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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 28th, 2023 - 7:00AM
Expires
Wed, March 1st, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A storm moving in will increase the avalanche danger to CONSIDERABLE above 2,500′ through the day and may reach HIGH overnight tonight. The danger is expected to remain MODERATE below 2,500′ today, where less snow and wind is expected, but will also increase in the overnight hours. Wind slabs are likely to develop by this afternoon in the higher elevations that may release on their own. Additionally, shallow storm slab avalanches will be possible to trigger in places seeing up to 6″ of new snow at any elevation. Paying attention to changing weather and avoiding slopes with active wind loading is recommended.

SUMMIT LAKE / LOST LAKE / SNUG HARBOR:  The eastern Kenai mountains are also expecting up to 10″ of snow or more in favored areas by tomorrow morning. Avalanche danger will be rising today and similar to Turnagain Pass, peaking overnight.

The National Weather Service has issued a Blizzard Warning for Girdwood, Portage, and along the Seward Highway to Moose Pass and Seward beginning at noon today, Tuesday.

Special Announcements

Chugach State Park:  Avalanche danger is expected to rise today into Wednesday as 4-8″ of snow with strong winds are expected from Eagle River, to Anchorage, to Indian. The National Weather Service has issued a Special Weather Statement – “…HEAVY SNOW AND BLOWING SNOW POSSIBLE FOR ANCHORAGE HILLSIDE AND SEWARD HIGHWAY ON TUESDAY…”

Tue, February 28th, 2023
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

There were no new known avalanches yesterday. The last avalanche was a snowboarder triggered shallow wind slab on Magnum three days ago, Saturday.

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

A cold winter storm is on the doorstep. Weather forecasts are showing anywhere from 4-6″ of low density (very light) snow to fall by sunset with an additional 6-12″ overnight and into Wednesday morning. Storm totals are looking to be in the 10-18″ range by tomorrow afternoon. Ridgetop winds will be picking up this morning and blowing 25-35mph with gusts in the 60’s by this afternoon. All that said – avalanche conditions will be directly related to the amount of new snow and wind loading.

With the storm slated to peak after sunset, today will be a day to keep a close eye on how much new snow falls and simply avoid being on or under any slopes with active wind loading. It should be the wind today, more than the snow amounts, that will cause avalanche issues. Once the snow starts piling up overnight, then we can expect to see all the different types of storm snow avalanches, including those in sheltered places like the trees. The new snow is falling on various old surfaces including weak faceted snow, wind crusts, and sun crusts. The faceted snow that formed during those last few cold clear days is a concern for the future as it often inhibits the new snow from bonding. Here’s a breakdown of all those storm snow issues:

Wind Slabs:  Strong easterly ridgetop winds today may move enough new and old snow around to create touchy wind slabs that could start releasing naturally by the afternoon. This is most likely to occur in the higher elevations. Any fresh wind slab should also be quite easy to trigger. Overnight, slabs will get bigger and may release more often.

Storm Slabs:  Slopes in areas out of the wind with over 6″ of new snow may start forming very shallow storm slabs. It’s uncertain if enough snow will fall during the daylight hours to create storm slabs. Tonight however, when up to a foot or more of snow accumulates, storm slabs should be expected. The snow could be so light that it may not stick together enough to be a slab right away, hence it could be a day or two before we see avalanches breaking at the new/old interface.

Loose Snow Avalanches:  Naturally occurring sluffs in the new snow on steep slopes should be widespread due to the very light nature of the storm snow. Sluffs could be on the larger side in areas with over a foot of new snow.

 

An example of the surface snow yesterday on Magnum. This photo shows weak near surface facets that in some places had a very thin rime crust on top of it. Unfortunately once buried, this layer is likely to be a problem. 2.27.23. 

The charts below show Day 1 and 2 snowfall (Tuesday and Wednesday). These are provided by the National Weather Service and can be found on their Avalanche Weather Guidance page.

 

 

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

There are multiple old layers of facets and buried surface hoar in the upper 2-3′ of the snowpack, but they have become so stubborn and unreactive that triggering an avalanche in one of these layers is unlikely. With new snow and wind on the way, we’ll again be looking for any storm snow avalanches that might step down into these old layers. There could be some surprises out there, even after the storm settles out.

Weather
Tue, February 28th, 2023

Yesterday:  Sunny skies were over the region with light west to northerly winds (5-10mph) along ridgelines. Temperatures were cold, single digits at all elevations. Daytime warming did warm lower elevations into the low teens for a few hours.

Today:  Clouds have moved in overnight ahead of storm system that should bring heavy snowfall and strong easterly winds staring late morning and peaking overnight tonight. Anywhere from 10-18″ is forecast with the highest amounts near Portage Valley. Temperatures should warm into the teens and low 20’sF which will keep snow to sea level.

Tomorrow:  The storm looks to abate tomorrow midday and head out by the afternoon, if not earlier. As soon as the storm exists, models show strong NW outflow winds impacting our region from Wednesday afternoon to very early Thursday. A break is stormy weather is likely on Thursday and possibly into the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 7 0 0 65
Summit Lake (1400′) 0 0 0 37
Alyeska Mid (1700′) 8 0 0 69
Bear Valley – Portage (132′) 10 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 2 W 7 20
Seattle Ridge (2400′) 5 N 4 15
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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.