|UDOT crews working tirelessly to clear the road to Alta after the historic avalanche cycle that left inhabitants trapped in Little Cottonwood Canyon for 52 consecutive hours. Photo by Rocko Menzyk
From Feb. 6 – 7, 48 avalanches were recorded in Little Cottonwood Canyon, 21 of which hit the highway, keeping inhabitants of the town of Alta trapped indoors on an “interlodge notice” for 52 straight hours. It was the longest time Alta had been interlodged in its history. The Tanner’s Gulch slide path released a size D4 avalanche that was triggered naturally and had traveled all the way down to highway 210, burying a section of the road. This is only the seventh time in recorded history that Tanner’s has slid all the way to the highway.
“When you see trees and snow on the road from Tanner’s, an avalanche path that from the ridgeline to the road is over a mile and a half and over 4,000 feet above us, even a small amount of trees and debris on the road is a big avalanche,” said Utah Department of Transportation avalanche forecaster Mark Sauer.
The avalanche cycle which occurred from Thursday, Feb. 6 to Friday, Feb. 7 has now been labeled by meteorologists as a “historic weather event,” and to understand how it happened, one must look at the events leading up to this over the course of the entire winter.
Utah Avalanche Center forecaster and education specialist, Trent Meisenheimer, said that this all goes back to mid-October of 2019 when the Wasatch received its first snowstorm of the winter season that deposited cold, low-density snow. The next snow would not fall until Nov. 21 and would be much warmer than the previous October storm. Because of this, a weak layer of faceted snow formed in the snowpack, which is angular snow with poor bonding created from large temperature gradients in the snowpack.
“Faceted snow is the jihadist of our snowpack. It’s bad and we don’t want it,” Meisenheimer said.
By Dec. 15, faceted snow was still a major concern in the Wasatch’s snowpack. It had created a persistent weak layer on north-facing slopes, which would remain an avalanche problem on the UAC’s daily issued avalanche forecast until Jan. 19, 2020 only to be reinstated on south-facing slopes the following day, Jan. 20. Then, on Feb. 3, an “atmospheric river,” hit the Wasatch, that, in conjunction with strong, sustained winds from the northwest, would be the catalyst for this historic avalanche cycle.
Michael Wessler, a meteorologist and Ph.D. student at the University of Utah, described an atmospheric river as “a firehouse of moisture,” where a little pathway of moisture is transported north out of the tropics towards the west coast of North America by very strong winds.
“Atmospheric rivers are not unique events,” Wessler said, and not all locations see the same weather impacts as others when dealing with one. What makes atmospheric rivers special for a place like the Wasatch are the mountains themselves.
“The key piece here in the Wasatch is that we have terrain. That terrain acts to basically take this atmospheric river and wring it out like a sponge,” Wessler said.
When the atmospheric river first entered the Wasatch, it deposited cold, low-density snow in the mountains, with large snow totals being found on valleys and benches. Then, temperatures rose and winds picked up, sustaining an average wind speed of about 50 mph for 36-48 hours, according to Wessler. This is the timeframe when higher density, heavier snow was being deposited on top of the lighter, lower density snow thus creating an extremely unstable weak layer. This is what meteorologists refer to as an “upside-down storm,” drastically increasing the likelihood for avalanches. It was at this point on Feb. 6 that avalanche forecaster Trent Meisenheimer decided to call his boss and director of the UAC, Mark Staples, and tell him to issue an “avalanche warning,” for Little Cottonwood Canyon.
“Winds above me are just nuking sideways, it’s snowing super hard, and I say ‘Hey Mark, I think it’s time to pull the warning,” Meisenheimer said.
The avalanche warning along with the interlodge notice for the town of Alta would remain in effect until 9:30 a.m. on Saturday, Feb. 8. This was the first time that Alta residents were allowed to step outside in 52 hours. The following week was greeted with sunshine and temperatures in the low to mid-twenties (°F) providing excellent skiing conditions. But the snowpack is already beginning to change again. Cold and clear conditions the week of Feb. 9-16 have furthered the development of faceted snow, and with a new storm system moving into the Wasatch on Sunday, Feb. 16, there is potential for another weak layer forming and raising the avalanche hazard.