Some Oddities About This Deluge

Impressive rainfall numbers are coming in from this latest storm period.  Total accumulated precipitation since very early Friday morning (~5 AM MDT 26 April) is now over 1.1 inches at the University of Utah.

A look at accumulations for the Salt Lake Valley region shows the largest accumulations in the Tooele Valley and Magna areas and secondarily along the east bench of the Salt Lake Valley.  Water equivalent totals in the central Wasatch are actually lower than at the high precipitation sites along the east bench. 

If you are wondering, my guestimate for Alta-Collins based on automated measurements is about 6 inches of creamy snow from 0.85" of water.  I suspect snowfall amounts at the base are lower given that some of the precipitation may have fallen as rain yesterday.  

Let's talk about some of the oddities of what is happening this morning.  We operate profiling radars in the Salt Lake Valley (at Highland High) and at Alta.  Each peers up through the storm and we generate time-height sections with them showing the evolution of the vertical structure of the storm.  In the 3rd row below are profiling radar time-heights from Highland High showing nearly continuous precipitation over the three hour period ending at 16:40 UTC (10:40 AM). In contrast, at Alta, there was only a brief period of precipitation.  

So if you drove up to Alta expecting dumpage, I suspect you were disappointed.  Hopefully the cream on crust skied well.  

So, my back yard this morning looked like coastal northern California.  Foggy, rainy, and green.  The deer are laying low.

But up at Alta, you could actually see blue skies at times this morning.  The web cam image below was taken at 7:42 AM MDT.

Of course, the clouds appeared to move in and out and you could have been in the ping pong ball at times.  Still, a different world up there than in the valley.  

What also caught my attention was how shallow the radar echoes were above Highland High.  For most of the morning they have been below 2000 m above ground level or about 3350 meters above sea level.  That's just under 11,000 feet.  That makes it a very shallow storm, at least in the Salt Lake Valley (it may be deeper to the northwest where echoes in the KMTX radar are stronger.  Temperatures this morning at 11,000 feet were around -5°C.  At such temperatures, it is hard to get ice to form.  There is a decent chance that the rain this morning in the Salt Lake Valley was produced by warm-rain processes.  Instead of there being ice crystals that grow and then fall out and melt on their way to the ground, as often occurs when it rains around here, cloud droplets colliding and coalescing was likely the main pathway for generating rain drops.  This is hard to do over continental interiors.  It's the sort of thing that one typically sees in coastal areas (e.g., northern California) or the windward side of the Hawaiian Islands.  

Anyway, that's my hypothesis.  I can't rule out the possibility that the storm was deeper than indicated by radar, but my impression, based on the droplet characteristics, that we saw warm-rain processes predominating this morning.  I await additional observations from you readers.