5:30 PM update on Friday...its snowing in Everett...intense convergence zone convection plus cold temps aloft have brought thundersnow to the ground....
The numerical forecast models are consistently calling for a major pineapple expresss/atmospheric river event starting Sunday and extending through early Wednesday. These are amazing phenomena....I have an active project now studying them with a graduate student Michael Warner. Huge flux of moisture in what appears from satellite imagery (vertically integrated water vapor, above) as a narrow plume of moisture.
Check out the 24-h precipitation from the 12-km WRF run at the UW for the periods ending 4 PM on Monday and Tuesday (graphics). Vancouver Island and the Cascade/Coast Mountains on the mainland get hammered with a number of areas getting 5-10 inches and more. The Olympics and the northern Cascades also get large amounts..but less. The central Cascades receive far less (good for the Green River).
A major issue is that the freezing levels will move way up, with warm rain falling on some of the nice snow in the Cascades. Well, we can't escape Cascade concrete for long!