I have a lot to talk about.
Some of you have been commenting about the potential for a big event next week and the National Weather Service and the media have been hinting about it. I will analyze the possibility in this blog. Taken literally, the latest model runs based on the U.S. global modeling system suggest a major lowland snowstorm late on Wednesday and on Thursday. And lets be clear up front: the event is a long ways off (Wednesday/Thursday) and uncertainties are still large. Exactly who will get the most snow is not clear. Portland, SW Washington, Puget Sound? And in this blog I will discuss how we estimate and deal with the uncertainties.
Act I, the Cool Down: Lets begin with today. During the afternoon we had a marvelous cold frontal passage, with a large windshift from southerly to northerly, sharp pressure trough, and moderate rain. As the front passed by and the winds swung to northwesterly along the coast a nice Puget Sound convergence zone formed (see radar below), dampening the bicycle commute home for many.
Although temperatures are rapidly falling aloft, it is still way too warm for snow over the lowlands, and will continue to be too warm until tomorrow in most locations.
This is the first act of our little weather play.
Act II, The Tease: Colder air will move into British Columbia and Washington tomorrow and by tomorrow evening it will be cold enough to snow over the Puget Sound lowlands (even earlier in the day above 500 ft and where precipitation intensity is greatest). Later tomorrow cold northeasterly winds will start pushing southwestward through the Fraser River Gap. Several convergence bands will be established tomorrow and early Sunday and some scattered showers will also occur. Many of these showers will be snow. At most places it will be flurries and light, but in some of the convergence bands perhaps an inch or two will be possible. And Sequim and Port Angeles could get some upslope snow again. Very similar to a week ago. The models suggest that the Portland area could get some light snow on Sunday from a weak disturbance moving down the coast.
Act III: Waiting-Cold and Dry
As the cold air pushes south, the showers will end over northern Washington and the drying will push south during the day. By Monday, the region will be cold and dry. Temperatures Monday and Tuesday morning will drop back into the 20s and the the highs will only reach into the 30s. Sunny, but cold.
Act IV: Snowapocalypse?
The cold air will be in place and then a Pacific weather system will approach from the west and southwest, pushing moisture over the region...as snow north of the Oregon/Washington border. Here is a surface chart showing what the UW WRF model expects for Thursday morning at 4 AM:You can see the Pacific low pressure system off of southern/central Oregon and the cold air (blue colors over Washington). Cold air will be moving into NW WA through the Fraser Gap and over lower terrain at this time. Precipitation will move into Oregon late on Wednesday and snow will extend northward. Here is the snowfall for the 24h period ending 4 AM on Thursday. Eastern Oregon and the Cascades down into the foothills get snow. Portland gets snow as does southern WA.
The snow will then extend northward. Here is the next 24-h:
Several inches of snow gets to Seattle, but the heaviest stuff is to the south, with roughly a foot in the Portland area and roughly half a foot to Olympia. The previous run had the heaviest snow over Seattle. This is all followed by snow showers and continued cold.
Bottom line: Although each run is a little different in low intensity and position, all have the same general solution with some area in the NW lowlands being hit by significant snow. These are all based on the American GFS (Global Forecast System) model, downscaled with the high-resolution WRF model. The European Center Model, which is generally superior to the GFS, takes the low north of us and we get rain. So does the Canadian GEM model. The weather will crucially depend on the position of the low--if it goes north of us we get rain, just south of us, a big snowstorm, too far south, nothing. We need to wait to really know what is going to happen. Hopefully, by Monday the solutions will pull together and our confidence in an evolution will increase.