The Storm Ahead

Change is on the way as a deep upper-level trough is expected to move into the western United States early this week.  The GFS forecast for 0000 UTC 29 October (6 PM MDT Monday) puts the surface front over northern Utah and the Salt Lake Valley in cooler, northwesterly, post-frontal flow.  

This storm reminds me a good deal of the last with the trough digging into California and the bulk of the "dynamics", or large-scale lift ahead of the trough, moving across southern Utah and Colorado.  However, there is potentially one important difference.  Instead of closing off over central Utah, the trough remains what we call an "open wave" which means the GFS ultimately puts us into a colder, moister, northwesterly flow on Tuesday, as illustrated below.  

Our GFS-derived forecast for Little Cottonwood Canyon ultimately generates just over an inch of water and 16 inches of snow through 6 AM MDT Wednesday.  This is a colder storm than the previous one two, with temperatures forecast to drop to 10°F at the top of Mt. Baldy and snow to liquid ratios ultimately incresing to about 20:1 at the end of the storm.  

All in all, the GFS is producing a pretty good right-side-up Goldilocks storm.  If only we had a base for it to fall on.  

That said, the GFS is a bit more excited than most members of the Utah Snow Ensemble.  Both the GEFS and ENS mean through 6 AM MDT Wednesday (12Z 30 Oct) are right around 0.7" of water and 10" of snow.  If you like to wishcast, there are 3 members (out of 82) that produce more than 1.5" of water and 20" of snow.  

We've been working internally to produce snow forecasts using the experimental Rapid Refresh Forecast System (RRFS) that is being developed by the National Weather Service.  It only has 6 members and only goes out to 00Z 30 Oct (6 PM MDT Tuesday), so it doesn't capture the entire storm period, but through that time, the members are putting out 4-12" of snow.  For comparison, the GFS is putting out 15". 

So, to summarize, most of the model guidance at the moment is giving us a modest storm, with the GFS and a relatively small percentage of ensemble members going for amounts above 15".  There really are two wildcards with this storm.  One is will the frontal precipitation with the system, which is advertise to be sort of disorganized over northern Utah, come together for a while over the central Wasatch.  The second is will we get a decent shot of northwesterly flow post-frontal magic.  I'm inclined to be thinking 8-14" for a storm total through Wednesday morning, but we will see how things come together over the next couple of days.