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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 1st, 2022 - 7:00AM
Expires
Fri, December 2nd, 2022 - 7:00AM
Forecaster
Jake Kayes
The Bottom Line

3” of new snow along with strong winds overnight created dense wind-hardened snow capable of producing small avalanches.

Winds are forecasted to continue throughout the day. Expect the avalanche danger to increase throughout the day. The snowpack is already very weak and will not adjust to an even bigger load. 

Small human-triggered wind slabs are possible at mid and upper elevations on W thru NE aspects. Natural avalanches are unlikely. 

Small to large human-triggered persistent avalanches will be likely by the end of the day. 

Coverage is still thin and getting caught in any size avalanche could have severe consequences.

Thu, December 1st, 2022
Upper Elevation
Above 3,500'
Considerable (3)
Avalanche risk
Mid Elevation
2,500'-3,500'
Moderate (2)
Avalanche risk
Low Elevation
Below 2,500'
Moderate (2)
Avalanche risk
Upper Elevation
Above 3,500'		 Avalanche risk
Considerable (3)
Mid Elevation
2,500'-3,500'		 Avalanche risk
Moderate (2)
Low Elevation
Below 2,500'		 Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

No Recent Avalanches have been observed or reported since 11/16.

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

Winds from the SSE-SSW lasted for 7 hours with gusts up to 24mph. These winds transported 3” of new snow that fell last night in addition to low-density faceted snow that formed during last week’s cold spell and has created slabs 2-4” thick. These wind slabs will be found at mid and upper elevations on West thru Northeast aspects near ridgeline and leeward features on slopes 35º or steeper.

These wind slabs formed on weak faceted snow. Because the wind slabs have formed on weak snow we anticipate this avalanche problem to gain stability very slowly. 

North- Northeast winds are forecasted throughout the day with sustained wind of 20mph and gusts up to 33mph. If this weather forecast verifies you can anticipate wind slabs at upper elevations on southerly aspects and leeward features.

After a week of stable conditions with lots of users traveling in avalanche terrain, stability has changed. 

If you decide to head out in the mountains today be on the lookout for flagging or plumes of snow near ridge tops. 

Wind slabs often have smooth, pillowed, or rounded features. They often sound hollow or drum-like. To identify this avalanche use pole probes, hand pits, and formal stability tests.

Shooting cracks and collapsing are red flags for this avalanche problem.

Continue to use safe travel protocol in avalanche terrain:

  • SPREAD OUT when ascending.
  • Descend ONE at a time and regroup in identified safe zones.
  • Avoid riding above or near cliffs, rocks, gullies or other hazards.
  • As always, carry a transceiver, probe, and shovel, and know how to use them!

Coverage is still thin in many parts of the forecast zone.

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

The poor structure that has been discussed for the last few weeks should be back on your radar.

Strong winds from the N-NE are forecasted for today with sustained winds up to 20mph and gusts up to 30 mph. If this forecast verifies, snow transported by wind will create additional stress on the weak faceted snow at the ground. Small to large human-triggered persistent slab avalanches 1-3’ thick will be likely at upper elevations near ridge tops and leeward features on slopes 30º or steeper. 

Over the last week, we have observed a thinner, shallower, and weaker snowpack on southerly aspects. Southerly aspects are where today’s wind will be transporting snow. Be on guard if traveling on south-facing slopes. 

Shooting cracks and collapsing will be red flags for this problem. Use pole probes, hand pits, and other formal stability tests to identify this avalanche problem. 

This persistent slab problem will be slow to heal, expect this avalanche problem to gain strength very slowly. 

Weak sugary faceted snow can be observed near the ground. This structure will need time to adjust to any additional load.

Weather
Thu, December 1st, 2022

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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