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Observation: Turnagain

Location: Corn Biscuit

Date:
Observer:
Route & General Observations

Our plan was to ascend Corn Biscuit’s West Face to evaluate the current snowpack, hoping to dig on a variety of aspects and elevations. We specifically targeted Corn Biscuit knowing that the South end of Turnagain Pass experienced dryer/thinner early season conditions, compared to the North side of the pass. However due to crowded slopes today we decided it was not possible to maintain safe travel protocol due over a dozen people on the West face at the same time, both skinning up and skiing down. We changed plans and traveled along the NW shoulder (far lookers left) out from under the run-out of the West face. We dug a pit at 2000′, NW aspect and found exactly what we expected, a 3′ slab sitting on fist hard facets that required a lot of force to fail, but still indicated propagation potential. We also found a thin area, only 15″ deep that had chained depth hoar. This thinner zone along a wind exposed part of the ridge was a good (low angle) example of a possible trigger spot. Unfortunately we had to end our tour short due to a binding failure.

Red Flags
Red flags are simple visual clues that are a sign of potential avalanche danger. Please record any sign of red flags below.
Observer Comments

No obvious signs, except sun triggered roller balls on SE-S-SW aspects today. Most notably on SE aspects.

Weather & Snow Characteristics
Please provide details to help us determine the weather and snowpack during the time this observation took place.
Weather

Sunny and clear
Temps at road level (6F -17F)
Temps in the Alpine (low 40F's)
Winds were calm

Snow surface

900'-1400': 1cm + surface hoar sitting on 3-6" near surface facets sitting on a melt/freeze crust

1400' - 2000' very small surface hoar ~2mm sitting on 8-10 loose snow or 1" light wind skin in places near the NW shoulder.

Snowpack

Dug a hand pit at 1800' in a thin area, only 15" deep. Found a pencil hard (supportable) old wind slab sitting on well developed chained depth hoar (5-6mm in size.) Hand shear failed on isolation.

Probed around a lot at 2000' on a NW aspect, height of snow was between 100cm and 140cm. Could feel with probe that the last 10 to 20cm, near the ground were really weak. In a pit at this same elevation the height of snow was 120cm (3.9'), we found a 95cm (~3') slab (1F-4F hard) sitting on Fist hard facets, 20cm (8") thick. These loose dry 2mm sized facets were on moist facets on the ground. Our tests results were variable. Only one Extended Column test propagated and two didn't fail, even when additional forced was added. ECTP26, ECTXx2. We also did a propagation saw test and the weak layer failed at 45cm. PST 45/100End. These results show that it is very had to initiate the weak layer in a thicker part of the snow pack, but should you find just the right spot, there is potential for a slab to propagate. See video. We also noted on shovel shear test, 15" below the surface, a density change/old wind slab interface that produced a clean shear. We had no other results on this layer including a compression test, which failed on 19 taps on basal facets.

Photos & Video
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