A strong cold front passed overhead during the morning commute (notice the wind suddenly dropped off?) and now lies down in the Valley. Just behind it it is cooler air with the freezing level down around 2,000′. Normally you might expect snow just a little below that, but with the very heavy precipitation rates (.85″ @ PDX since 4am!), the snow level drags all the way down to near sea level. Sticking snow appears to be falling to around 500′, especially onthe eastside of the Metro area where the precipitation is heaviest. I already have 2″ at home. It appears that the dropping snow level has bottomed out with temps holding steady the last hour or so. If you don’t have snow now in the metro area you probably won’t get it.
This will continue all the way through tomorrow morning as the front becomes almost stationary over Northwest Oregon. Combine the great atmospheric forcing with a stationary front and you have a tremendous snowstorm that’s just getting going. Heavy snow has begun falling in the Cascades and will continue through Tuesday morning…imagine a total of 2-3′ in just 24 hours! Our RPM model forecast for the next 18 hours is down below. When you read maps like this one, don’t get too hung up on exact location of contour lines. The graphic is “machine-made”, so it interpolates between points. It knows 30″ at the crest of the Cascades and nothing in Portland, and generally sees a “smoothed” elevation map. But it can’t account for every ridge and valley. For example, it isn’t going to snow 10″ at Multnomah Falls at sea level with a west wind, but a graphics program can’t handle that kind of detail for drawing the contours. More importan is that this is a 4km model. That means the grid points are 4km across…wider than the Gorge. It sees the Gorge as a low pass. All models do that to one degree or another, depending on their resolution.
Down in the lower elevations, I expect the sticking snow level to climb a bit to 1,000′ or even a little higher by this afternoon, but it won’t go above 1,500′. So how much snow in places like Sandy, Detroit, or the hills above Washougal, Corbett, and Estacada? There’s no reason a foot of snow couldn’t fall in those areas, but at least 6″. This happened one day in February 2008 when I got 9″ at 1,000′ at home and it was significantly deeper just 500′ higher, but none in Portland. This setup also tends to give power outages with the falling trees in the Hoodland area from Sandy to Rhododendron.
Beyond that, a quick jump in freezing levels tomorrow afternoon ends the snow threat at the lowest elevations. The big windstorm for Wednesday is a bit farther offshore, so just windy here in the Valleys with strong wind at the Coast.
Chief Meteorologist Mark Nelsen