Snow and Cold are Coming

Update 10:30 PM: Lots of report of ice pellets, graupel, and wet snow in the convergence zone and in the foothills. New model runs in...still very threatening for snow
Update 7:15 PM--the convergence zone has formed and I can hear ice pellets on my window!

I am going to go out on a limb now...I think most of you ...even those in the lowlands ...will see some snow before the week is up. And much colder temperatures.

A front is now crossing the coast, with a switch to northwesterly flow behind. You can see this on the latest surface chart:

You can see the lowering of the clouds and start of precipitation over the Olympics in this video:




Colder air will move in behind the front, causing the atmosphere to be less stable (more convective showers) , and there is a good chance of a convergence zone over Puget Sound forming tonight and tomorrow morning behind it. The freezing level will be low enough that if the CZ has sufficiently intense precipitation, wet snow could reach sea level with the zone.

So don't be shocked by some flakes during the next 18h or so. This system will SURELY bring snow to the mountains...in fact, enough to improve the skiing a bit (6-12 inches). I was skiing in the pass on Sunday and lets say it needed to be freshened up a bit. Here is the 24-h snow ending 4 PM Tuesday. And yes, it is showing some snow over the eastern Puget Sound lowlands...places a high elevations and away from the water.


But this is just a warm up (really just the opposite!!). The air will get colder on Tuesday into Wednesday and then an upper level trough will move towards us from the north (see map). This is close to a classic upper air snow configuration. Not quite perfect (little too much extension over the Pacific), but close enough. And by the time we get to Wednesday we won't have to worry about being cold enough to snow.

Associated with this upper trough will be a low pressure center that will form in the lee (south) of Vancouver Islands. (map). The real primo cold air will be locked in the interior of BC, blocked by the mountains.

Virtually all the models are on board with this solution--enough that I think it has a good chance of it happening (YES, there is always uncertainty in a forecast). Virtually all indicate strong NE winds will exit the Fraser starting on Wednesday...and then it gets interesting and more uncertain.

There is little doubt that there will be some snow showers with this system, but serious snow demands more. If the low remains offshore of the north WA coast the easterlies will move down the Strait and some of them will plow into the Olympics, hitting the north Olympic communities with snow. Those retired folks will have to switch from golf to x-country skiing for a while! If the low moves southward, then southerlies moving up the Sound could plow right into them, give decent lowland snow over the Sound. In any case, there will be an interface between the northeasterlies and southerlies that will produce a significant snowfall somewhere, most probably over NW Washington.

Here is what the WRF model run at the UW is showing for two 24-h time periods--ending 4 PM on Wednesday and Thursday:

You can see the heavier snow associated with the interface--stretching across Whidbey Island to the north Olympic peninsula.

I was looking forward to giving my talk at Ivar's on Wednesday evening....so I hope the snow does its typical mirage act.... But the bottom line is that exciting things are possible during the next 48-h..so be prepared. I know Jim Forman is.