This Was More Than a Downslope Windstorm

The windstorm of 8 September featured a remarkable coverage of strong winds both along the Wasatch Front but also westward from the base of the Wasatch Range.  Although there was certainly an important downslope wind component to the event, with the strongest winds observed at the base of the Wasatch Mountains, it's important to recognize that the event was more than just a downslope wind event.

Before looking at what happened on 8 September, lets' take a look back at another event that occurred on 1 December 2011.  You may recall this event as it produced a 101 mph gust in Centerville, Utah and quite a bit of damage in the Centerville and Farmington area.  Power outages affecting 50,000 customers occurred along the northern Wasatch Front through North Ogden, but were somewhat more limited in the Salt Lake Valley, althouth there were some in Millcreek, Murray/Holladay, and Midvale.

Power Outages from the 1 December 2011 event. Source: Salt Lake Tribune

Analyses of the event show a very typical pattern for northern Utah downslope windstorms with a developing upper-level trough in northerly flow closing off and centering in the Las Vegas area as depicted below at 700-mb (about 10,000 feet, or crest level for the Wasatch Range).  This leads to easterly flow across the Wasatch Range.  If you were to average the pattern for many Wasatch downslope windstorm events, it would look a lot like this.  

In contrast, the 8 September 2020 event produced power outages for 170,000 people with tree damage well west of the base of the Wasatch in areas such as Rose Park.  It was much more widespread.  A nice graphic produced by the Salt Lake Tribune showing peak wind gusts based on observations provided to the National Weather Service shows the strong winds along the base of the Northern Wasatch, but also remarkably strong winds in other areas including 67 mph in the northwest Salt Lake Valley, 77 mph at the Salt Lake Airport, 74 mph near East Canyon Reservoir, 61 mph at Parley's summit, and 54 mph at Kimball Junction.  

Thus, this was a regional event.  the strongest winds were at the base of the mountains in a few locations, but strong winds were observed everywhere.

A look at the 700-mb analysis for 1200 UTC (0600 MDT) 8 September shows the closed low that was found in the 1 December 2011 and other events, but note its location is near the Utah Colorado border.  Additionally, a strong gradient wraps around the low across northern Utah and over the West Desert.  A look at the water vapor imagery shows unusually high "brightness temperatures" (indicated by purple color fill) on the back side of the low.  All of this is consistent with intense upper-level front development and mid-level cyclogenesis (i.e., low pressure formation).  

That closed low developed rapidly over northern Utah.  Understanding this event and its widespread coverage requires not only thinking about mountain wave dynamics, but also the role of the developing closed low and upper-level front in producing strong regional-scale winds across northern Utah.  

At least that's my hypothesis.

I need to go and teach class, so apologies if this post is a bit uneven.  I just wanted to get this out to illustrate that the widespread nature of this event requires avoiding what I call "mountain myopia" and also considering larger-scale processes.