February 24th – March 2nd: Powerful Storm Causes Widespread Natural Slab Avalanche Cycle

By Arden Feldman            CBAC Intern

After the cold front on February 23rd, the Crested Butte area was stuck in cold, northwest flow until the 26th. Embedded disturbances within the flow kept clouds and convective snow showers overhead during this period but did not result in significant accumulations. Temperatures were cold with a high at Elkton of 6 degrees Fahrenheit on the 24th, reminding ourselves how soft we’ve gotten during this unusually warm winter. Ahead of an incoming storm, southwesterly flow increased on the 26th resulting in gusty winds. The combination of a cold front, the jet stream bringing in subtropical moisture, and an upper level low pressure trough further adding instability, resulted in a powerful winter storm on the 27th and 28th. By the evening of the 28th, 8 to 25 inches of snow fell across the forecast area. Schofield Pass picked up 25” of snow (2.4” SWE) while Kebler Pass received 18” of snow (1.5” SWE). For the rest of the week, the Crested Butte area remained in dry, cold, northwest flow allowing for light winds, sunny skies, and cold temperatures.

2/27/17 – Satellite image showing the major storm system impacting Colorado.

 

Wind slabs from strong winds the week prior and continued winds this week remained the primary avalanche problem through the first half of the week. On February 25th, the persistent slab problem from the surface hoar layer buried January 19th was finally put to rest and taken off the problem list. This was after over two weeks without a persistent slab avalanche being reported in our forecast area, and 11 days since one was reported in the Aspen Zone. We were allowed two days without a persistent slab problem until a new persistent weak layer became active during the powerful storm on the 27th. A layer of near surface facets that formed during the previous dry weather was now being put to the test. On the 28th the avalanche danger was raised to Considerable at all elevations and we were dealing with a significant natural slab avalanche cycle. These slabs were primarily failing on fragile facet and crust-facet layers 1-3 feet deep, most commonly on easterly facing wind-loaded aspects above treeline. The storm also brought about wind and storm slabs that were quick to heal and off the problem list by March 2nd.

 

3/1/17 – Large persistent slab avalanche on a SE aspect ATL.

 

3/1/17 – Multiple slab avalanches on E aspects ATL.

 

3/1/17 – Natural D2 slab avalanche on a NE aspect.

 

2/26/17 – Snow pit showing an example of the concerning crust facet sandwich in our snowpack.

 

2/23/17 – Small wind slab on a SE aspect NTL.