Saturday, December 14, 2013

Odds of a White Christmas

With Christmas only 11 days away, it's time to start thinking about the chances for a White Christmas.  First, let's look at climatology.  In other words, what are the chances of a White Christmas in any year, based on what has happened in the past.  The National Climatic Data Center produced a nice map of the 48 states earlier this week.  It showed the climatology of at least 1" of snow on the ground on December 25th.   Below is the map, or if you want to read their article, you can find it at:

Here's a zoomed image of the Pacific Northwest:

While the mountains are almost assured of a White Christmas, the valleys (where most of us live) have much lower odds.  Here's a table of the probabilities for selected cities in the Inland Northwest:

Let's take at the analysis of snow cover from the National Operational Hydrologic Remote Sensing Center (NOHRSC):

If you see a shade of green, that's bad (if you want a White Christmas).  The light gray shading (less than 2") isn't much better.  While we're at it, let's refresh our memories of last year.  Here's the NOHRSC image of snow cover last year at this time:

A much better coverage of snow last winter than this year at this time.  Just about everyone had some snow on the ground, and some valleys had more than a foot.

But, there's still 10 days to pick up some snow.  Does the forecast offer any hope?  Here is the jet stream image at 4pm Saturday:

The shading is wind speed.  The wind blows parallel to the solid black lines.  So the jet stream comes off the Asian continent (left side of the image), but half way across the Pacific (center of the image) it is diverted north by a ridge of high pressure over western North America.  This diverts the Pacific storms north of our area into western Canada.  From there, they dive into the midwest US.  This is not a good weather pattern for precipitation in our area. 

Here's the same image, only it's a forecast for next Wednesday afternoon:

Doesn't look much different, does it?  There is a slight difference.  The ridge of high pressure is now over the Gulf of Alaska, giving us a slightly better chance of getting some precipitation.  This is our next best chance of snow. Still, not a great pattern for snow for our area.  And here's the forecast for Christmas Eve:

Again, not a lot of difference.  Still a huge ridge over the West Coast.  Not good for stormy weather.  However, this pattern does leave the potential for another cold arctic air mass to come into our area.  We're not saying that's going to happen, but there is the potential and we'll be keeping an eye on it.

We will see some systems move over this ridge of high pressure and give us a glancing blow.  So there will be some chances for precipitation.  The next chance will be in the middle of next week.  Not a great chance though.  A similar system is forecast to move through on Friday.  And there are potentially a couple more systems before Christmas.  Again, none of these are impressive-looking systems.  But 10 days out is pretty iffy to trust computer forecasts.

So, here's a summary of what we're looking at for the next 10 days:
  • No big snow storms.
  • Best chance for precipitation will be in the southern Idaho Panhandle (e.g. Moscow).
  • Very low chances of precipitation for the Columbia Basin (Wenatchee, Moses Lake, Tri Cities).
  • Near-normal temperatures (lows in the 20s, highs in the 30s) for the next week, but colder weather is possible.

1 comment:

  1. Just wanted to take a minute and say thanks for putting this blog together - there's some great info here. I can't claim to be a weather expert but all the posts here definitely are enlightening!