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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 13th, 2022 - 7:00AM
Expires
Wed, December 14th, 2022 - 7:00AM
Forecaster
Jake Kayes
The Bottom Line

Today another storm will bring up to 6-8” of new snow and .75” of SWE, the avalanche danger is considerable. 

Southerly winds with gusts up to 30mph are forecasted to begin in the afternoon. Pay attention for signs of active wind loading. Expect the avalanche danger to rise late this afternoon into Wednesday. 

Wind slabs 2-3’ thick will be likely to human trigger on West thru Northeast aspects at upper elevations on slopes 35º or steeper.

Human-triggered Persistent Slab avalanches 3-5’ deep will be possible at on slopes 30º or steeper on all aspects at all elevations. 

More snow is in the forecast for Wednesday and Thursday.

Tue, December 13th, 2022
Upper Elevation
Above 3,500'
Considerable (3)
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
Considerable (3)
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

Southerly winds are forecasted to increase in the afternoon, these winds will bring gusts up to 30 mph. 

Human-triggered wind slabs 2-3’ thick will be likely on West thru North East Aspect on slopes 35º or steeper. Natural Avalanches are unlikely.

Expect avalanche danger to rise if the winds increase today. Pay attention to any sign of wind transport throughout the day.

Any wind slab avalanche that occurs will have the possibility of stepping down and failing in old snow.

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

Yesterday the forecast area received 20″ of snow and 1.7″ of SWE, another 6-8” of new snow and .75” of SWE is forecasted for today.

This new snow will stress two different persistent slab problems. Weak sugary faceted snow exists at a rain crust in the middle of the snowpack and at basal facets near the ground. 

Human-triggered Persistent Slab Avalanches 3-5’ deep will be possible at all elevations and on all aspects on slopes 30º or steeper. Natural Avalanches are unlikely. 

The snowpack needs time to adjust to all the new snow.

 

Weather
Tue, December 13th, 2022

Yesterday over 20″ of new snow fell with 1.7″ of SWE.

Another round of new snow expected Wednesday thru Thursday 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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