Avalanche Forecast
Purcells
The snowpack is complex and there are many unanswered questions. Three active weak layers remain sensitive to human triggering. The easy solution is to choose simple terrain free of overhead hazard as the snowpack adjusts.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Be very cautious as you transition into wind affected terrain, wind slabs may be deep and touchy.
- Carefully investigate the bond of the new snow before stepping out into more challenging terrain.
- The new snow will require several days to settle and stabilize.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
The riding conditions may be great, but we can't forget about the persistent weak layers lurking in our snowpack that remain quite capable of producing very large avalanches. Keep the terrain choices reined in as the snowpack adjusts to the new load.
Travel and Terrain Advice
- Numerous large avalanches have run on these layers in well supported, treed terrain.
- Storm slabs in motion may step down producing large destructive avalanches.
- Good day to make conservative terrain choices and watch for clues of instability.
Avalanche Summary
Avalanche activity before the storm on Wednesday was limited to loose dry sluffing from steep alpine terrain.
On Tuesday explosive control work produced a size 1.0 persistent slab avalanche that failed on the mid-December weak layer on a north/northeast facing slope around 2400 m in the central portion of the region
In the south-central portion of the region on Monday a storm slab avalanche released naturally on an east facing feature between 2200 and 2500 m, failing on the mid-December weak layer. Explosive control work in the central portion of the region produced a size 2.0 avalanche on a west facing slope at 2000 m. This avalanche initially failed on the early January interface before stepping down to the late November crust. A size 3.0 wet slab avalanche was reported from a steep south facing alpine feature in the mid afternoon when warm temperatures and direct solar input were most intense.
Snowpack Summary
The current snowpack is complex, we now have three active Persistent Weak Layers (PWL) that we are monitoring.
Wednesday and Thursday's highly convective storm left 10 to 25 cm in its wake. This snow buried the mid-January PWL that is a crust and/or surface hoar interface. Prior to Wednesday's storm the crust was reportedly widespread, high elevation north is likely one of the few crust-free zones. The now buried surface hoar is 10 to 30 mm in size and was reportedly present at all elevations before the storm.
The second PWL is known as the early January interface, it is 40 to 80 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is present at all elevation bands. Recent snowpack tests have shown sudden results with moderate loads.
The third PWL is the mid-December facet/surface hoar/crust combination that is now buried 40 to 110 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine.
The November rain crust is 90 to 150 cm deep and is thought to have gone dormant for the time being.
Weather Forecast
The low that recently delivered significant wind, snow and rain to the interior is expected to weaken on Friday resulting in less frequent and less intense convective flurries. Saturday offers a brief lull in the action before another system takes aim at the interior Sunday.
FRIDAY: Scattered cloud cover, freezing level beginning at valley bottom rising to around 900 m in the afternoon, light to moderate west/southwest wind, 1 to 3 cm of snow possible.
SATURDAY: Scattered cloud cover, freezing level beginning at valley bottom rising to around 1300 m in the afternoon, light to moderate west/southwest wind, trace of snow possible.
SUNDAY: Overcast, freezing level holding near valley bottom, moderate to strong southwest wind, 3 to 10 cm of snow possible.
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By Friday a slab 10 to 30 cm in depth will rest on a nasty combination of crust and surface hoar. The slab will be much deeper in wind exposed features and is expected to remain quite sensitive to human triggering.