Wednesday, December 18, 2013

Snow Update

Wednesday, December 18, 2013
11:26 a.m.

I was pretty happy when I saw the UW's WRF-GFS model this morning.

I wasn't quite stunned. If that was the case, this would be my reaction.

But hey, I'll take what I can get. This run was an improvement over yesterday's runs. I'm still a little skeptical for the same reason as I said in the previous post, as snow levels will be RIGHT on the edge, but the models have been pretty consistent, and as I inferred in the previous post, models can catch your eye, especially when the event is less than 48 hours out.

That being said, let's look at the setup.

Valid 04:00 am PST, Fri 20 Dec 2013 - 48hr Fcst:

One thing we look at when making a forecast for snow is the thickness of the atmosphere. The colder the atmosphere, the denser it is, and the lesser the distance between pressure levels (the 1000-500 mb/hPa levels in this map). The warmer the atmosphere, the thicker it is. The highest thicknesses appear at the center of hurricanes, where the air is extremely warm throughout the atmosphere. Around Seattle, we generally need thicknesses to be around ~522 to start talking about snow. Here, they are 543. So how is this same model still forecasting snow?

Well, the upper atmosphere may be warming, but this moisture is forecast to arrive right in the early morning hours, when temperatures are the coldest at the surface.

Valid 04:00 am PST, Fri 20 Dec 2013 - 48hr Fcst:

Another thing - Thursday is expected to be mainly cloudy, but the clouds look to decrease Thursday night. This keeps the temperatures down during the day and lets radiational cooling take over at night. It's like a 1-2 punch.

That's the thing though. This situation is SO dependent on timing. If things get shifted by a couple hours, then the lower atmosphere will likely be too warm, and we'll just get rain. Right now, the timing looks PERFECT - the precipitation starts to come in at 4 a.m. That means the models are showing the best-case scenario. But let's not think about that. :)

Valid 04:00 am PST, Fri 20 Dec 2013 - 48hr Fcst:

Valid 04:00 pm PST, Fri 20 Dec 2013 - 60hr Fcst:

Not too bad, eh? Three inches of snow over Seattle. If you want more, go north to the Everett area. If you want a lot more, go to the mountains. If you want less, go south. And if you don't want any, swim to China.

Now, the moment, you've all been waiting for... my LAWES rating.


Meaning a snow event within 3 days with up to 4 inches of snow with one consistent model showing it. Other models besides the WRF-GFS are also showing snow, but I decided not to include them because they are not showing as much.

This event, save perhaps a snowy Puget Sound Convergence Zone, is the hardest type of snow event to predict here in Western Washington.



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