Monday, December 22, 2014

Odds for a White Christmas: Update

OK, we're only 3 days away from the Big Day, and snow is still hard to come by in my Inland Northwest locations.  Is there any hope for a White Christmas?  Here's the current snow depth analysis:

Pretty pathetic.  Folks in the Cascades are doing well, as are those in some of the northern valleys (e.g. Omak, Republic, etc).  Otherwise, lots of bare ground.  

Just to refresh your memory, here was last year's snow cover at this same time. Not stellar, but better than this year.

But there's still time to change this, right?  Actually, the answer is "yes", for some locations anyway.  As we've been discussing in earlier blogs, the computer models have been suggesting a Christmas Eve storm for over a week now, but they've been waffling on the exact timing, track, etc.  And these small differences play a HUGE role in who will get precipitation, and in what form will it fall.  But now, the models are in fairly close agreement. So let's take a look at what they're saying.

Below is the GFS forecast precipitation for the 12 hours ending 4pm Wednesday.

While this looks encouraging, unfortunately almost all of this will fall in the form of rain.  Snow levels during the morning hours will be around 6000' but will rapidly fall through the day as cold air moves in from the north.  Can the snow levels drop all the way to the valleys before the precipitation ends?  Maybe, but this is typically a poor pattern for valley snow.  And even if it does snow in the valleys, it will have a hard time accumulating on the ground during the afternoon/evening hours.

Here's the probability of 1" or more snowfall on Christmas Eve. 

Things are looking up for folks in the Idaho Panhandle.  Meanwhile things look bleak for locations in central/eastern Washington, with locations west and south of Spokane having next to no chance for snow.

So, looks like a brown Christmas for many folks.  The interesting climate tidbit in all this is that we could set an obscure record at Spokane.  The latest that it's ever taken for the Spokane Airport to have it's first 1" or greater snowfall from one storm is Dec 24th 1976.  Unless the forecast changes in a hurry, we'll break that record.  But not by much.  Why you ask?  Because it's is now looking very good for a decent snow event after Christmas.

Here's the 24-hour precipitation forecast from the GFS ending Saturday evening:

Looks similar to the Christmas Eve storm, right?.  The difference is that just about all of this will fall in the form of snow instead of rain.  The reason is that while the Christmas Eve storm might not bring much snow, it will bring colder air.  So by the time this second storm arrives, we'll be much colder.  Different models have different amounts of snow, but in general they all have a solid 2-4" for most locations with the potential for higher amounts.

Here's the snowfall forecast from the NAM (red) and GFS (blue/purple) for Spokane. 

There are different forecasts here using different techniques.  But in general they give a similar message.  Snow will start Friday evening and continue into Saturday evening.  The ECMWF (not shown) has a similar story, but doesn't start the snow until Saturday morning and continues it into Sunday.  Either way, there is the potential for several inches of snow.

It's too early to hang-your-hat on this forecast.  But one of the things we like to see is that each model forecast is similar to the previous one (GFS runs every 6 hours, ECMWF every 12).  And they've been saying a similar message for about the last 24 hours.  So that's a good sign and gives us confidence that this forecast could indeed pan out.  But the overall pattern for this storm isn't necessarily a favorable one for heavy snow in the Inland Northwest.  The mountains will almost assuredly pick up some snow from this, but the lower elevations might not see much. Stay tuned to the forecast, especially if you plan to travel this weekend.

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