This is not a vague threat.....moderate precipitation in a Puget Sound convergence zone has started to bring wet snow to the ground from north Seattle up to Everett tonight.
If you take a look at Seattle SnowWatch, supported by the City of Seattle, you can view the temperatures above the city based on data from aircraft arriving and leaving from Sea-Tac. Here is the latest (the dashed horizontal line shows the freezing level). The green dots are the aircraft temperatures. The blue dots are temperatures from another source: the Sand Point profiler over north Seattle. Temperatures are cooler to the north (freezing level about 500 ft) because of the cooler effects of the precipitation in the convergence zone.
Paine Field in Everett switched to snow around 6 PM and the Seattle Forecast Office in north Seattle is reporting mixed rain and snow. Here is the proof from a cam in Mountlake Terrace: