Observation: Brighton Perimeter

Observation Date
12/23/2017
Observer Name
B

Region:

Location Name or Route
Brighton Perimeter
Weather
Sky
Obscured
Precipitation
Heavy Snowfall
Wind Direction
Northwest
Wind Speed
Moderate
Weather Comments
Early morning winds were light with moderate gusts out of the WSW and then they veered out of the NW. By mid afternoon these winds ramped up the to high end of the moderate range with gusts in the strong to intense range. Transport was intense at mid to upper elevations and the wind blown was getting into terrain that is usually more protected. Temperatures appeared to drop by 1300. Snowfall from 0800 until 1800 was at least S2 with daytime totals in the 8 to 10 inch range
Snow Characteristics
New Snow Depth
16"
New Snow Density
Medium
Snow Surface Conditions
Powder
Wind Crust
Snow Characteristics Comments

New snow overnight was at least 8 inches and in the early morning hours the riding was good in right side up light density snow. By 1030 there was a noticeable density inversion due to the high PI rates from 0700 on. Riding in the mid to late afternoon was much slower and thick due to a combination of graupel and the winds. 

Red Flags
Red Flags
Recent Avalanches
Heavy Snowfall
Wind Loading
Cracking
Collapsing
Poor Snowpack Structure
Red Flags Comments
Collapsing was observed on a moderate sized wind loaded pillow on the north ridge of Pioneer. See above for heavy snowfall description. Wind loading was intense and appeared to reach a critical point around 1400 when the Pioneer Main Chute naturaled. Wind slabs of up to 30 inches were observed and cracking was easy to initiate above steep rollovers. Widespread natural avalanche activity was observed with at least three different cycles occurring from 0830 until the Main Chute observation at 1430. Cracking and propagation was easy to initiate on steep test slopes with aspects ranging from N, NE, E, and SE.
Avalanche Problem #1
Type
Persistent Slab
Trend
Same
Problem #1 Comments

The first avalanche cycle appeared to be a Storm Slab Problem that was reactive due to a density change from the overnight light density snow to the Heavy PI rates in the mid morning hours. These slides appeared to be 6 to 8 inches deep and many were observed in more protected terrain in the Lower Mary Chutes areas on aspects ranging from N, NE and E. The second cycle appeared to have been reactive around 1030, and this problem was more likely a Persistent Slab issue. Multiple naturals were observed on the Lower Dog Lake Ridge, and both the Dog Lake Chute number 1 and number 2. The aspects involved were NE, E and SE; and these appeared to be running on the old snow/new snow interface in NSF's that were buried by the initial 5 inches that fell on Wednesday night. These slides were running pretty far on the old m/f pre-storm crust surface. The third cycle was previously described and noted as the Pioneer Main Chute natural and it most likely was not an isolated event. It appeared to be a Persistent Slab Problem aided by cross loading from the intense NW afternoon winds. At 2000 hours it appears that these winds are still active at upper elevations, though they appear to have subsided at the mid to lower elevations. 

Avalanche Problem #2
Type
Wind Slab
Trend
Decreasing Danger
Problem #2 Comments

The winds were active as previously noted and getting into protected mid elevation terrain. This problem may be decreasing by Sunday at mid elevations, but the upper elevations may still be of concern despite not exhibiting the reactivity and sensitivity observed on Saturday. 

The overall danger on Saturday appeared to be a solid Considerable, and there may have been heightened periods of High around 1500 and later as observed by the large natural in the Pioneer Main Chute. 

If the winds die down and stay in the Light range, and up until later on Sunday when the new snow begins to accumulate the danger may descend to a high consequence Moderate. Without the presence of naturals as aided by the Saturdays intense winds, widespread avalanche activity may briefly subside. Regardless, we appear to be at a tipping point where the risk of triggering large slides may still be a reality. 

Today's Observed Danger Rating
Considerable
Tomorrows Estimated Danger Rating
Moderate
Snow Profile Coordinates