ThingS are expected to be fine Saturday morning for BWI and Washington DC. After about noon time, all bets are off as conditions are expected to worsen quite rapidly. The following is forecast maps here
http://community.webshots.com/album/255662789RgcvtO depicting that snow will develop around midday Saturday and linger through Saturday night and possibly even into Sunday morning if the coastal low brings in some additional moisture. However the key to this forecast is the later we go into Saturday and into Sunday morning, the worse it gets because not only will snow be a factor, but strong winds with gusts of 35-40 mph Saturday night into Sunday. Thic should result in near white out conditions at times from DC and Baltimore and up through Philly and New York as well.
As for accumulations, expect 8-12 inches in Baltimore City points north to the Mason Dixon line from midday Saturday through about Sunday morning. Expect 6-8 inches at BWI and 5-8 inches at Reagan National in Washington. Some of the heaviest snow may fall from Philly up to New York City where 12-18 inches is a possibility with strong winds to boot. Near blizzard conditions maybe the result especially Saturday night into Sunday.
Jim