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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, February 8th, 2020 - 7:00AM
Expires
Sun, February 9th, 2020 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

HEADS UP! A significant storm system will impact Hatcher Pass today. New snow and strong winds will continue to keep the avalanche hazard elevated today through Sunday. 18″ to 34″ of new snow is forecasted over the next 24 hrs.

A CONSIDERABLE avalanche hazard exists today for natural slab avalanches up to 2 feet deep, at mid to upper elevations, and growing in size, up to 3 feet deep by the end of the day and into Sunday, due to strong winds and increasing snowfall. A MODERATE avalanche hazard exists at low elevations on isolated terrain features.

We just received 8-12” of new snow on February 5-6th, followed by natural and human triggered avalanches. The snowpack has not had enough time to adjust to this week’s loading and today’s storm will continue to overload and tip the balance for avalanches.

Recent avalanches, whumphing or collapsing, and shooting cracks, are all red flags for today’s avalanche problems.

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Sat, February 8th, 2020
Upper Elevation
Above 3,500'
Considerable (3)
Avalanche risk
Mid Elevation
2,500'-3,500'
Considerable (3)
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
Considerable (3)
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

Natural and human triggered avalanches were observed on 2/5-6, size D1-2, on all aspects, at mid to upper elevations, after HP received 8-12″ of new snow with light to moderate winds.

2/5-6 – Eldorado Lower Rolls, East, 3600′. Natural slab avalanches failed during storm

 

2/6 – Microdot, South, 4550′. Unintentional skier triggered avalanche, approximately 12″ deep, contained to terrain feature. See observations for more details.

 

2/6- Eldorado, NE, 4350′. REMOTELY triggered by rider at marked white X. Crown marked in red.

 

2/6 – Eldorado Headwall, East, 4650′. Intentional ski cut triggered at white X, no involvement, crown in red. 6″ deep x 175′ wide x 1200′ vertical

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
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

 

As of early this morning, strong gusts have picked up to SE 30-49 mph at 4500′ for 6 hrs.  New snow and strong winds today have and will continue to build dangerous, sensitive wind slabs. Wind slabs will be a CONSIDERABLE hazard on steep slopes, on leeward aspects, generally West to North, at mid to upper elevations. Natural avalanches will be possible and human triggered avalanches are likely. Expect wind slabs to be up to 12″ to 24″ thick this morning and growing in size and sensitivity throughout the day, potentially reaching 3 feet deep by Sunday.

As wind slabs build in depth today, they will potentially overload persistent weak layers deeper in the snowpack, which, if triggered, will be larger and more dangerous.

Below is a picture of the Marmot snow stake early this morning, showing wind effected snow.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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 avalanche hazard for persistent slabs, at mid to upper elevations, on slopes 35° and steeper, is CONSIDERABLE this morning on W to NE aspects. Strong winds and new snow will overload the existing snowpack, with slabs reaching 2-3 feet deep by the end of today. The persistent slab hazard will be MODERATE this morning on windward aspects, rising to CONSIDERABLE this afternoon, on all aspects, with additional new snow accumulation and strong winds.

Persistent slabs may allow you to enter slopes and break well above you, making escape difficult to impossible.

At low elevation, the hazard is LOW, and will rise to MODERATE this afternoon, on slopes 35° and steeper, in specific locations, on leeward aspects, West to North, and on terrain features such as gully side walls and on steep roll-overs.

The pattern of distribution is complex and highly variable. Persistent slabs are sitting on a variety of bed surfaces, including combinations of crusts, smooth wind board, and the January High Pressure Facets.

Below is a video recorded on 2/7 which shows the snowpack setup leading up to this weekend. New snow loads will likely overload these layers.

 

Avalanche Problem 3
  • 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
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

Approximately 18” to 35″ of new snow is expected above 1000’ by late today. Storm slabs will be building through the day on all aspects, at all elevations.

We expect storm slabs to initially fail on the new/old snow interface. If we get more than 12” of new snow, expect the potential for storm slabs to fail and step down deeper to the persistent slab, up to 2-3 feet deep, which will be larger and more consequential. By end of day today, Naturals avalanches will be possible, and human triggered avalanches will be likely, on all aspects, at mid and upper elevation, on slopes steeper than 35°.

Dry loose avalanches will be an additional new avalanche problem on steep slopes and growing in size through today and into tomorrow.

Weather
Sat, February 8th, 2020

1-2″ of new snow accumulated at Frostbite Bottom Snotel (2700′) in the late afternoon on 2/7, 2″ at the Marmot snow stake (3100′), 3”  at IM Snotel (3550′), and 4″ at Gold Cord weather station (4050′). The marmot snow stake showed 2″ of new snow between 3-7pm yesterday evening in calm to light winds. Yesterday from 8am-1pm winds up at 4500′ were SE 15-24mph, and from 4am-1pm gusting 21-35mph. Skies became increasingly overcast in the mid morning, with poor visibility through the day above 3000′. Overnight a brief clear off occurred around 1am followed by a steep increase wind speeds tat 4500′, gusting SE 49mph by 5am at 4500′.

Today:

NOAA is calling for 8-15″ of new snow with a high pf 31°F at 1000′ and 24°F at 3000′. This storm will start out warm and then cool.

Tonight:

NOAA is calling for 10-19″ of snow with a high 28°F at 1000′ and 23°F at 3000′.


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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