An End to Endless Summer?

Believe it or not it is looking increasingly likely that we will see an end to endless summer later this week.  The models are advertising the movement of an upper-level trough over the western later this week.  We may first begin to feel it's effects Wednesday with an increase in southerly flow and a last warm day as the trough approaches and then cooler air moves in later in the week.  The GFS forecast for 1200 UTC 18 October has a bonafide cold front over northern Utah.   

It's a bit soon to lock in on that solution, so let's have a look at the Utah Snow Ensemble for Alta-Collins.  All of the ensemble members are dry through 0000 UTC 17 Oct (6 PM MDT Wednesday).  Then nearly all produce precipitation at some point during the 2.5 day period through 1200 UTC 19 Oct (6 AM MDT Saturday.  

Timing and amounts vary with the ECMWF ENS members generally being drier than the GEFS.  Through 1200 UTC 19 Oct (6 AM MDT) the former produces an average of about 0.75" water and 6" of snow at Alta Collins.  The latter is at about 1.6" of water and 16" of snow.  A look at the plan view plot for the Wasatch Front at that time shows a mean for the ENS and GEFS combined of 10.6" at Alta Collins and even some light amounts in the valley (<1").  

The spread is still large though, so we will see how this comes together in the coming days.  However, our long stretch of days with above average high temperatures, which extends back to September 22, looks to be over later next week and probably after Wednesday unless the trough and cold front are slow to arrive.