The overnight Utah Snow Ensemble forecast for Alta-Collins may be the crazies I've ever seen. Most of the European Ensemble (ENS) members are producing little to no precipitation over the next 10 days including during during the period from 10-11 December (more on those dates in a minute). The wettest ENS member puts out about 0.8" of water and 11" of snow.
Some members of the American Ensemble (GEFS) have very different ideas. There are 31 GEFS members and 10 of them put out more than 1.5" of water just through 00Z 12 December. Several more put out between 1 and 1.5" of water. There are some that track with the ENS (hard to see but they are there), but many GEFS members are much wetter.
What gives?
I don't plot or attempt to look at forecast plots like our four panel synoptic diagnostic for all 82 members of the Utah Snow Ensemble. Nobody has time for that. So I'm going to start here with the ECMWF HRES forecast that serves as the ENS control and is valid 0000 UTC 11 December. This forecast captures many of the salient details of the pattern that is going to prevail over western North America the next few days. A broad, low-amplitude upper-level ridge is centered near the Pcific coast (upper left) with an atmospheric river rolling over this ridge and across the northern US Rockies (lower right). Heavy orographic precipitation occurs where the atmospheric river crosses major western mountain barriers.
So in the HRES the central Wasatch is just to the south of the action. There are a few dribs and drabs that make it in, but accumulations are scant. Most ENS members have a similar forecast.
Some of the GEFS members, however, shift the storm track far enough south to get us in on the action.
Diagnosing the why this occurs in this case is beyond my abilities. I'll just say that I lean heavily toward the view that the central Wasatch may see some dribs and drabs over the next week but it's unlikely we'll see the game changing major accumulations that we need.
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