Wednesday, December 17, 2014

Are the chances of a White Christmas improving?---UPDATED 12/18/14

...Here's an update to yesterday's blog. We replaced the images with the latest weather forecasts...  

 Since our last post, we have gotten a little more resolution on the prospects of seeing a White Christmas across the Inland Northwest. But before we answer that question we need to deal with a weak weather system tonight and Friday and then a much stronger system for the weekend. As of our latest forecast (issued 3pm Thursday). Tonight and Fridays weather system is calling for light snow, mainly confined to the mountains. Valley snow chances will be reserved for the Cascade valleys, Okanogan Valley, and a small part of the Waterville Plateau. These locations will generally see amounts ranging from 1-3 inches with locally heavier amounts near the Cascade Crest. There will likely be some travel concerns going over the Cascade passes.

Snow forecast for tonight-Friday

The storm system for Saturday and Sunday stands a much better chance of producing significant snowfall as it will contain much more atmospheric moisture. This will be care of a very well-defined atmospheric river that has its sights set on the Pacific Northwest. Here's what the atmospheric river is expected to look like by Saturday morning. This river will draw its moisture from well south and west of Hawaii.
Atmospheric river forecast for Saturday morning. The darker the colors, the more moisture content. 

While the river will assuredly produce widespread precipitation, it will also deliver steadily warming temperatures. Our confidence is high that snow levels will be low enough to produce moderate to heavy snows near the Canadian border and in the Cascades. It would not be surprising to see snow amounts approach a couple feet in the Cascades.  Meanwhile, the forecast for the nearby valleys is a much tougher call as temperatures will be critical. If they warm much above 32°F the snow accumulations could be less than forecast, in fact much less. Right now, we are forecasting temperature very close to freezing over most of these lower elevation locations and putting moderate snow accumulations in the Cascade Valleys. Most of this valley snow threat would occur between Saturday morning and early Saturday evening.

Weekend Snow forecast

By Sunday afternoon, temperatures will warm significantly as the warmth associated with the sub-tropical moisture continues its northeastward surge. Snow levels will rise to 4000' or higher over the entire Inland Northwest. This will translate to melting snow over most of these valley locations. The big question is can we melt what falls? That depends on how much accumulates over these locations. The warming will be accompanied by breezy conditions which will help melt the snow especially as the dewpoint temperatures and nighttime lows rise above freezing. And that's what we are expecting as of our latest forecasts. Here's a look at the high-temperature forecast for Sunday.

Sunday Forecast Highs

 So based on the current forecast the only valley locations which stand a chance of snow before Christmas Eve would be near the Canadian Border, or the Cascades. How about the remainder of the region? Well, as we talked about in our last blog entry we are expecting a big pattern change, right around Christmas Eve. That's still true. Our mild west-southwest flow of late will take a decided cooler turn to the northwest. Here's what the upper-level pattern is going to look like (at least something like it anyway). The wind flow will be parallel to the yellow height lines and it will deliver cooler air into the area (blue and purple shading is cold air, other colors are relatively warm) via the Gulf of Alaska. Meanwhile, the remnants of the weekend moisture stream will get hung up somewhere over the Pacific Northwest. The big question is where is that going to happen? Most likely it will occur immediately downstream or east of the curve in the yellow lines. In the picture below that suggests that will be somewhere over extreme southern Washington and northern Oregon.

500 mb Heights and temperatures

So is there good model agreement in where that curve is going to form? That is the key to this forecast. As of the latest model runs, consensus is growing. Here's a look at four various model runs all looking at the afternoon hours of Christmas Eve. We placed the precipitation forecast on top of the 500 mb yellow height lines. Notice, they all show a similar curve in the height lines or flow, however they vary on how far north to place it.  Model trends continue to support the band forming well south of I-90. Notice some don't place any precipitation over eastern Washington, while others only give us light precipitation amounts. Based on the trends, our confidence in seeing precipitation remains highest for locations near the Oregon/Washington border, such as Lewiston and Pullman. However, even in these locations temperatures may remain just a little too warm for snow.

Various weather models for Christmas Eve. Shading represents the precipitation. 

So to answer our original question. Are the chances for a White Christmas improving? For locations such as Mazama, Republic, or Plain the answer is a decided yes. For Lewiston or Pullman the answer is a maybe. Folks in Spokane, Couer d'Alene, Moses Lake, or Wenatchee the odds are not great. Of course, Christmas Eve is just under a week away and much can change between now and then. So stay tuned and maybe just maybe the yellow curve will decide to set up over the entire Inland Northwest resulting in a White Christmas for all. 

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