The media is beginning to talk about it and the emails are coming in, so perhaps it is time to talk about the potential for cold air and snow later this weekend and early next week.
My general policy is to wait until such events are about 120 hour out, because their predictability is often very low beyond that. And although some models have been indicating a cold event for a while, their forecasts have been inconsistent--suggesting large uncertainty. So now that we are getting close enough to see "the whites of their eyes", what do the latest model predictions indicate.
First, snowstorm 101. It is hard to snow around western Washington and Oregon. We can be warm and wet, and sometimes cool and dry. But only rarely are we wet and cold--which you need for snow. The trick is to get moisture off the ocean and cold from off the land, without letting the ocean warmth turn it to rain. I did a paper on NW snowstorms with Garth Ferber, Mike Patnoe, and Gary Lackmann. We found that most snowstorm events looked very similar, with an upper level (500hPa) trough moving
southward along the eastern flanks of a large north-south ridge in the
northeastern Pacific. Like this. The trough provides lift to produce clouds and precipitation.
So what are the latest WRF model runs showing? Here is the 500 hPa upper level map for 4 PM Sunday. Nice, strong trough, but a bit more extended east-west than the canonical snow trough. But perhaps close enough.
Looking at several other models (including the ECMWF) it really looks like frigid cold air is quite certain. But the run to run consistency of the details (strength and position of the trough and surface low) are quite variable, indicating lots of uncertainty.
Now the snow forecasts. Here is the total snowfall for the 24h ending 4 AM on Monday. Lots of snow over the mountains (foot plus over the Cascades), with several inches over Seattle to Everett. This lowland band is associated with a convergence zone and the leading edge of arctic air coming out of the Fraser gap. Note snow on the NW side of the Olympics--classic with Fraser outflow.
These kinds of situations are reminiscent of the details of setting off explosions. Although you need to have a match, explosives, and dry conditions, there is no certainty everything will come together to get the big bang if the components. We have all the ingredients...now the question is how they will come together. And to get significant lowland snow over western Washington, the requirements are exacting and relatively rare.
What about Thanksgiving Day? Mainly dry, but cloudy. Perhaps a few sprinkles late in the day. Friday will be cloudy with a few light showers. The big action will be on Sunday and early Monday...unless the models change, again. You coming home from east of the Cascade crest? Perhaps better to return on Saturday.
More Smartphone Pressure Observations Needed
Have an Android smartphone with a pressure sensor (such as Samsung Galaxy III or Galaxy IV)? Would you like to help with weather prediction and be able to minitor your local pressure information? Then download a pressure app for your phone.
A good one is PressureNet4 (free)
Another is WeatherSignal. (free)
Need a Good Weather Calendar?
Like to Help UW Atmospheric Sciences students go to conferences?
Then buy the Washington Weather Calendar! You can order online (about $15. plus shipping) here. A few dollars goes to the UW Chapter of the American Meteorological Society for each calender. A fine holiday gift for the weather lover.