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Observation: Snowbird periphery

Observation Date
2/27/2017
Observer Name
B
Region
Salt Lake » Little Cottonwood Canyon » Snowbird periphery
Location Name or Route
Snowbird Periphery
Weather
Sky
Obscured
Precipitation
Heavy Snowfall
Wind Direction
West
Wind Speed
Strong
Weather Comments
Another fast hitting/major storm moved through starting Sunday night. Intense PI rates were sustained during the mid morning hours through late afternoon. Strong west winds were occuring at the upper elevations above 9600 feet with intense transport and or wind blown. At mid elevations the winds were vacillating between light and moderate with the incoming squalls and impulses. Still at mid elevations there was moderate transport observed in the most exposed terrain. Temperatures appeared to drop significantly with frontal passage around 1130.
Snow Characteristics
New Snow Depth
22"
New Snow Density
Low
Snow Surface Conditions
Powder
Wind Crust
Snow Characteristics Comments

Overnight there was around 6 inches of snow that fell. This first wave of moisture came in as moderately heavy density wise, and as the storm progressed through the morning hours the snow immediately lightened up. By 0930, snowfall rates were hitting S4 and S5 at times, and the storm snow was becoming rightside up providing superb powder riding. By late morning, it was "getting in your face" even on 30 degree slopes. By 1500 storm totals were easily hitting 22 inches. Exposed windward terrain was getting scoured in the upper elevations.

At 8000 feet in BCC storm totals were 14 inches by 1800 with very light snow still falling

Red Flags
Red Flags
Recent Avalanches
Heavy Snowfall
Wind Loading
Cracking
Poor Snowpack Structure
Red Flags Comments
Widespread natural and human triggered loose snow dry avalanching was observed in the mid and upper elevations due to the intensity of the incoming storm. Slope cuts were able to get significant amounts of snow moving and entraining more snow on slopes with angles > 35 degrees. Slope cuts were also able to initiate soft slabs up to 14 inches deep on steep rollovers that had received even modest amounts of wind loading. One slope cut on a ENE aspect around 9300 feet easily triggered a 30 foot wide propagating avalanche that appeared to run on a pencil hard ambiant melt freeze crust that formed Sunday. The weak layer appeared to be mixed forms and graupel, and the bonding in this isolated area was poor with a clean shear plane. Cracking was observed in the new snow on other rollovers. Wind loading at the upper elevations was evident and roadside observations revealed widespread loose snow naturals in many of the steep chutes and gullys from the Y Coulior down.
Avalanche Problem #1
Problem
Wind Drifted Snow
Trend
Same
Problem #1 Comments

The wind slab problem may not have been the most widespread, but it was probably the most dangerous in the upper elevations that were receiving combinations of wind and snow. And, even in the mid elevations where the storm snow was only receiving modest amounts of wind these soft slabs were a definite concern. Overnight on Monday the forecast for ample wind speeds appears to be enough to keep this problem active and and issue for Tuesday.

Avalanche Problem #2
Problem
New Snow
Problem #2 Comments

The significant amount of new snow that was being entrained in the sluffing made this a problem of concern on Monday. The light density of the new snow should keep this problem active as well on Monday, though it may not be quite as reactive and or easy to trigger. Regardless, any slope that is 38 degrees and greater should continue to have this problem.

Cornices continue to be something of concern and with the additional loading from this latest storm they most likely will be quite sensitive again on Tuesday.

Danger appeared to be at least Considerable during the high PI and wind on Monday. As snowfall rates subside and in areas unaffected by the wind the Danger may be Moderate on Tuesday; yet in the areas that continue to receive wind speeds capable of loading the danger may remain Considerable on Tuesday.

Today's Observed Danger Rating
Considerable
Tomorrows Estimated Danger Rating
Considerable