May 2002
Late Spring Avalanche Discussion
So far, there have been eight avalanche-related fatalities in Colorado this 2001-2002 season. With good planning and decision making, there will be no more. Here is the last word from the CAIC concerning the outlook for additional avalanche potential for late spring; that is, late April to mid-May.
First, the state of the snowpack. It was a very poor year for snowfall, but let's put some numbers on it. Generally, the better snowfalls were in the northern mountains, and the worst were in the south. All our data sites were below normal, but Steamboat and Copper Mtn came closest to hitting their long-term averages (90-95%), while Purgatory and Wolf Creek were near 50% of average. The majority of our data sites varied from 56-86% of normal snowfall. For several, this was the driest winter since either 1976-77 or 80-81, two infamously dry seasons.
Current SNOTEL data show the water content of the mountain snowpack varies from 45-68% of normal, with the higher numbers in the northern basins and the lower numbers in the southern basins. One final statistic: at Gothic (just north of Crested Butte) there was 9" of snow of the ground on April 22, while the 27-year average is 48". Other snow study plots (located between 9,500 and 11,500 feet) show the following snow depths: Vail, 53"; Keystone, 39"; A Basin, 34"; and Bear Lake, 28".
So the logical question is: With so little snowpack remaining, can there still be avalanches, especially human-triggered avalanches? The answer is yes, but they will likely be small, and confined to slopes above 11,000 feet elevation, facing NW-E, and 35 degrees or steeper. Most slopes that face south and west have melted out, or have a very shallow, consolidated snowpack. It's a sure bet the corn season will be abbreviated.
The season-long shallow snow cover is a major culprit in the lingering avalanche potential. This is not your normal spring snowpack. Beneath a strong surface crust, it remains weak, riddled with damp or wet depth hoar (sugar snow), and prone to collapse with a trigger heavy enough to crack the crust. And during periods of thaw, the crust softens and the snowpack has little strength from top to bottom.
One other question arises: What would a spring snow storm do to the avalanche potential? Small storms (4-8") will have no additional effect, but strong wind with the storm would produce pocket-releases. Medium storms (10-20") would produce a few soft-slab avalanches in the storm-snow layer only (wind would increase the potential). Large storms (24" or more) would produce many soft-slab releases in the storm layer, plus a few deep slabs breaking to the ground (wind would create a significantly worse situation).
We can't predict if, when or where triggered avalanches will occur, other than to say we remained concerned with the unusually weak snowpack. Hopefully this discussion will help in your backcountry decision-making through May.
Further information concerning avalanche facts, accidents, and reporting can be found on the website: http://geosurvey.state.co.us/avalanche/. Become a Friend of the CAIC and get the daily avalanche report delivered via e-mail every day during the season.