Snowpack Summary

This season re-defined “feast or famine” conditions, suggesting—once again—that abnormal may in fact be the new normal.

We were off and running when a warm storm near the end of October provided a supportable 2-3’ snowpack. We (wrongly) assumed the early start to the season was a harbinger of an epic, record-setting winter—but we’ll get to that later. In the following weeks, very warm temperatures and sunny skies reduced the snowpack to dirt on many slopes. The snow that lingered on shaded slopes turned to hard melt-freeze crusts.

The next month delivered a few small storms interspersed with both very warm and cold spells. Particularly dry weather during late November and early December produced a widespread layer of facets sitting atop a hard crust—at least where the snowpack survived the early season thaw. This facet/crust layer became the season’s primary weak layer when it was buried on December 11th.

Snowfall finally returned—with a vengeance. December 11th to January 7th was one of the snowiest periods in memory. An estimated 100” (on Baldy) to 200” (Vienna Mine) of snow fell during this period, coming in three distinct 5-7 day storm cycles. Most SNOTEL stations were sitting at 70-80% of normal snowpack prior to this period and were 140-150% of average when the dust settled on January 7th.

Not surprisingly, these large storms falling on a well-developed weak layer produced dangerous avalanche conditions. Our first Backcountry Avalanche Forecasts of the season came out on December 13th with HIGH danger in three out of four zones. An Avalanche Warning was issued the next day with HIGH danger in all zones. The avalanche danger remained elevated—CONSIDERABLE or HIGH—for most of the first month of the forecast season.

Avalanche activity during this period was fairly extensive. The first avalanche cycle came with the initial multi-day storm from December 12th-18th, including four slides triggered remotely from lower angled terrain. More avalanches occurred with the “smaller” storm cycle from December 22th- 27th, but the largest avalanche cycle of the season came with the January 3rd-8th storm event. When the storm cleared, we observed 45 avalanches D2 or larger. Given the often atrocious visibility during this snowy period, the number of actual avalanches was undoubtedly much higher.

Up until this point, the winter was a forecaster’s dream: a well-developed persistent weak layer, major storm cycles, numerous avalanches, and no close calls that we heard about. But like many good things, it wasn’t to last: winter was here, and then it wasn’t. The table on the following page compares the SWE that fell at stations across the forecast area during the first and second halves of the winter.

Very little snow fell between January 7th and February 27th. In the Wood River Valley, only a few inches accumulated while wetter areas near Smiley Creek and Stanley picked up 12-16”. The exceptionally dry conditions led to extensive near-surface faceting on many slopes. On January 20th, just 6-8” of new snow and strong N winds resulted in several small, human-triggered slides in the Banner Summit zone. On February 28th- March 1st, a very warm storm delivered rain up to 9,000-9,500’ and 4-8” of dense snow to the highest elevations.

As weak layers that formed during dry periods were (very) slowly buried by minor snowfall and wind events, the rest of March had us worrying about human-triggered slides. The right combination of slab and weak layer was relatively isolated, and the danger remained MODERATE; the conditions seemed ripe for getting surprised. On March 18th, riders in Baker Creek triggered three medium-sized (D1.5) avalanches, but no one was caught (observation, observation).

The next significant weather event didn’t deliver snow, but rather heat and sunshine. March 23th-28th brought the warmest temperatures of the winter, and upper elevation temperatures stayed above freezing for five consecutive nights. Fortunately, clear skies provided a decent freeze most nights, preventing a major wet slide/wet slab cycle. Still, countless small wet avalanches occurred, and a few large wet slides hit Warm Springs Road near Ketchum.

Once the snowpack froze hard on March 29th, we returned to relatively stable conditions. The first 10 days of April saw a mix of LOW and MODERATE danger. We issued our last (and 119th) daily Backcountry Avalanche Forecast on April 10th. Owing to the three month long drought from January 8th-April 9th, we issued twice as many LOW danger forecasts compared to the previous two seasons; over half of the Soldier & Wood River Valley zone’s danger ratings were LOW. Our snowpack had dipped to 60-80% of normal by early April.

The proverbial faucet turned on shortly after publishing our final daily Backcountry Avalanche Forecast. From April 10th through early May, the mountains received 3-6 feet of snow. The low snowfall overloaded a near surface facet/crust+facet weak layer on northerly, upper elevation slopes, resulting in skiers and snowmobilers triggering four persistent slab avalanches in an eight-day period in mid-April. Three skiers were caught in two separate events, and a snowmobiler was able to outrun a large slide he remotely triggered. In early May, a group of snowmobilers remotely triggered a 1000 ft wide avalanche on this same weak layer. Fortunately, no one was injured in the late season rash of human involvements.

While this season’s peak SWE was well below normal, a cool and wet spring allowed for a slower melt. When we issued our final General Avalanche Information product on May 9th, the snowpack contained more water than normal for that date.

Despite April’s close calls, we’re grateful to finish our operating season with no avalanche fatalities or injuries in our footprint.