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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 10th, 2022 - 7:00AM
Expires
Fri, February 11th, 2022 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′ where strong west winds have built fresh wind slabs up to 2+’ deep overnight. Human triggered avalanches are likely and natural avalanches are possible on wind loaded features. Areas that are prone to harboring wind slabs include upper elevation ridgelines, cross loaded gullies, and convex terrain features. Identifying areas with fresh wind slabs may be trickier than usual because the overnight winds blew from the opposite direction than during the last storm (late Tuesday into Wednesday morning). Look for shooting cracks, hollow feeling snow, and active wind loading to help identify areas with reactive wind slabs.

Below 2500′ the avalanche danger is MODERATE. In treeline areas exposed to the winds from last night human triggered avalanches are possible and we recommend evaluating any terrain features that appear to have been wind effected carefully.

SUMMIT LAKE: Strong winds overnight have formed fresh wind slabs on top a weak existing snowpack and increased avalanche danger is expected. A human triggered avalanche on a layer of buried facets on top of the Halloween crust yesterday is a wake up call that old weak layers are becoming active again. These persistent weak layers are widespread in the Summit Lake area and should be evaluated carefully before entering avalanche terrain.

Thu, February 10th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

Outside our forecast area in Summit Lake on Fresno Peak a group from the Pro 2 avalanche course triggered an avalanche on a layer of facets on top of the Halloween melt freeze crust. This is a significant avalanche because it is the first time we have seen this old weak layer re-activate in a skier triggered avalanche since mid-December. The snowpack is shallow and the structure is very poor in the Summit Lake area, which is supported by multiple instability tests showing propagation (see additional obs here and here), and the potential exists for additional human triggered avalanches on old buried weak layers.

Skier triggered hard slab on facets above the Halloween crust. Photo from AAS Pro 2 Course 2.9.22Hard slab debris 3-6′ deep from skier triggered avalanche on Fresno. Photo from AAS Pro 2 Course 2.9.22

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

Wind is the name of the game today. Starting yesterday afternoon west winds have been blowing across the forecast area with average wind speeds in the 10-30 mph range and gusts up to 80 mph on ridgetops. This follows a period of strong winds late Tuesday and early Wednesday that blew out of the east, which could lead to different generations of wind slabs existing on a variety of aspects today. In Turnagain Pass yesterday we found that the earlier generation of wind slabs that were created by the east winds were less reactive than we expected and we did not see any avalanche activity during the periods of good visibility. Despite the prior wind loading there was still plenty of soft snow on the surface yesterday for the next generation of winds to transport, especially in the treeline elevation band.

Today the west winds will have created fresh wind slabs up to 2+’ deep that are likely to be reactive to human triggers and possible to produce natural avalanches. The biggest question is whether the overnight winds were strong enough to reach down into the treeline elevation band. We recommend approaching areas with potential recent wind loading cautiously and evaluating how reactive wind slabs are on small terrain features before entering avalanche terrain. There were notably higher wind speeds and widespread pluming along Turnagain Arm yesterday, so extra caution is warranted in areas exposed to these stronger gap winds.

A human triggered avalanche on facets above the Halloween crust in Summit Lake yesterday (see ‘recent avalanches’ for details) is a potent reminder that areas with a thinner overall snowpack are still harboring these old buried weak layers and they are possible to be triggered be a person. While most of our forecast area has a substantially deeper and stronger snowpack than Summit Lake, areas along the southern periphery – like Silvertip Creek, Lynx Creek, and Johnson Pass – could have a snowpack more closely resembling Summit Lake. We recommend evaluating the overall snowpack structure and considering the potential for avalanches on deeper buried weak layers if you are travelling in an area with a thinner snowpack.

Glide Avalanches: A glide crack recently opened on Repeat Offender above the Seattle ridge uptrack that is worth being aware of and trying to minimize exposure time. Glide cracks are notoriously unpredictable and can produce very large and destructive avalanches.

Sunburst winds showing the periods of wind loading from the east Tuesday afternoon into Wednesday morning (red box) and west Wednesday afternoon into Thursday morning (blue box). Photo 2.10.22

Signs of recent wind effect on the snow surface at the top of Tincan Common yesterday. We saw a few small shooting cracks on wind loaded features but overall a lack of reactive wind slabs. Photo from Andy Moderow 2.9.22 

 

 

Weather
Thu, February 10th, 2022

Yesterday: The latest round of snowfall tapered off yesterday morning and skies partially cleared during the day as west winds moved into the area. Those west winds were transporting snow along Turnagain arm starting in the afternoon and continued overnight. In areas not as exposed to gap winds the wind didn’t pick up until around 5 pm with average wind speeds in the 10-30 mph range and gusts up to almost 80 mph overnight. Temperatures dropped significantly overnight as well dipping into the positive and negative single digits.

Today: Temperatures should climb back into the teens to single digits today and winds will diminish throughout the day before switching back to the east and increasing again overnight. Wind speeds during the daylight hours should be in the 10-20 mph range with gusts possible into the 30s. No significant new snowfall is expected today and cloud cover should be thin until the next pulse of precipitation enters the area around midnight.

Tomorrow: A short pulse of precipitation could bring 3-9″ of new snow to the area overnight tonight accompanied by strong east winds. The snowfall should end Friday morning and the winds will drop off throughout the day Friday. Saturday night should bring another short pulse of precipitation with warmer temperatures and calmer winds leading into a cloudy and relatively calm remainder of the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 17 0 0 92
Summit Lake (1400′) 13 0 0 36
Alyeska Mid (1700′) 16 0 0 93 (estimate)

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 W 11 73
Seattle Ridge (2400′) 11 W 4 20
Observations
Recent Observations for Turnagain Pass
Date Region Location
02/25/24 Turnagain Observation: Kickstep NE Bowl
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02/22/24 Turnagain Avalanche: Lynx Creek
02/22/24 Turnagain Observation: Turnagain, Seattle, Mt Ascension
02/21/24 Turnagain Observation: Tincan Trees
02/21/24 Turnagain Observation: Sunburst
02/20/24 Turnagain Avalanche: Tincan
02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
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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.