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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 23rd, 2023 - 7:00AM
Expires
Fri, February 24th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger will rise to CONSIDERABLE above 1000′ as a storm impacts our area today. Strong winds and up to 6-12″ snow will make human-triggered avalanches likely and natural avalanches possible. This new snow will also be loading several suspect layers in the upper snowpack, making larger avalanches possible. Dangerous avalanche conditions will require cautious route finding as the storm unfolds, especially avoiding steep, wind-loaded terrain. The danger will be MODERATE below 1000′.

PORTAGE VALLEY: As usual, the Portage area will end up on the heavy end of this storm. Expect dangerous conditions on all steep terrain as storm slabs a foot deep or deeper develop through the day.

SUMMIT LAKE: The Summit area will also be hit hard by this storm, with 6-8″ possible through the day. This snow will be loading a thinner, weaker snowpack than our core advisory area. Anticipate dangerous avalanche conditions with an increased likelihood of natural and human-triggered avalanches through the day, and choose your terrain cautiously.

Special Announcements

Front range/Chugach State Park: The storm is expected to hit the Front Range and Chugach State Park much harder than our core advisory area. Be aware of very dangerous avalanche conditions in this zone as the storm develops.

NWS Warning: The National Weather Service has issued a Winter Weather Advisory for a large area just to the west of our advisory area, including Anchorage, with this approaching storm.

Hatcher Pass Avalanche Center has issued an Avalanche Warning with the approaching storm. You can read their full advisory at hpavalanche.org.

Thu, February 23rd, 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

There were no new avalanches reported yesterday. The last known avalanche was the skier-triggered avalanche in the Library on Sunday. More details from that avalanche in this observation.

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

Avalanche danger is on the rise today as a large storm impacts the area. It is looking like this storm will favor the Front Range and Hatcher Pass over our advisory area, but we will likely still see enough  new snow and strong winds to make for dangerous avalanche conditions. We should see 6-8″ new snow near Girdwood, 6-12″ around Portage and Placer, and 4-6″ at Turnagain Pass during the day. This new snow is arriving with sustained easterly winds of 20-30 mph with gusts of 30-40 mph.

It will be likely that a person can trigger an avalanche a foot deep or deeper  on wind-loaded slopes as the storm continues through the day. The most likely places to find sensitive wind slabs will be in steep terrain below ridgelines, convexities, and in steep gullies. We’ve seen a fresh layer of surface hoar develop up to around 2000′ during the past few days of clear weather (more details in John’s observation from Eddie’s yesterday). This will make the new and wind-loaded snow a bit more reactive than usual today.

For today, the best way to travel will be to avoid steep, wind-loaded terrain. Avoid slopes with stiffer slabs near the surface, and be on the lookout for clear warning signs of instability like shooting cracks as you travel. It can be incredibly informative to hop off your machine or take a few steps off the skin track to see how the snow is behaving. Plan for increasing avalanche danger through the day, and pay attention for higher danger on all slopes if snow totals end up on the higher end of the forecast.

 

This system will favor the Front Range and Hatcher Pass over our advisory area, but it is looking like we should still get a decent refresh today and tonight. Graphic courtesy of NWS Anchorage.

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

This new and wind-loaded snow will be adding stress to several suspect weak layers in the upper 2-3′ of the snowpack. We suspect one of these layers played a role in the skier-triggered avalanche in the Library on Sunday. Although storm totals may be modest for our area today, we will still be seeing a new load adding stress to the snowpack, nudging the needle towards instability. This will make it possible that a person could trigger a larger avalanche involving more than just today’s new snow.

There is a high amount of uncertainty with these newer layers (there are potentially three weak layers that have formed in the past two and a half weeks), which calls for extra caution while choosing terrain. As mentioned above, all wind-loaded terrain should be considered dangerous today. In addition to the wind-loaded slopes, look out for warning signs for these deeper weak layers. As Wendy mentioned yesterday, it can be useful to assess the upper foot or so of the snowpit with quick hand pits as you travel, but getting down to the layers that are buried 2-3′ deep will require taking out the shovel and digging a quick snowpit. These layers can be tricky to assess, so give yourself a wide margin for error with terrain selection. With higher uncertainty around these layers, the safest bet would be to avoid steep terrain entirely.

This was an unstable test result in John’s snowpit on Eddie’s yesterday. The test suggests it is possible a person could trigger an avalanche on slopes with similar structure. This test failed on the layer of facets that was buried by the 2/5 storm. 02.22.2023

Additional Concern
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

In areas with a thin snowpack (i.e. Silvertip and the southern end of the forecast region to Summit Lake) there are various weak layers near the base of the snowpack that remain a concern. For these areas, triggering a larger avalanche is not totally out of the question and a more cautious mindset is recommended.

Weather
Thu, February 23rd, 2023

Yesterday: Skies were mostly sunny over our advisory area until later in the afternoon when high clouds moved in. Winds were light out of the west for most of the day, before switching east just before midnight and picking up to 15-20 mph with gusts of 25-35 mph. High temperatures were in the upper 20’s at higher elevations and mid 30’s near sea level.

Today: Precipitation is just starting this morning, with around an inch of snow equaling 0.1” snow water equivalent at weather stations as of 5:00 AM. Through today we will likely see 6-8” snow near Girdwood, 8-12” near Portage, and 4-6” at Turnagain Pass. Winds will be out of the east at 20-30 mph with gusts of 30-40 mph. High temperatures should be in the mid 20’s to 30 F, with the rain line staying down near 200’ for Girdwood and Turnagain Pass.

Tomorrow: The storm should pass tonight, with clearing skies tomorrow and some low level clouds likely. Winds will be light out of the west at 5-10 mph with gusts of 15-20 mph, gradually increasing through the day. Temperatures should be in the mid to upper 20’s F. No precipitation is expected during the day tomorrow.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 1 0.1 65
Summit Lake (1400′) 26 1 0.1 36
Alyeska Mid (1700′) 27 1 0.12 71
Bear Valley – Portage (132′) 27 2 0.2 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 W-E* 9* 35
Seattle Ridge (2400′) 23 W-SE* 7* 20

*Winds shifted to the east and increased to 15-20 mph just before midnight last night.

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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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