Prospects for Early Snowfall

 

Perhaps the meme above is correct, but it's a pretty exciting forecast if you ask me.  It's been a long summer, which followed on the heels of a so-so COVID winter.  The return of the storm track this fall should be cause for celebration.  Let's have a look, focusing on what may happen over approximately the next week.  

There are three major systems of concern.  The first is and upper-level trough that this morning was located just off the Pacific coast (dashed line below).  The second is an upper-level trough and surface cyclone over the Gulf of Alaska and Bering Sea (circled).  

This second feature is associated with a strong Pacific jet stream and is fast moving, in contrast to the relatively slow moving upper-level trough along the Pacific coast.  Ultimately, these two features merge and move slowly inland as an upper-level trough (dashed line below) that is located just upstream of Utah at 0000 UTC 9 October (6 PM MDT Friday).  

Meanwhile, upstream of that feature, the North Pacific jet is locked and loaded and about to transition from zonal (i.e., predominantly westerly) to a wavier, more amplified flow.  The loop below illustrates this transition and how it ultimately leads to a very deep upper-level trough over the western U.S. at 0000 UTC 13 October (6 PM Tuesday).

This results in two possible storms over the next week for the Cottonwoods, as illustrated by the downscaled NAEFS ensemble plume below for Alta-Collins below.  

Through 0000 UTC 13 October (6 PM Tuesday) the downscaled NAEFS ensemble generates a range of 18 to 45 inches of snow with a mean of about 30 inches.  Mean water equivalent is around 3 inches.  

One factor contributing to uncertainty during this period, especially for the second storm, is something known as downstream development.  Amplification of the upper-level pattern over the Pacific reflects the downstream propagation of energy at jet-stream level, ultimately leading to the trough development over the western U.S. next week.  The details of that trough development are very difficult to nail down and the precise position of the upper level trough will influence snowfall in the Cottonwoods.

For example, below is a comparison of the GFS and ECMWF 500-mb height forecasts from Pivotal Weather for 0000 UTC 13 October.  The GFS (top) puts the trough over northeast Utah, whereas the ECMWF (bottom) has it over the Utah-Arizona boarder.  

These small differences in track and position ultimately affect the placement of precipitation features associated with the trough and flow interactions with, for example, the local terrain and the Great Salt Lake.  

Regardless, I'm looking forward to this period as it has been a long time since we've seen real winter weather around here.  I often express concern about early season storms since if the snow doesn't keep coming, we often go through a period where the south aspects melt out and the snowpack on the north aspects weakens, creating a persistent weak layer that is problematic for avalanches during the season.  However, we are in such serious drought that I think we can't be picky anymore.  We need moisture.  Even if most of this snow were to melt, the water will help increase soil moisture prior to the development of the seasonal snowpack.  If it does hang around, it will give us an early start to the snow accumulation season. 

Perhaps the most important question is will we be ski touring later next week?  Much will depend on whether or not we end up in the upper-end of model projections and your tolerance for low-tide conditions.  Best case scenario is we get walloped.  Worst case is that next-week's trough digs to the south, we get some snow with the first trough, but the second is a bust and we have several inches of snow on dirt.