Hot and Dusty with Avalanches and Flooding

These are eventful times, and there is much to talk about today.

First let's talk about the temperatures.  The monster swing over the past 10 days or so is well illustrated by the graphic below which is produced by the National Weather Service Forecast Office in Salt Lake City.  It shows the range of average temperatures (1991-2020) in green, record highs (top of red), and record lows (bottom of blues).  The range of observed temperatures each day are the dark blue bars.  On April 4th, the high temperature was only 33˚F.  By April 11th, it was 83˚F, a record high for the date. 

Source: NWS

Yesterday was a touch cooler, although we still hit 78˚F, marking three days in a row above 70˚F and five days in a row above 60˚F.

As readers of this blog are well aware, this led to elevated avalanche hazard as we transitioned from dry to wet avalanches, a prolonged closure of Little Cottonwood Canyon, and regular afternoon closures of Big Cottonwood Canyon.  

🚧 #SR210
Can’t take the suspense? Results of yesterday’s (4/11) @UDOTavy heli-bombing mission, the slide stopped just short of the road.

The trees, however, got more than they bargained for.

📍: White Pine, China Wall pic.twitter.com/FA8XmAON1l

— UDOT Cottonwood Canyons (@UDOTcottonwoods) April 12, 2023

My view is it's been too hot to ski tour, so I've been perfectly happy down in the valley dreaming of cool nights and warm (not hot) days for a good corn cycle.

Unfortunately, there is a fly in the ointment for that and that is the dust that was blown into the area yesterday.  Observations from the University of Utah snowed elevated PM2.5 levels, peaking at around 25 ug/m3 during the day yesterday. 

Source: MesoWest

This dust was kicked up and transported to the area by the strong southerly and southwesterly flow.  I had hoped that with such a wet season, we might escape without such dust, but alas, that's not to be. 

Finally, there was localized flooding in several areas of northern Utah yesterday.  In Salt Lake City, water was spilling out of the Emigration Creek fed Wasatch Hollow area and onto surface streets near 1700 East and 1700 South.  Much thanks to volunteers working to sandbag the area. 

Citizen volunteers worked into the night to protect against flooding along 1700 South in Salt Lake City last night after Mayor Erin Mendenhall issued an emergency order.https://t.co/zItZ3KY7UX pic.twitter.com/J46xZoPUEq

— The Salt Lake Tribune (@sltrib) April 13, 2023

The hydrograph for Emigration Creek, taken near Hogle Zoo, shows the flow peaking above flood stage at 155 cfs.  If the data I obtained from the NWS site is correct, the peak flow at this site is 164 cfs on May 13, 1984. 

Source: NWS/CBRFC

1984 is not a typo.  As much attention as 1983 gets because of the iconic photos of water flowing down State Street, 1984 was also a monster snowpack and runoff year.  At the Parley's Summit SNOTEL, for example, peak water equivalent in 1984 (blue line) exceeded that in 1983 (brown).  This year (black line) the peak was just above 1984.

Source: USDA

The good news is that we are cooling off today, and the expectations are for the flows on Emigration Creek, after dropping overnight, to continue to drop today.