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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 1st, 2023 - 7:00AM
Expires
Thu, March 2nd, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1,000′. Human triggered avalanches are likely on slopes with more than 6″ of new snow that have seen, or are seeing, active wind loading. Wind slabs are expected to be 1-2′ deep and easy to trigger. On slopes out of the wind, the danger is MODERATE. Here we may be able to trigger shallow storm slab avalanches and sluffs. These will be the depth of the new snow; the more snow the larger the avalanche. Cornice falls are also possible with continued winds in the high elevations.

A cautious mindset is recommended. This includes paying close attention to signs of unstable snow: recent avalanches, wind loading, and cracking in the snow around us.

SUMMIT LAKE / LOST LAKE / SNUG HARBOR:  The eastern Kenai mountains received 4-8″ of snow or more in favored areas. Strong NW winds are forecast today that will keep the danger elevated. Extra caution is advised in these areas.

The Blizzard Warning issued by the National Weather Service for Girdwood, Portage, and along the Seward Highway to Seward continues today until 3pm.

Special Announcements

Chugach State Park:  Dangerous avalanche conditions are expected due to continued snowfall and strong winds from Eagle River, to Anchorage, to Indian. The National Weather Service has extended its Winter Weather Advisory – “Total snow accumulations of 4 to 10 inches. Highest amounts along the Upper Hillside.”

Wed, March 1st, 2023
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

We do not yet know the extent of avalanche activity from the stormy weather yesterday, or last night. However, we can expect storm snow avalanches were occurring to some degree. These would most likely be wind slabs at the higher elevations and sluffs in the new snow in steep terrain.

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Marvelous March begins with a bang. Not only do we have 10 hours and 24 minutes of daylight today, a storm system just brought a dose of very light (low density) powder snow. The storm looks to taper off today, only adding a couple more inches of snow. Here are the estimated snow amounts:

Estimated snowfall totals
Girdwood Valley: 6-10″
Portage Valley:  10-14″
Turnagain Pass:  6-10″
Summit Lake:  4-6″

It looks as though the winds will again be a large player in avalanche issues. They were easterly during the storm, but as of 6am began a shift to the northwest where they should blow in the 20-30mph range with stronger gusts. It won’t take much wind to blow all the loose new snow around, creating fresh wind slabs. Another player is the new/old snow interface. The new snow has fallen on various layers of loose faceted snow, wind crusts and sun crusts. All these layers will keep the snow from bonding right away so that will make wind slabs and storm slabs that much easier to trigger. The good news is, we should be able to see all these issues as they are ‘surface instabilities’ related to the storm snow and wind loading.

Wind Slabs:  Not only are there wind slabs left over from the storm when the winds were easterly, but new slabs forming with today’s NW winds on different aspects are also a concern. They are most likely found above treeline or in exposed areas in the trees that saw, or are seeing, wind loading. Watching for areas where winds are affecting the snow will be key, along with feeling for stiffer snow over softer snow or seeing cracks that shoot out from us.

Storm Slabs:  On slopes out of the wind we could trigger shallow storm slabs. These would be the depth of the new snow, so not on the smaller side. Because the old snow surface was a mix of facets and crusts, we are expecting poor bonding with the new snow. If the new snow is too loose, then we may not see storm slabs. But as the snow settles, these could become an issue in the future; something to look for.

Loose Snow Avalanches:  Sluffs again should be easy to trigger and could run further than expected. They could gain a lot of volume in areas that saw higher snow amounts.

Cornices:  With the new snow and winds, chunks of cornices can fall naturally and trigger sluffs or slabs below. Once travel becomes possible along ridgelines, remember to give them an extra wide margin.

 

It is still snowing in Portage Valley. Conditions at 5:30 this morning along the Seward Highway at the Portage Valley curve (MP 78.9). Image from the AK DOT & PF’s RWIS road weather cameras. 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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.

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

Buried in the top 3′ or so of the snowpack are various old layers of facets and buried surface hoar. These layers have shown themselves to be unreactive, meaning triggering an avalanche in one of them is unlikely. However, once skies clear enough to take stock of the avalanche activity that occurred during the storm, we’ll be looking to see if any storm snow avalanches stepped down into these various layers. This would produce a larger slide and tell us a layer has again become reactive. The storm didn’t have a lot of water weight to add much stress to the old layers as the snow was so light, so we are not expecting any step down avalanches, but there can always be surprises out there.

Weather
Wed, March 1st, 2023

Yesterday:  Stormy weather was over the region. Anywhere from 6-14″ of snow fell that was very light and low density. Ridgetop winds were easterly in the 15-25mph range with gusts into the 50’s. Winds backed off dramatically overnight. Temperatures were chilly, in the 20’s at the lower elevations and near 10F along the peaks.

Today:  The storm is moving out this morning with another 1-2″ of snow that could fall. Ridgetop winds will be swinging around to the NW where they are expected to be in the 20-30mph range with stronger gusts. Skies may start to break up later in the day as temperatures remain cool with NW flow.

Tomorrow:  A break in weather is expected Thursday and into the weekend. A ridge of high pressure builds in that will bring clearing skies. Right now, it looks like there could be some NW outflow winds for Thursday and even Friday. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19 8 0.4 72
Summit Lake (1400′) 19 5 0.3 41
Alyeska Mid (1700′) 18 8 0.4 73
Bear Valley – Portage (132′) 25 10 0.75

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 ENE 18 46
Seattle Ridge (2400′) 12 SE 14 27
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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.