Thanks to NWS forecaster Dewey Walston for the following directions to the
NWSFO. 

From points south...

Take I-95 north to the National Capital Beltway (I-495/95). Get on the
Beltway heading towards Tysons Corner (west side of Beltway). You will go
about 10 miles, then get on the Dulles Toll Road (267 West). 

Drive west on the Toll Road (toll is $.85) to the last exit before Dulles
Airport, Sully Road (VA Rte. 28). Go north on Rte. 28 about 1 mile, to VA
Rte. 606. Turn left on Rte. 606 and drive about 4 miles, past the exit for
the Dulles Greenway, to the entrance to the NWS Forecast Office property
(it's marked with a NOAA sign on the cyclone fence). Make a left turn on
Thunder Road and drive straight to the end of the road. The building in
front of you is the Forecast Office. Find a space in the parking lot (on
the right side of the bldg.) and come in. You will have to sign in at the
front desk. 

Dewey