A Storm Worth Watching

Enjoy today's mild weather because things are gonna change. I can feel it.  

By tomorrow morning, we will be in colder air.  The GFS shows that by 1200 UTC (6 AM MDT) tomorrow morning the surface cold front is through and we are in colder, westerly flow.  

Temperatures and snow levels will be dropping overnight and in the wake of the cold front.  Our GFS-derived forecast product for upper Little Cottonwood shows forecast highs for Alta-Collins today in the low 50s, fairly close to yesterdays 54°F max, and then temperatures falling overnight and remaining in the low 30s tomorrow.  

Concurrently, the wet-bub zero level drops to below 8000 ft by 9 AM tomorrow morning and near 7000 ft by 6 PM tomorrow afternoon.  That puts the snow level around 7500 feet in the morning and perhaps 6500 ft by afternoon.  

The GFS generates periods of snow with and following the front, adding up to about 8" at Alta-Collins by  6 AM Thursday.  The HRRR produces just under 8" by 6 AM Thursday, so that's decent agreement.  

There are, however, some real wildcards for this one.  The models are generating a prolonged period of cold, unstable post-frontal flow through Thursday afternoon.  The GFS has WNW flow at crest level (700-mb) during that post-frontal period.  

This raises the possibility of orographic and lake-effect snowshowers.  In the case of the latter, our GFS-derived lake-effect guidance is generating decent lake-effect likelihoods late Wednesday night and Thursday morning.  

The GFS large-scale forecast conditions in terms of instability and flow direction looks pretty good for the Cottonwoods.  However, that model can't really resolve the Great Salt Lake or the central Wasatch terrain.  The HRRR, on the other hand, can better resolve the Great Salt Lake and the central Wasatch.  However, it favors more northerly flow.  As a result, it produces a lake band, but more for the Oquirrhs than the hallowed ground of Cottonwoods.

The HRRR also produces a powerful lake band.  That might come through, but models like the HRRR tend to produce strongly banded features more commonly than Mother Nature, who favors more disorganized systems.  

We'll have to see how this one plays out.  I think that 6-12" for Alta-Collins is likely through Thursday afternoon, but we could do better if we can get the right flow and productive orographic and/or lake-effect snow showers late Wednesday night and Thursday.  

I usually conclude posts like this with "keep your fingers crossed," but it is unclear what to root for so early in the year.  I've seen these early season snowfalls turn to freaky facets too many times.  This is a storm that perhaps needs to go big or go home.