Pro Field Report
Basic Information
Observation Details
Observation Date:
January 15, 2021Submitted:
January 15, 2021Observer:
SAC - VandenBos (off duty), MawhinneyZone or Region:
Sawtooth and Western Smoky MtnsLocation:
McDonaldSigns of Unstable Snow
Recent Avalanches?
YesCracking?
IsolatedCollapsing?
IsolatedMedia/Attachments
Advanced Information
Weather Summary
Increasing clouds through the day, snowing S1-S2 by 1530, 4-5cm by 1900. Winds calm until onset of precip, then blowing moderate out of the NW.
Avalanche Observations
Have not had a chance to look closely at photos, but the very large slide on N side of the main arm of McDonald had a few places where the bed surface and debris appeared to be dirty. I've been waiting to see if we see any avalanches failing below 12/11 on 11/17 interface (which is quite dirty). Low confidence that this is what happened at this point, but will add photos to avalanche ob when time allows.
Snowpack Observations
Wanted to identify weak layer in slide observed from highway, map presence of rain crust, and look at low elevation snowpack in this zone.
Slide in elevator shaft started as a wind slab and stepped down to 12/11 facets.
Rain crust quickly fades out between 7,900 and 8,000'.
Pit at 7,200' on N aspect: Snowpack is thin and faceted, likely thanks to frequently inverted temperatures over the past month. HS =80cm. 1/13 rain crust presenting locally as fragile, <1cm thick IFrc. 1cm snow/graupel on top of crust from after cold front passed. 2-3mm SH present on top, but limited to low elevations, more of an inversion phenomenon. Slab of snow over 12/11 is 40-50cm thick, mostly F- to F with a thin interval of 4F-. Boot pen to ground, ski pen to near ground while skiing. ECTP 8 and 14, CPST and PST both in low to mid 20s to END, could not measure as blocks kept toppling into the pit. In one ECT I had simultaneous failure and propagation on 12/21 rain crust and 12/11 facets.
Pit at 8,200 on E: HS=120 cm. Above the inversion line the snowpack is stronger, but still concerning (based on direct observations and widespread avalanche cycle. No propagation in ECTs during standard loading steps, but managed to initiate and propagate fracture on 12/11 (down 80cm) in nonstandard steps. The lower portion of 12/11 facets are gaining strength, but a 2-3cm interval of cry, F facets are still present at the top of the interval. Snowpack is 4F+ and moist at the ground. CPST 30, PST 32, both END down 80cm.