Can You Help Explain the Overnight Snows

Sometimes those with investment portfolios and trophy homes get the last laugh and that was the case last night with Deer Valley the big winner in the central Wasatch.  Below is a look at the Ontario Snow Stake Web Cam at 8:05 AM showing a solid 8".

Source: https://www.deervalley.com/explore-the-mountain/webcams

Meanwhile, on the other side of the Wasatch, at a ski area also frequented by people with investment portfolios and trophy homes but better known for deep powder, pickings were much slimmer.  

Source: alta.com

This is a pattern that does sometimes bless the Deer Valley side, although I confess I don't exactly know why, in part because of poor radar coverage, poor radar estimates, and limited observational data.  

The issues with radar coverage are apparent in the plot below, which shows the accumulated precipitation estimated from the National Weather Service Radar (KMTX) for the 6-hour period ending at 1400 UTC (7 AM MST).  The radar thinks the heaviest precipitation is in upper Big Cottonwood, in Brighton Basin, rather than to the east in the Deer Valley Ontario area. 

Source: https://mrms.nssl.noaa.gov/

In part, this reflects differential orographic blocking of the radar, which results in weaker returns (all else being equal) in the Deer Valley area.  It could also reflect overshooting by the beam if the growth of snow crystals in this situation is shallow.  Finally, the correlation between radar reflectivity and snowfall rate (including water equivalent rate) is much lower than it is for rain, so there are times when radar estimates are simply out to lunch.  In any event, the National Weather Service radar is not all that helpful for understanding what is happening in these events.  

There is also a complete lack of upper-air observations near Deer Valley, so we have to make due with the sounding from the Salt Lake City International Airport.  This morning's sounding shows southerly winds at low levels and westerly flow at 700 mb, roughly 10,000 feet. At issue is whether or not the flow in the Heber Valley in such a pattern is lifted and produces local, shallow snowfall enhancement on the Deer Valley ridgeline.  Some have speculated this is the case, but the hypothesis has not been carefully evaluated. 

Source: SPC

Perhaps a conflicting piece of evidence in this case is that the flow direction on Mount Baldy was not southerly overnight but southwesterly.  

If it was southerly or southeasterly, it would fit this hypothesis a bit better.  Of course, there's always the possibility that wind direction is affected by local conditions and the overall flow in that area is actually ascending out of the Heber Valley.  

I have another hypothesis, although it might not be as compelling as the flow direction one.  As shown in the sounding above, the crest level flow in this case was westerly and we had near saturated conditions through a deep layer, with strong flow in the upper troposphere.  On the other hand, the low level atmosphere in the Salt Lake Valley was dry with a relatively high cloud base.  A look at radar echoes for this period showed that they were not evident right over the immediate western face of the Wasatch, but somewhat downstream. 

My hypothesis is that in this event, we are seeing a situation where there is weak orograhic lift over the western Wasatch, but it is is deep, resulting in ice crystal generation aloft. Those ice crystals are carried downstream and fall out preferentially downstream of the Wasatch Crest over Deer Valley.  

There are examples of this happening over other ranges.  The best example I can think of is a case examined by Geerts et al. (2015) in the Range of Wyoming.  They flew through the storm in an aircraft with upward and downward pointing cloud radars (the dashed line below is the aircraft flight level).  These radars don't scan, but instead collect a continuous curtain of radar data above and below the flight track, allowing the detailed vertical structure o fthe storm to be observed.  The top panels are two different flight flight tracks during the storm.  In both cases, there are no low echoes upstream of the mountain and on the windward (left) side of the crest, reflectivities are highest aloft.  They calculated the streamlines of ice crystals and showed that those generated in this windward area aloft were carried downstream and fell out on the lee side of the mountain. 

Source: Geerts et al. (2015)

That paper is a favorit of mine because it shows how you sometimes need to think beyond where the mountains are forcing rising motion. You also need to think about transport and fallout.  This is particularly important when there is crystal generation aloft, possibly well above the crest.  

Anyway, that's my story and I'm sticking to it.  Perhaps you have other ideas and can help explain the overnight distribution of snowfall.