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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 18th, 2022 - 7:00AM
Expires
Sat, February 19th, 2022 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations. Another round of new snow and wind overnight has built sensitive slabs, making it likely a person could trigger an avalanche up to 2′ deep. Avalanches triggered in the upper elevation start zones have the potential to run down to valley bottoms. Mixed rain and snow up to 1000′ will make loose wet avalanches likely at the lower elevations. Cautious route finding will be essential today.

Roof Avalanches: With warm temperatures and rain on snow at lower elevations overnight, roof avalanches are likely. Be sure to keep an eye on children and pets, and be mindful of where you park your vehicles.

Special Announcements
  • The Seward Highway is closed North and South of Girdwood. Crews will be conducting Avalanche Hazard Reduction work from mileposts 100 to 90 north of Girdwood, mileposts 88 to 83 on the Seward Highway south of Girdwood, and near milepost 5 and Bear Valley on the Portage Glacier Highway. Visit 511.alaska.gov for updated information.
  • Hatcher Pass has extended their Avalanche Warning. Visit hpavalanche.org for more info.
Fri, February 18th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Recent Avalanches

Seward Highway: A large natural avalanche put 10-15′ debris on the Seward Highway north of Girdwood early this morning. No people were involved with the incident.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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

The latest in our parade of storms moved through the area last night, bringing 6-12″ snow to Girdwood and 4-8″ snow to Turnagain Pass and Summit Lake. Easterly winds have been blowing 15-25 mph at ridgetops with gusts of 35-45 mph, making for dangerous avalanche conditions. It is likely a person could trigger an avalanche up to 2′ deep, especially on slopes with fresh wind slabs. Upper-elevation slopes will have the most reactive snow with the potential for the biggest avalanches. Keep in mind, with up to a foot of new snow in the mountains near Girdwood it may also be possible to trigger a storm slab avalanche on sheltered slopes.

Clouds are expected to break up through the day today, with winds already starting to die down this morning. Don’t let the fair weather catch you off guard- new wind slabs will be reactive through the day. If the sun ends up making an appearance, we will likely see some solar-triggered activity. On the small side this could look like rollerballs falling off trees and rocks, but there is also the potential that small wet loose avalanches will trigger bigger slabs. Be careful with your terrain choices, avoiding steep slopes near ridgelines, below convex rolls and in steep gullies, and be aware of the terrain above you. Pay attention to the classic warning signs of dangerous avalanche conditions- shooting cracks, fresh avalanche activity, and collapsing. With one large avalanche putting debris on the highway just north of Girdwood early this morning, we know we are dealing with a dangerous setup.

Loose wet avalanches: Mixed rain and snow up to 1000′ will make loose wet avalanches possible in the lower elevations.

Cornices: With continued snowfall and sustained moderate winds, cornices continue to get bigger and more sensitive. As always, give them plenty of room when traveling above ridgelines, and limit time spent below them.

Weather data from the Sunburst station. Strong winds overnight will make for dangerous avalanche conditions today. 02.18.2022

 

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

There are multiple layers of weak faceted snow associated with crusts that formed earlier in the season that are still giving us cause for concern in the areas with a relatively thin snowpack surrounding our forecast zone. We’ve seen poor snowpack structure in the Crow Creek area near Girdwood, the south end of Turnagain Pass towards the Lynx Creek and Silvertip Creek drainages, and throughout the Summit Lake area. This setup produced avalanches during the avalanche work in Summit Lake on Wednesday, and we saw evidence of natural activity in the Summit area as skies cleared for a moment yesterday.

The good news is that this layer does not appear to be a widespread concern for our core advisory area. If you are planning on getting out in the periphery zones, don’t forget about this poor structure. A relatively small avalanche triggered near the surface will have the potential to step down to weak snow buried deeper in the pack, making a large and dangerous avalanche. We have been getting mixed test results in our snowpits on this weak layer, but the recent avalanche activity is all the indication we need to know the snowpack is dangerous and capable of producing large avalanches.

A sample of large sugary facets, buried 3′ deep in a snowpit on Colorado yesterday Photo: Heather Thamm. 02.17.2022

Weather
Fri, February 18th, 2022

Yesterday: High temperatures were in the upper 20’s to low 30’s F under cloudy skies. Snow started falling in the afternoon, with 6-12″ in Girdwood, and 4-6″ accumulating in Turnagain Pass and Summit Lake overnight. Easterly winds were blowing 10-25 mph, gusting 40-45 mph, with the strongest winds starting around 8 p.m. last night. Rain made it as high as 900-1000′.

Today: The system is passing this morning, and skies are expected to clear during the day. Winds are already calming down and are expected to stay around 5-10 mph, switching westerly during the day. We might see a trace of precip this morning. High temperatures are expected in the mid 20’s to low 30’s F during the day, with lows dropping back down to the low 20’s overnight. We might see some lingering low level clouds before things clear up later in the day.

Tomorrow: It is looking like another day of quiet weather is on tap for tomorrow, with light westerly winds at 5-10 mph and mostly clear skies. High temperatures will be in the 20’s F, and no precipitation is expected.

PRECIPITATION 24-hour data (6am – 6am)

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 7 0.4 97
Summit Lake (1400′) 30 6 0.4 48
Alyeska Mid (1700′) 31 11 1.1 N/A

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 E 13 44
Seattle Ridge (2400′) 25 SE 15 32
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.
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