Thursday, December 10, 2009
Steam Fog, Snow over the Weekend and Beyond
Another really cold day, although it warmed up a bit this afternoon. The air aloft was warmed significantly--by roughly 10F over the last 24 h, and surface stations are about 2-5F warmer than yesterday at the same time.
While bicycling into work this morning (it was 18F when I left my home!), I was warmed by the sight of steam fog over Lake Washington. It was surreal...sort of like the steam and fog coming out of some large witches brew. Devin Ellis sent me a picture snapped on his phone-as shown above. Steam fog occurs when cold air passes over much warmer water. The air temperature was around 20F and the water temperature according to the King County buoy was around 48F. You see steam fog during our primo cold air outbreaks...particularly early in the season when the water is fairly warm.
OK, but steam fog is not what you really want to know about...its the potential for snow. My friends in the National Weather Service have a "chance of snow" in the forecast for the entire weekend and on Monday. Can't argue with that...but lets try to be more specific. The computer models are not in agreement on Sunday and Monday, leaving us with some uncertainty.
There are basically two shots at snow: Sunday and late Monday. On Sunday, our first chance comes as a short wave disturbance aloft moves southward and crosses the region (graphic). The current forecast driven by the GFS model is not favorable for a general snowstorm..the disturbance is a little too far east and the amount of precipitation is light (check my book section on the optimal snow set up). The other NWS model (NAM) has more forcing over us. Considering the normally superior GFS-WRF run (WRF is a high resolution local model run at the UW), there would be only some scattered snow showers around here, with heaviest lowland snow perhaps in a Puget Sound convergence zone north of Seattle (graphic).
But we get a second chance at the white stuff! On Monday a warm front will approach, with lots of precipitation (graphic, green and yellow are warmer air).
Cold air will be in place--particularly over the northern portion of the state and it could start as snow...and perhaps last as snow for several hours (graphic).
Eventually (on Tuesday) the atmosphere here will warm and the cold and snow will be gone..but before that we have two shots at it. I wish I could tie this up neatly for you...but I can't. Clearly, a better chance up in NW Washington. Mountains will surely get lots of snow, as should eastern WA.
And did I mention the potential heavy rain and a pineapple express on Tuesday? I better not talk about this right now...enough to think about....
PS: I will be giving a public lecture--"The Secrets of Snow"--at Third Place Books in Lake Forest Park on December 17th at 7 PM, followed by a book signing.
Posted by Cliff Mass at 8:31 PM