A Snowstorm of Two Characters

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The latest forecast models runs suggest a snow event of two characters.  Over the mountains and over southwest Washington there will be large quantities of snow.  And substantial snow over the eastern slopes of the Cascades.  But over Puget Sound and northwest Washington this will be a very modest affair.  In fact, many of you over the north Sound and Whidbey Island got WAY more snow today than you will get tomorrow.

Right now two signs of the incipient action are evident.  First, the Langley Hill coastal radar is now showing the leading precipitation/clouds.  You really have to love this radar!

And the 8 PM surface charts show true Arctic air moving into Bellingham.  14F temps at Bellingham Airport with a dewpoint of 10F.  Cold air at Blaine and really frigid stuff (single digits and subzero) over southern BC.   This air has not made much progress into Washington yet, but that will change as the low moves south of us.
Now to the important part.  I have reviewed the latest National Weather Service forecast model output (4 PM), the forecasts from the UW high-res models, and the output from several ensemble prediction systems.  The situation is similar to this morning, and IF ANYTHING DRIER WITH LESS SNOW OVER PUGET SOUND.

Here is the latest 24-h snowfall ending 4 PM tomorrow (the storm is over by then west of the Cascades) from the 4-km UW WRF model for both the region and western WA.  Huge (like 3 feet+) snowfalls over the southern WA and Oregon Cascades.  8-15 inches over portions of SW Washington.  7-8 inches near Olympia, 2-4 inches over Seattle, 2 inches over Everett. Very little over Bellingham. Big snows over SE Washington.

I think this forecast is the best my discipline can give you for a single forecast.  The latest update from the Seattle NWS office is consistent with the above numbers.  Far better would be a probabilistic forecast:  X% for 0-2 inches, Y% for 2-4 inches, Z% for 4-6 inches, etc.  But that is for a latter blog.  And yes, THERE is some uncertainty with this forecast--my gut feeling is there is perhaps a 20% chance it could go wrong in a serious way.

Finally, there is been a lot of talk about really strong winds tomorrow.  My examination of the weather models suggest that the winds will only be strong (30 knots or more) over portions of NW Washington downstream of the Fraser River gap, the Strait, and offshore of Grays Harbor.  The northerlies in the Sound will increase rapidly (to 20-30 mph over water) as the low center moves south of us, resulting in a large pressure difference between high pressure in southern BC and the low.  Here are the predicted winds at 10 AM. 

I wanted to end with an editorial comment.  There were a number of folks that were concerned/upset about the fact that my and others forecasts have changed over the past few days (from slush to snow to lighter snow).  One individual told me I needed to show more "character" and be strong enough to stay with my original forecast.  An ill-informed (and somewhat insulting) writer for the Seattle Times wrote:

"How much snow could fall in the Seattle area Wednesday now appears to anybody's best guess." and then made fun of one of my colleagues at the National Weather Service. 

The very nature of forecasts is that they tend to change in time.  They become more accurate the closer you get in time to an event.   A good forecaster will change the forecast as more information and new guidance becomes available.  What would you have them do?

In one lecture of my senior weather forecasting class I talk about the psychology of weather forecasting and how they have to push previous forecasts out of their minds, not compensating for past errors.   Forecasters need the mindset of one individual in particular:

Vulcans Make Excellent Weather Forecasters
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