Odds of a White Christmas

Yes, Christmas is still more than 2 weeks away.  So it's kind of a stretch to be talking about White Christmas odds.  But since the computer weather forecast models make predictions out to 15 days in the future, we do have some educated guesses we can talk about.

First, we should start with climatology.  On average, how often do we get a White Christmas (defined as at least 1" of snow on the ground)?  By now you've probably seen this map that the National Weather Service put together about a dozen years ago and has updated.  It shows the historic probability of a White Christmas (not a forecast for this year on any year).

First, a few observations.

• The vast majority of the lower 48 states have very low probabilities of  a White Christmas for any year.
• The highest odds of a White Christmas are actually in the high mountains of the western US (but not too many folks live there).
• The Inland Northwest is actually one of the better places for having snow on the ground on December 25th, ranking up there with Minnesota, Michigan, Wisconsin and the Northeast.

But let's zoom in a bit to our part of the world.

• Yes, the odds in the mountains are near 100%.  But the valleys (where most folks live) are quite a bit lower.
• If you live west of the Cascades, forget about it.  Odds are 3% for a White Christmas in Seattle or Portland.
• The mountain valleys of the Inland NW have much better chances than the Columbia Basin.

Here's a table of some of the locations that go into this analysis.

• Spokane - 48%
• Sandpoint - 70%
• Priest River - 89%
• Colville - 64%
• Northport - 82%
• Republic - 93%
• Davenport - 48%
• Lind - 33%
• Moscow - 49%
• Lewiston - 11%

So what about this year?  Remembering that this is a strong El Nino winter, our hopes should not be very high for a white Christmas.  But there's always a chance.

Snow is a sure bet for the storm arriving this evening.  How much and where is more debatable.  The computer forecasts aren't necessarily all over the place, but there's a fair amount of variation.  Here's a few different forecasts.

First, from the European model (which is typically very good).  It would favor the southern ID panhandle, with more snow for Spokane than points north.

The US GFS model is even farther south with the snow, favoring the Palouse with a heavy dumping.

But there are other forecasts that are more "traditional", putting the heaviest snow in the Panhandle and northeast WA.

How much snow we get from this storm could factor heavily into whether we'll have a white Christmas.

For the upcoming week, the weather looks rather dry with a strong, warm high pressure over the western US.

Going out a bit farther in time (14-19 December), high pressure has nudged eastward with a cold trough well off the West Coast.  This pattern could bring weak but mild weather systems into our area.  Temperatures will be crucial in determining rain vs snow.

Then for the days leading up to Christmas, things are still looking mild in our area.  Again, Pacific systems coming into our area in this pattern could bring rain or snow, depending exact timing and temperature.

So the screaming message is this: I don't see a cold, snowy pattern setting up in the next two weeks.  Our temperatures will be on the mild side, which means just about any weather system could bring rain or snow to our area.  Given the overall patterns, I'd say rain is favored over snow in general.  That doesn't mean we won't get any more snow before Christmas.

So the odds of a White Christmas could very well hinge on how much snow we get tonight and Sunday and how much of it can survive then next two weeks.