The Jackson Hole News&Guide checks in with the Bridger-Teton Avalanche Center to bring readers weekly snowpack reports and summaries. Find avalanche reports, snow conditions and new features at JHAvalanche.org. Backcountry travel in avalanche terrain is inherently dangerous. This report is not meant to replace your own information gathering, only to be one source, and is not a replacement for taking an avalanche safety course.
Weather: The story of 2020, so far, has been snow. Each day of this year, as of Tuesday morning, registered at least 3 inches of snow, so the skiing has been phenomenal.
Rendezvous Bowl at Jackson Hole Mountain Resort has seen 44 inches of snow with nearly 5 inches of snow-water equivalent since the New Year. Unfortunately for backcountry skiers, wind has accompanied the storms, moving snow around onto leeward slopes.
Wind speeds have remained constant in the range of 20 to 30 mph near 10,000-foot ridgelines in the Tetons, with gusts creeping northward of 60 mph on Jan. 2 and 5. According to the University Corporation for Atmospheric Research, a free training resource for geoscientists, wind speeds above 20 mph can transport high-density snow, sometimes doubling or quadrupling deposition rates on leeward slopes.
The roughly 9% density snow that fell in the past week was right in line for this time of year. Avalanche Center Director Bob Comey said that on the whole the snow so far this year has been heavy, with December showing an average of 11% density, 2 or 3 percentage points higher than typical.
Snowpack: Few avalanches had been reported to the Avalanche Center’s website as of Tuesday morning, but Comey was under no illusions that means slides aren’t happening.
“I think there have been a lot of slides,” he said. “Everybody that’s been out, guides and forecasters, said we’re seeing evidence of them.”
The observations the Avalanche Center has received indicate some slopes have avalanched in the fresh storm snow, particularly leeward slopes where the 40 inches of recent snow could be more like 80. Comey said people have reported seeing odd piles of snow at the base of slopes, but the crowns aren’t visible because wind-transported snow covered them following the slides.
A break in the weather would allow forecasters to observe some of the slides Comey suspects are happening, but no such stoppage is expected. Comey pointed to a piece of data that could help predict if an avalanche cycle from this storm system or the snow that is falling this week will step down to the persistent weak layer.
The Avalanche Center’s Snowpack Tracker has a section on historic weather data, and it offers insight in how severe an avalanche cycle might be. One of the graphs shows the 10-day cumulative snow-water equivalent (how much water weight is in the past 10 days’ snow), an indication of how heavy the storm snow is. Spikes in cumulative snow-water equivalent correlate with increases in reported avalanche activity.
“When we have a persistent weak layer,” Comey said, “it takes as much or more than the previous storm cycles to reach that weak layer.”
Previous storm cycles this winter that produced widespread avalanche activity had 10-day cumulative totals between 4 and 6 inches of water. As of Tuesday the 10-day cumulative snow-water equivalent was 5 inches.
Outlook: More snow is on the way, according to National Weather Service forecasts. A winter weather advisory starts Wednesday morning, and the storm is expected to bring a foot of snow to the upper elevations.
“For the next 10 days or two weeks,” Comey said, “we’re going to have a fair amount of moisture coming out of the Gulf of Alaska and the Pacific Northwest.”
Those Pacific storms are going to drop the bulk of their moisture over the Cascade Range in the Northwest, but Comey expects the remnants that reach the Tetons to still bring several inches of snow a day. He said the denser snow is building the snowpack and making it more difficult to trigger the persistent weak layer, which ultimately is good, though it comes with storm slab danger.
“With all the wind and the dense snow, we are getting more stability slowly,” he said, “but there is still some pretty good danger out there.”
— Tom Hallberg