December 21
Bowman-Yellow Jacket and Alexander Basin
Elevations, slope angles and aspects
6400-10k, angles approaching 40°, all aspects but south facing.
Snow conditions
Storm total from the all blow was around 6". West facing at mid elevations received enough heating early to get damp and a little sticky. Upper elevations saw good settlement from warmer temperatures. Some snow was moving in the afternoon from gusty winds.
Weather
Partly cloudy to overcast. Wind increased in the afternoon, gusting over 20 mph, from the southwest. Moderate temperatures, about 20° warmer than yesterday.
Avalanche activity
Avalanches were found in Yellow Jacket, middle bowl in Alexander, mid elevation rolls in Alexander, Toots Sweet and Toots to Boot,(mid elevation Alexander). These were both human triggered and natural, the majority from strong wind following the storm, late Friday and Saturday, 18-19th of December. Aspects were mostly ne facing with some wrap around on east, north and northwest facing aspects. Path lengths were up to a thousand vertical feet.
Natural activity:
Yellow Jacket had a slide about 100' wide, with a crown about 3' in places. It was on the ne facing headwall, initiating near the peak.
Middle or Depth hoar bowl had two slides, one initiating from near the summit and about 75' wide. The second slide, adjacent to the first, ran slightly later, similar dimensions.
4 slides were observed on mid elevation breakovers, below the upper bowls in Alexander. These were 50-100' wide, ne facing pockets, initiating at about the same time, running short distances onto lower angled terrain.
Human triggered
Yellow Jacket
Triggered on Saturday. This slide was about 200' wide, maybe larger. A visible stress fracture was observed running another 50' or so. Crown was from 1-5' deep, initiating both above and below the rain crust. It stepped to the ground in several areas, leaving very little snow on the bed surface. It ran through a grove of trees, leaving fingers of undisturbed snow and large piles of debris behind some trees. Depth of the debris was over 8' feet in places. Path was about 800 vertical feet.
Middle bowl
Triggered shortly after the Yellow Jacket slide. Avalanche was down the ridge from the two naturals, connected by stress fractures. Slide was over 100' wide, running a thousand vertical, with a crown of up to 8' deep in places. I'd consider it a climax or full depth avalanche, since it ran to ground.
Toots Sweet and Toots to Boot
Crowns were observed on both paths, Sweet a large pocket and To Boot wall to wall. Crowns were 1-3"(observed from a distance) If both paths are included, the slide was about 150' wide. Length of the path was 800-1000'. This slide may have been triggered by a separate group than the Yellow Jacket-Middle bowl slides.
Photos
Looking at the step down
Grandeur peak in the distance. Stress fracture and step down.
Fifty inch crown
A skier's view of the left side of the slide.
Fracture in the middle bowl of Alexander Basin. Crown is over eight feet, if the width of the fallen cornice is included.
A portion of the crown in Alexander, middle bowl slide, stepping to the ground.
Middle bowl human triggered slide path. Naturals are lookers left, out of view.
Pocket in Toots Sweet with part of Toots to Boot, visible.
Toots to Boot
Zoomed Toots to Boot showing a portion of the crown and possible cornice drop trigger.
One of the slides in the mid elevation roll overs, Alexander Basin.
Apologies for the quality of several photos. Wind and flat light are not conducive to photography.
Snow pit
Evaluation
A large natural avalanche cycle in the area was immediately followed by helpful volunteer stability testers. Results were impressive. Stability remains marginal with a bridge or cap now resting over the rain crust-faceted layering. Slides may initiate in upper hard slab layering and step down. The final result of triggered slides may well be climax, full depth avalanches.
Proceed with caution.
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