The Coming Storm Must Come Through!

I don't want to put any pressure on Mother Nature, but the coming storm that begins tonight must come through.

Let me first explain why.  In its wake, the models are advertising the dreaded split-flow pattern with a humungous ridge over the northeast Pacific Ocean.  There are some modest differences between the GFS and ECMWF HRES forecasts for 1200 UTC 14 December (5 AM MST Friday), but this a pattern from which I'd expect scant precipitation.  

Now let's get back to the coming storm.  Today we will see strong southerly flow and predominantly overcast skies in the mountains with perhaps a few fits and starts of snow.  At 10 AM, ridgetop winds were gusting to as high as 76 mph.  

Precipitation moves in tonight, first in southerly to south-southeasterly crest-level flow, which for a while may favor the Wasatch Back and upper Big Cottonwood.  The crest-level flow is forecast to shift to southwesterly and then *possibly* westerly tomorrow.  More on this in a minute.

Through 0000 UTC 13 December (5 PM MST Monday), GFS-forecasted water equivalent precipitation is 0.9" at Alta Collins and 0.65" at Deer Valley-Ontario.  The more excited HRRR, plotted below, has 1.23" at Alta–Collins and 0.82" at Deer Valley-Ontario.  

As noted above, the crest-level flow may shift to westerly tomorrow.  At least the GFS  is into such a solution with moist, unstable, WNW-W flow most of the day tomorrow through tomorrow night.  The forecast below is one that I generally like, although I would like it locked in a bit more to northwesterly.  

The HRRR, however, has other ideas and only briefly brings the flow to WNW following frontal passage and then flops things around to southwesterly again.  

The ECMWF HRES (not shown) is a bit more GFS like, so perhaps the W-NW flow wins on penalty kicks?  Let's hope so.

Taking a look at the GFS-derived Little Cottonwood forecast, we see the fits and starts of snow today, with a miserly 0.03" of water equivalent and 0.3" of snow through 5 PM.  Things pick up tonight through tomorrow morning with an additional 0.36" of water and 5.4" of snow through 5 AM Monday.  This is followed by an additional 0.52" of water and 9.1" of snow through 5 PM Monday.  Note that the snow-to-liquid ratio is decreasing throughout the storm and gets down to about 18:1 (6% water content) by Monday morning.  Thus, tomorrow we will be near or in cold-smoke territory.  

Through Tuesday afternoon, the GFS puts out 1.23" of water and 21" of snow.  

I have to say, however, that this is a forecast that gives me a good deal of heartburn.  The trough characteristics are such that the details of what happens after the frontal passage are a bit uncertain.  The HRRR and GFS put out over an inch of water each, although with differences in storm characteristics.  The ECMWF HRES seems to give a favorable post-frontal flow pattern, but with scant precipitation.  If cold northwesterly flow comes through, there is a prolonged period where the lake could get going, adding another wildcard into the mix.  Finally, the large snow-to-liquid ratios means we could see a good deal of snow per unit of water equivalent.  All of this makes me think there are a wide range of possibilities.

Indeed, this is precisely what the short-range ensemble forecast system (SREF) is doing.  The forecasts for Alta are relatively evenly distributed through 0000 UTC 14 December (5 PM Tuesday), especially from about 12" to...wait-for-it...38".  

If this seems like a rambling, incoherent blog post, that's because it is!  If you want confidence, look elsewhere.  Periods of snow through at least Tuesday and possibly into Wednesday.  I'll go for storm totals through Tuesday night of at least 15 inches at Alta, but possibly a good deal more if everything comes together.  Let's hope it does.