just below the ceiling. You know the area well enough
to do a contact approach safely, even though the MSA for the area is
3000 ft MSL (2000 ft AGL).

Seems like the right thing to do in such a situation is to call ATC,
tell them your situation (although I am not sure precisely what one
would say), and ask for an on-the-fly IFR contact approach clearance.
It seems like seeking a full IFR approach causes one to go up into the
clouds and risk icing, and so would be a less safe alternative.
Opinions?

-Sami
N2057M, Piper Turbo Arrrow III