Avalanche Forecast
Purcells
This warm, wet and windy storm is expected to form a touchy slab that rests on crust and surface hoar. There is potential for rain as high as 1600 m and winds will quickly form potentially deep slabs. Choose simple objectives without overhead hazard.
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.
- Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
- Carefully monitor the bond between the new snow and old surface, touchy slabs are expected.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
As new snow begins to stack up 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 reigned in as the snowpack adjusts to the new load.
Travel and Terrain Advice
- Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.
- Numerous large avalanches have run on these layers in well supported, treed terrain.
- Storm slabs in motion may step down producing large destructive avalanches.
Avalanche Summary
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.
On Sunday a natural persistent slab avalanche cycle was observed on south and southeast facing terrain between 2100 m and 2500 m, producing avalanches to size 2.5. These were running on both the early January and mid-December weak layers. Loose wet avalanches were reported on southwest, south and southeast facing terrain between 1700 m and 2800 m to size 2.5.
Snowpack Summary
Recent warm temperatures and strong solar input have left a crust that has been widely reported from many locations, high elevation north is likely one of the few crust-free zones. Fresh surface hoar up to 10 mm in size has been reported to ridgetop. We can probably expect a touchy storm slab to form on this interface as storm totals exceed 10 cm.
There are three significant and active Persistent Weak Layers (PWL) that we are monitoring.
The first PWL is known as the early January interface, it is 10 to 60 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 moderate planar results at this interface.
The second PWL is the mid-December facet/surface hoar/crust combination that is buried 40 to 100 cm deep and is most problematic at and below treeline. This interface has also shown sudden planar results in recent snowpack tests.
The third and most deeply buried PWL is the late November rain crust. It is found 90 to 150 cm below the surface. This may be triggerable in thin spots around variable snowpack areas in the alpine.
Weather Forecast
A classic winter storm smashed into the coast Wednesday morning and that system spilled into the interior Wednesday afternoon. By Wednesday night strong southwest flow aloft will allow the freezing level to rise to around 1600 m as waves of convective precipitation move through the region. Snowfall should persist into Thursday and the freezing level is expected to fall back towards valley bottom Thursday night. Friday and Saturday look cool and calm with another system set to move into the interior on Sunday.
WEDNESDAY NIGHT: Freezing level between 1300 and 1600 m, 5 to 10 cm of snow across the region, favored convective pockets could see 15 cm or more, moderate to strong southwest wind.
THURSDAY: Broken cloud cover, freezing level beginning around 1000 m rising to 1500 m in the afternoon, moderate to strong southwest wind, 5 to 10 cm of snow, favored convective pockets could see 15 cm or more.
FRIDAY: Scattered cloud cover, freezing level beginning at valley bottom rising to around 1300 m in the afternoon, light west/southwest wind, trace of snow.
SATURDAY: Broken cloud cover, freezing level around 1300 m, light west/southwest wind, trace of snow.
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By Thursday morning a slab 5 to 20 cm in depth should rest on the old surface which is a combination of crust and surface hoar. The new snow is expected to be quite touchy and sensitive to human triggering as a result.