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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 12th, 2022 - 7:00AM
Expires
Tue, December 13th, 2022 - 7:00AM
Forecaster
Allie Barker
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
The Bottom Line

Avalanche Danger is HIGH today above 3500′. A massive storm has rapidly dumped approximately 2 ft of snow with 1.5″ SWE with increasing temperatures and wind over the past 24hrs.

Natural avalanches are likely today. Human triggered avalanches are very likely above 3500′.

The avalanche danger is CONSIDERABLE at mid and low elevation.

Wind slabs 2 to 3ft thick will be very likely to human trigger on West to Northeast aspects at upper elevation on slopes 35º and steeper. Natural avalanches are likely.

Persistent slabs 3 to 5 feet deep will be likely to human trigger in specific locations at all elevations on slopes 30º and steeper. Natural avalanches are possible.

A second round of snow is forecasted for Tuesday which will continue to elevate the avalanche danger. 

 

Mon, December 12th, 2022
Upper Elevation
Above 3,500'
High (4)
Avalanche risk
Mid Elevation
2,500'-3,500'
Considerable (3)
Avalanche risk
Low Elevation
Below 2,500'
Considerable (3)
Avalanche risk
Upper Elevation
Above 3,500'		 Avalanche risk
High (4)
Mid Elevation
2,500'-3,500'		 Avalanche risk
Considerable (3)
Low Elevation
Below 2,500'		 Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

A massive storm over the last 24hrs accompanied with east winds gusting 17 to 26 mph have built WIND SLABS 2 to 3′ thick on leeward aspects WEST TO NORTHEAST above 3500′ today.

Large natural avalanches are likely, human triggered avalanches are very likely on slopes steeper than 35º.

Wind slabs will have the potential to step down into the basal facets at the ground, increasing the size and consequence of any avalanche.

Weather stations showed minimal winds overnight below 3000′.

Winds have diminished this morning. The snowpack needs time to adjust to the new load. We do not recommend traveling in avalanche terrain today.

Still snowing at 6am as shown here on the Marmot snowstake with approximately 2ft of new snow in the past 24hrs.

 

 

 

 

 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Persistent slabs 3 to 5 feet deep will be likely to human trigger in specific locations at all elevations on slopes 30º and steeper. Natural avalanches are possible.

Two persistent slabs exist on all aspects and at all elevations. One is the new snow sitting on a rain crust and the other is the entire snowpack sitting on the basal facets we have been talking about all season. Both are problematic.

We recommend giving the snowpack time to adjust to the new load.

Another round of precipitation on Tuesday will continue to stress and overload weak layers in the snowpack and elevate the avalanche danger.

 

 

 

 

Weather
Mon, December 12th, 2022

Snowfall rates are expected to gradually taper off this morning from west to east. However, it is an active pattern and more snow is expected on Tuesday, but not nearly as much as the storm from yesterday into this morning.

NWS AVG Forecast here.

NWS point forecast here.

Marmot Weather Station here.

Independence Mine Snotel here.

State Parks Snow Report and Motorized Access information here.

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