Friday, December 1, 2023

Buckle Up

The much anticipated storm cycle is upon us.  It is already snowing what appears to be pixie dust up at Alta.  Automated observations show 2" of new snow from .03" of water, which would be less than 2% water content if accurate.  The latter may be underdone, but it certainly looks light and fluffy on the snow-stake cam and pictures never lie, right? 

The models are now in pretty good agreement on the general characteristics of the storm through 0000 UTC 3 December (5 PM MST Saturday), so I'll start with that period.  There will be a crest-level (10,000 ft) trough passage at around 2 PM MST this afternoon and then a second one at 11 PM MST tonight.  I've identified each of these in the HRRR-derived guidance for Little Cottonwood Canyon below.  Then there's a third trough passage at around 5 PM MST Saturday, and this is where we start to see a good deal of model divergence. 

Through 5 PM MST Saturday, the models are calling for persistent, moist, westerly to northwesterly flow at crest level.  From 7 AM MST this morning through 5 PM MST Saturday, the HRRR generates 1.06" of water and 16.4" of snow at Alta Collins.  The GFS is a bit behind with 0.78" and 13.6" of snow.  A look at the downscaled SREF shows most members through 5 PM MST Saturday (03/00Z) are tightly clustered around 0.6 to 1.4" of water and 12 to 24" of snow (I'm eyeballing).   

Thus, I'm feeling pretty good about totals through 5 PM MST Saturday at Alta Collins 0.75-1.5" of water and 12-20 inches of snow.  

After that, I get an ulcer.  As can be seen in the SREF plume above, there is enormous spread in what happend after that.  Some members produce little precipitation, some a ton.  This is what I'll call the Atmospheric River stage of the storm. A look at the GFS forecast at 1500 UTC 3 Dec (8 AM MST Sunday) shows the situation with a broad, low-amplitude ridge over the Pacific States and moist, northwesterly flow extending into northern Utah.  True atmospheric river conditions (integrated vapor transport greater than 250 kg/m/s) never quite make it to the Wasatch, but we're just a bit below that.  We also see warm air advection (i.e., the transport of warmer air) at crest-level.  Much of the precipitation being produced by the GFS in our area is orographic and due to mountain uplift.  

Atmospheric rivers are very fickle in Utah.  Some generate heavy precipitation.  Some little.  As noted above, the precipitation amounts being produced by the various models and ensembles vary quite a bit after 0000 UTC 3 December.  One thing they agree on is that it is going to get warmer.  A look at the GFS-derived guidance for Little Cottonwood shows the wet-bulb-zero level rising from the valley floor on Saturday morning to a bit above 7000 feet by Sunday night.  This would mean snow levels to perhaps 6500 feet by Sunday night.  Temperatures climb to the the mid 20s at Alta Collins by Sunday afternoon and consistent with this is a decrease in snow-to-liquid ratios to values around 10 to 1.  

So, expect an upside down snow on Sunday assuming the storm delivers.  It looks like the Utah Avalanche Center has started issuing forecasts again and just in time as if we start seeing substantial water totals and upside down snow on the mid and upper elevation facets that exists in many areas, things are going to get interesting real quick.  

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