The AIM defines “Known Icing Condition” at AIM 7.1.22
Atmospheric conditions in which the formation of ice is observed or detected in flight.
Note: Because of the variability in space and time of atmospheric conditions, the existence of a report of observed icing does not assure the presence or intensity of icing conditions at a later time, nor can a report of no icing assure the absence of icing conditions at a later time.
jmuelmen, I see we are neighbors (4 miles SE of BFI) 
