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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 15th, 2022 - 7:00AM
Expires
Fri, December 16th, 2022 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′ today. With 4-8″ of snowfall overnight and moderate winds at upper elevations wind slabs up to 2′ deep are likely for human triggering and possible for natural avalanches. Common areas to find wind slabs include higher elevation ridgelines and cross loaded gullies. Below 1000′ the avalanche danger is MODERATE.

SUMMIT LAKE:  About 5″ of snow and 0.5″ of water fell overnight on a very weak existing snowpack in the Summit Lake area. Avalanche conditions will be elevated today, especially in areas receiving active wind loading.

LOST LAKE / SEWARD / SNUG HARBOR:  Very limited information exists for these areas. Cautious travel in avalanche terrain is recommended and please let us know what you see if you head to these zones!

Special Announcements

Chugach State Park:  Last night’s storm system brought more snow to Anchorage’s front range than to Turnagain. Expect avalanche conditions to increase today in areas that received heavy snowfall and active wind loading.

Hatcher Pass: The winner from last nights storm is Hatcher Pass, where 0.9″ of water and 10″ of snow fell in the past 12 hours. Check out the Hatcher Pass Avalanche Forecast for more information.

Forecaster Chat #1: Come join forecaster John Sykes tonight (Dec 15, 7-8pm) at Alaska Mountaineering and Hiking! He will be discussing current conditions, how to submit quick and quality observations, and decision-making during complex snowpack conditions. Admission is free and all are welcome!

Thu, December 15th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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

The last observed avalanche activity was on Monday, December 12th when Andrew saw several natural avalanches along Seattle Ridge that had released during the storm. These likely released at the interface of the new snow and old snow or on a weak layer a little bit below the old snow surface. We also had observations of avalanches on Tincan and Sunburst during this cycle. Since then visibility has been poor, so observing avalanches is very challenging.

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

A quick storm over the past 24 hours has added about 0.4 to 0.7″ of water to the snowpack which should be roughly 4-8″ of new snow. This storm produced higher snowfall totals for the Girdwood valley compared to Turnagain Pass. A brief period of elevated winds from 6pm to 12am with wind speeds of 10-20 mph and gusts of 30-40 mph will have quickly formed fresh wind slabs, especially in higher elevation areas.  There is a lot of light snow on the surface right now, so even though this storm only produced 3-8″ of new snow in our forecast region wind slabs could be up to 2′ deep due to potential for additional wind transport.

The overnight storm system should move out of our area pretty quickly today and winds will switch to the NW at 15-25 mph in areas exposed to gap winds. Likely areas to see stronger winds today include higher elevation ridgelines, Turnagain Arm, Seattle Ridge, and Crow Pass. To identify areas with fresh wind slabs look for active wind transport and feel for areas with stiffer hollow feeling snow on the surface. Using small test slopes can be a good way to check whether wind slabs are reactive in the area you are travelling. We have limited information about conditions at higher elevations since the last big snowfall on Sunday/Monday, so we recommend adopting a conservative mindset if you venture into alpine terrain today.

Loose dry avalanches (aka sluffs) will be a concern in steeper terrain today. To manage this hazard it is important to be mindful of the terrain you are travelling in and make sure your sluff won’t push you into an undesirable location.

Snowfall predictions from 6pm Wednesday to 6pm Thursday from NWS Anchorage. 12.15.22

Weather
Thu, December 15th, 2022

Yesterday: Light snow throughout the day with winds averaging 5-10 mph for most of the day except a period from 6pm to 12am when wind speeds increased to averages of 10-20 mph with gusts of 20-40 mph. Snowfall totals range from 4-8″ across the forecast area. Temperatures briefly climbed above freezing at sea level from 8pm to 3am so the precipitation may have come as rain at the lowest elevations.

Today: Snowfall is expected to taper off quickly in the morning and transition to clear skies in the afternoon. Winds will shift from SE to NW as the storm passes and a high pressure system moves into the area. Winds speeds should be in the 15-25 mph range in areas exposed to NW gap winds, like Turnagain Arm and upper elevations. Temperatures will drop from the twenties to teens at upper elevation this afternoon and evening.

Tomorrow: Clear skies should remain through the start of the weekend with winds decreasing on Friday to the 5-15 mph range and temperatures dropping into the teens and single digits over the next few days.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 5 0.4 46
Summit Lake (1400′) 25 5 0.5 35
Alyeska Mid (1700′) 27 8 0.7 48
Bear Valley (Portage) (132′) 30 2 0.35 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 ESE 9 39
Seattle Ridge (2400′) 25 SE 7 18
Observations
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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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