Active Pattern with a Lot of Possibilities

There are a lot of ways to look at the downscaled NAEFS plume for Alta-Collins below. 

One could focus on the upper-end forecasts.  What could be better than 5+ inches of water and 75+ inches of snow?  

Or one could look at the low end forecasts.  Those aren't so bad, but give a pretty different result with less than 20 inches of snow and 1.5 inches of water.  

Or one could see the period beginning on Friday morning (about 12Z 1 Dec) through Monday (about 18Z 4 December) as a stormy period in which there are a wide range of possibilities depending on details that cannot be confidently predicted at this stage.  

Let's take a look at what some of the models are saying. 

The GFS is pretty much right in the mean of the NAEFS forecast and puts out a total of 2.75" of water and 41.8" of snow for Alta-Collins.  A close look at the accumulated precipitation and snowfall traces shows a series of three waves moving through, with precipitation and snowfall waxing and waning with the passage of these systems.  

In addition, one can see that this is a period in which temperatures initially fall through late Friday, but then climb again into Sunday, when the winds also increase.  As a result, snow-to-liquid ratios maximize late on Friday and then decline on Saturday and Sunday.  Thus, should this forecast pan out, we will probably be dealing with upside down snow on Sunday.  

The Euro, on the other hand, is quite a bit drier.  I don't have an equivalent plot for the Euro, but totals for water equivalent in the Alta area are around 1.56".  It's not unusual for the Euro to be drier than the GFS, but there's actually a fairly large difference between the two models in terms of the storm track and strength of embedded features with the GFS favoring a somewhat more southward track with stronger troughs.  

For example, the GFS forecast for 0000 UTC 3 December (5 PM MST Saturday) has a strong, sharp, short-wave trough at upper levels over Idaho.  The 700-mb (crest-level) along the Idaho–Nevada border is 50 knots.  

In contrast, that same trough and flow in the Euro are weaker and precipitation, especially in northern Utah, is also weaker.  

Later in the storm cycle, at 0600 UTC 4 December (11 PM MST Sunday), the GFS brings another strong trough through.

But the Euro has a very different forecast with a trough up in the Pacific Northwest.  

These are very different forecasts and they reflect the challenges of forecasting the evolution of features (e.g., troughs and frontal systems) developing in and emerging from an intense jet over the western and eastern North Pacific and ridge off the coast of California and Baja California.  

I can't pick one of these forecasts over the others.  The troughs above are currently weak features embedded in the Pacific jet.  The models producing different forecasts tell us that their evolution over the next few days is someone chaotic and uncertain.  A stormy period beginning Friday looks likely, but just how big is hard to say and will depend a lot on how things come together over the weekend.  A GFS-like solution might give us a major storm cycle with 30-45" of snow through Monday.  A Euro-like solution is more subdued and perhaps in the 10-20" range.  Stuff at the upper end of the NAEFS is probably unlikely.  

Breath.  Keep expectations low and hopes high.