Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, April 15th, 2021 - 7:00AM
Expires
Fri, April 16th, 2021 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

A MODERATE hazard exists for PERSISTENT SLAB, WIND SLAB, and WET-LOOSE avalanches today. It will be possible to human trigger avalanches, naturals are unlikely. Spring has sprung, finally, and temperatures have significantly increased since Monday and will continue to steadily increase throughout the week.  Observations have been focused at low and mid elevation due to the road closure.  Variable conditions exist including buried and exposed crusts, wind board, corn, and very moist snow up to 3200′ or higher.

DOT plans on doing avalanche mitigation on Friday. Clearing the road will follow mitigation which is anticipated to happen next week.

Special Announcements

HPAC’s last forecast will be Saturday, April 17th. We will continue to monitor the observation platform through the end of the season.  HPAC will continue to publish updates as necessary.

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Thu, April 15th, 2021
Upper Elevation
Above 3,500'
Moderate (2)
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
Moderate (2)
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 in the HPAC forecast zone since Sunday, April 4th. There is a chance some thin wind slabs avalanches occurred at upper elevation this week but no one has had eyes on that terrain.

One large avalanche was reported on the Willow side near Dogsled Pass on April 9th. We believe the Willow side got more wind during that period.

Snowmachine triggered avalanche near Dog Sled Pass 4/9

 

 

Avalanche Problem 1
  • 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

Persistent slabs,1 to 3 feet deep, will be possible to human trigger today on all aspects, at all elevations, although more likely on East to South to West aspects on slopes above 30º.  A warming trend since Monday, with temperatures increasing from single digits to 33º, has resulted in significant settling of the snowpack.  This large temperature increase, combined with the intensity of the sun, has made the persistent slab more cohesive.

Numerous persistent slabs, mostly on E to W aspects already avalanched, failing on the facet/sun/drizzle/facet crust during the Easter Storm. However, slopes that did not propagate will be more suspect moving forward. An additional concern lies with the poor structure at the bottom of the snowpack. With more intense warming and the lack of freezing for several nights, be on the lookout for the initiation of a wet slab cycle which could easily step down into deeper weak layers and fail at or near the ground, increasing the size and consequence of any avalanche.

If you are getting out, continue to practice safe travel protocol, ski/ride one at a time, spot your partner, and get out of Harms way:(

 

HS 135 cm

 

Avalanche Problem 2
  • 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

Winds have diminished this morning since an increase last night, gusting SE 24 to 41mph for 10 hours at 4500′. Thin WIND SLABS will be possible to human trigger today on West to North aspects at upper elevation. Naturals are unlikely. Although we have limited information from upper elevation since the road closure, weather stations report that winds have been gusting SE 20-41 mph on and off since Sunday. For the most part, HP has been spared compared to the Mat Valley. The other factor limiting the size of wind slabs is the limited amount of snow available for transport. Most surfaces on southeast aspects are crusts or moist dense snow and leave little snow to move to leeward aspects. The wind slab problem will be short lived and improve within 24-48 hours.

Avalanche Problem 3
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Small LOOSE WET avalanches will be possible to trigger today on East to South to Southwest aspects, on steep slopes above 40º, in the afternoon. Snow surfaces are very moist on E to SW aspects with penetration up to 12″ deep below 2500′ and 3″ deep at 3200′. Limited information is available above 3600′. Loose wet avalanches can trigger slabs that break into deeper layers in the snowpack.

Wet avalanches are predictable and avoidable. Stay off of and away from these aspects during the heat of the day. Large roller balls are an indicator that it is time to change to a cooler aspect. Avoid skiing or riding above terrain traps which can increase the consequence of any avalanche.

Moist snow up to 3200′ is easy to make a snowball and is moist up to 12″ deep below 2500′.

Weather
Thu, April 15th, 2021

Temperatures have increased significantly since Sunday. Temps have risen from -4ºF to 33ºF this week. A few winds events occurred, mostly from the SE gusting 20-41mph for periods of time, off and on Sunday through early this morning.

Temperatures will be 33-41ºF at 1000′ and 29ºF at 3000′ today with light to variable winds. A 30% chance of snow today will bring mostly clouds with clearing and sunny skies Friday.


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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