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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 29th, 2024 - 7:00AM
Expires
Fri, March 1st, 2024 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists above 1,000′. Triggering a wind slab avalanche, formed by yesterday’s northwest winds, is possible. Watching for, and avoiding, slopes where winds have deposited snow, or places that are still seeing drifting snow today, will be key. Additionally, loose snow avalanches (sluffs) could be easy to trigger on steep slopes and may be larger than expected.

SUMMIT PASS:  Moderate northwest winds are forecast. Watch for both new wind slabs that form today, as well as those that formed yesterday. NOTE: a wind slab avalanche is this area could step down into a buried weak layer, creating a larger slide.

SEWARD/LOST LAKE:  Strong northwest outflow winds are expected to continue today. Natural wind slab avalanches are likely to occur as long as the winds persist.

Special Announcements

Chugach State Park:  Strong northerly winds are impacting Anchorage’s Front Range. Yesterday, blowing snow could be seen from town. Wind slab avalanches will be a concern in this region.

Thu, February 29th, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Fri, March 1st, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Fri, March 1st, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

There were no avalanches reported from the forecast zone around Turnagain Pass yesterday. However, Mik saw a natural avalanche in motion in the Summit area, south of the forecast zone. Yesterday’s strong northwest outflow winds likely caused many more natural wind slabs we did not see.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

For anyone in the mountains or just driving around yesterday, it was hard not to notice the strong northwest outflow winds. These have been impacting all elevations along the major gaps: Turnagain Arm/Portage/Whittier, Cook Inlet/Anchorage, and Seward. Lucky for Turnagain Pass, the winds were more sporadic, hitting some areas, like Seattle Ridge, but missing others on the non-motorized side. The good news is today the wind is supposed to back off (average 5-15 mph gusting 20-30 mph). Side note: strong winds are still expected for Seward, Whittier, and Anchorage’s Front Range.

Wind Slabs:  It’s no surprise wind slab avalanches are the main concern today. These are often easy to identify if paying attention to the snow surface and should be more likely to find in the higher elevations. Watching for areas of wind loading (smooth rounded surfaces and pillow-like features), feeling for stiffer snow over softer snow, and looking for cracks that shoot out from us are all great clues in sussing out wind slabs. Any wind loaded slope is suspect to avalanche. Wind slabs could be hard or soft, likely in the 1-2′ thick range, and vary in how easy or stubborn they are to trigger.

Loose Snow Avalanches:  The cold temperatures are weakening the surface snow, making it less cohesive. This means sluffs are getting larger by the day. We had one verbal report from a group stating “sluffs were running BIG today” at Turnagain. Heads up for those seeking the steeper sheltered slopes.

Example of cracking in the snow under us on a small test slope that’s easy to see has been wind loaded. This is from the Summit area on Raven Ridge. 2.28.24. 

 

Strong winds in the Summit Pass area yesterday were blowing snow off ridges and loading slopes. This is Fresno Pk. 2.28.24.

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

As we’ve been talking about for some time, there is a layer of faceted snow from January buried 1.5-3′ deep in the snowpack. In the Turnagain Pass area we have not been able to find any places it is concerning. That is great news. But in areas with a shallower snowpack near Summit Pass it has been an issue. The storm last weekend produced some natural wind slabs that stepped down into the layer, which tells us it is still there. For anyone headed south of Turnagain to the central Kenai Mtns, we want to be sure folks know this layer could produce a surprise avalanche without any warning signs beforehand. See Mik’s report from yesterday at Raven Ridge.

Weather
Thu, February 29th, 2024

Yesterday:  Sunny skies were over the region with cold northwesterly outflow winds.  Ridgetops and lower elevations along Turnagain Arm reported wind 10-25 mph with gusts in the 25-50 mph range. Temperatures were cold, anywhere between -10F and +15F. The winds have kept the lower atmosphere well mixed and there was no inversion, temperatures were colder with elevation (teens at sea level and around -5 on Sunburst for example).

Today:  High clouds, cold temps, and moderate northwesterly winds, are expected today. The winds along the ridgetops are looking to average 5-15mph with gusts 20-30 mph. Westerly gap winds along Turnagain Arm should remain strong.

Tomorrow:  Mostly clear skies return tomorrow, Friday into Saturday, with continued cold temperatures and moderate northwesterly outflow winds (5-15 mph gusting 20-30 mph). Temperatures look to remain in the -10 to +15 range. The next weather system is on the horizon for possibly early next week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 6 0 0 90
Summit Lake (1400′) 8 0 0 47
Alyeska Mid (1700′) 8 0 0 91
Bear Valley – Portage (132′) 16 0 0
Grouse Ck (700′) 14 0 0 66

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) -4 NW 5 29
Seattle Ridge (2400′) 2 NW 8 24
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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.