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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, November 28th, 2019 - 7:00AM
Expires
Fri, November 29th, 2019 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

It’s Thanksgiving Day. With heavy snowfall and strong winds overnight, continuing through today, we recommend avoiding avalanche terrain.

Dangerous avalanche conditions exist for wind slabs and storm slabs after a significant storm system brought strong winds followed by heavy snowfall overnight. Continued strong winds and heavy snowfall are expected today which could raise the avalanche hazard to HIGH later today.

Natural avalanches are possible and human triggered avalanches are likely, up to D2 in size, large enough to bury, injure, or kill a person. 

Accurate information on the amount of new snow is difficult to assess at this time with multiple weather station malfunctions and errors. However, we estimate that approximately 20″ of snow in 1.7″ of SWE, accumulated overnight and expect another 10-16″ of snow today with winds forecasted at 3000′ SE 11-33 mph. Winds are currently SE 15, gusting 26 at 4500′.

Winds were strong and sustained overnight, transporting snow into large drifts and dangerous wind slabs. Winds yesterday also contributed to wind slab development, with gusts at 3550′ reaching 41 mph and gusts at 4500′ reaching 55 mph. Wind drifting at 3000′ on the Marmot snow stake indicates a 2.5 foot drifts:

Plows will likely be delayed clearing the road to HP.

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Thu, November 28th, 2019
Upper Elevation
Above 3,500'
Considerable (3)
Avalanche risk
Mid Elevation
2,500'-3,500'
Considerable (3)
Avalanche risk
Low Elevation
Below 2,500'
Low (1)
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
Low (1)
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
    Very Large
    Large
    Small
    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
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

Consistent, strong, SE winds have transported snow at mid and upper elevations since yesterday afternoon, building dangerous wind slabs on West to North aspects, 2 to 3.5 feet deep. Continued winds and heavy snowfall today will maintain or increase the avalanche hazard today.

Avalanche Problem 2
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

Heavy snowfall overnight and continued snowfall today could bring storm slab depth to 30″+ inches. This is a rapid load and red flag on the snowpack. While we lack weak layers and expect bonding to be good over the long term, we expect natural avalanches to be possible and human triggering likely in the short term. Most avalanches occur within 24 -48 hours of a storm, and this storm will continue to deliver through today.

Loose Dry avalanches will also be a factor today, with plenty of available low density snow.  We expect natural activity, and for human triggering to be likely. The volume of loose dry avalanches may be substantial. Be cautious of terrain traps which could compound the hazard and result in enough volume to bury a person.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide avalanches will continue to be problematic. This is a low frequency/high consequence hazard. There is a high level of uncertainty with the timing and prediction for glide avalanche release. . The best advice is to avoid areas with glide cracks, and areas that have previously released. Old avalanche paths have the potential to re-activate, as hang-fire and the snowpack above these paths can release. With the new snow it will be extremely difficult to identify these locations. Review the seasons observations to familiarize yourself with problem area.

Weather
Thu, November 28th, 2019

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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