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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, December 19th, 2023 - 7:00AM
Expires
Wed, December 20th, 2023 - 7:00AM
Forecaster
Mik Dalpes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000’.  Strong easterly winds will make natural and human triggered wind slab avalanches 1-2’ deep likely below ridgelines, rollovers, and across gully features.  Below 1000’ the avalanche danger is MODERATE.  Winds are not as likely to form slabs at this elevation, but there is a small chance of triggering a large avalanche on a weak layer buried deep in the snowpack.  All of today’s problems can be avoided by enjoying some low angle powder in the trees.

 

SEWARD/LOST LAKE: It has already started snowing in the southern zones, and this area could see 8-10″ snow during the day today. Be on the lookout for increasing avalanche danger during the day, and keep in mind with heavy snowfall you may be able to trigger avalanches in sheltered terrain as well as on wind loaded slopes.

Special Announcements

The National Weather Service has issued a Winter Weather Advisory for the western Prince William Sound including Whittier, Seward, and Moose Pass as well as most of our advisory area.

Tue, December 19th, 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
Wed, December 20th, 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
Wed, December 20th, 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

The most recent avalanches reported were those observed on Sunday that likely occurred during Saturday night’s (December 16) storm.  There was widespread natural activity observed throughout our forecast areas from Girdwood to Seward.  Most of these avalanches failed in the new snow, but some appeared to fail on deeper layers in the Girdwood and possibly Summit areas.

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 new storm is arriving this morning bringing easterly winds that are forecast to increase to 30-50 mph by this afternoon, gusting into the 60’s mph later in the day.  We could see around 2-6” of snow today in most areas (with the exception of the Seward zone, which will likely see 8-10” before sunset). Most of the snow with this storm is forecast to arrive later in the day picking up in intensity after 3pm. This will make our primary concern today wind slab avalanches up to 1-2’ deep by the end of the day. 

Fresh wind slabs are often easy to trigger, but this avalanche problem is one of the easier ones to identify.  As you climb, watch for blowing snow that is likely accumulating in large pillows below ridgelines and across gully features.  Wind deposited snow may feel hollow or firmer beneath your feet or machine and may produce cracks that shoot out from you.  A small slope with no consequences is a great place to jump on the snow or dig a hand pit and see how well the newly loaded snow is bonding to the snow below.  Remember, no result on a test slope doesn’t always mean you won’t get a result on a larger slope.  You can avoid this problem by enjoying some powder in the trees today.

The incoming storm will favor the coastal areas, but it is looking like Girdwood could pick up as much as a foot of snow by tomorrow morning. Graphic courtesy of NWS Anchorage, 12.19.2023

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

Yesterday at Pyramid Mountain we found the same poor snowpack structure we have found throughout the forecast zone.  This includes a large slab of snow 3-5’ thick sitting on top of the firm Thanksgiving crust.  In many places we are finding a layer of weak faceted snow either above or in the middle of the crust.  This is the set up for an avalanche with a firm slab on top of a weak layer.  We have not seen much avalanche activity on this layer, which can make it tempting to let your guard down.  However, avalanches on this layer have been occurring as recently as Saturday, December 16 in the Girdwood (and possibly Summit) areas.  Snowpits have been producing mixed results on this layer.  Some are showing more potential for failure and large avalanches while others are not.  This is a reminder of how variable the snowpack can be and how you might find a stable result in one location and trigger an avalanche a short distance from your stable result.  As this layer is buried deeper it is becoming harder to assess with our normal tests and will not likely produce any red flags such as shooting cracks or whumpfing.  The consequences of an avalanche on this layer would be great and the only way to completely avoid this problem is to stick to slopes that are less than 30 degrees.  If you are getting into steep terrain be sure to ride one at a time, watch your partners, choose safe spots to group up that are out of runout zones, and avoid rocky terrain and slopes that end in creeks or gullies.

Snowpit at Pyramid Mountain.  12.18.2023

Weather
Tue, December 19th, 2023

Yesterday: Skies were mostly cloudy with scattered snow showers through the day. We received a trace to an inch of snow across most of the advisory area, with around 3” in Seward overnight and snow to sea level. Temperatures were in the high teens to high 20’s F with lows in the mid teens to low 20’s F.

Today: Stormy weather ramps up today, starting with wind before the snow arrives. We should see easterly winds with sustained speeds of 30-50 mph with gusts around 60 mph. Expect to see 2-4” snow for most areas before sunset, with 3-5” in Portage and Placer, and 8-10” near Seward. The storm should peak in intensity this evening, with another 6-10” snow near Girdwood, 3-6” for Turnagain Pass and Summit Lake, 12-14” for Portage, and only 2-4” near Seward. Rain lines during the storm will move between sea level and 300 feet, but it is looking like the storm will finish with snow to sea level. Daytime high temperatures should be in the high teens to mid 20’s F, with lows tonight in the mid teens to low 20’s F.

Tomorrow: The active weather should pass tonight, with a quiet day tomorrow. Skies are looking to be partly cloudy with light winds out of the south and west. Temperatures will drop tonight through tomorrow with daytime temperatures in the mid teens, warming to the upper teens to low 20’s tomorrow night. For now it is looking like a few days of quiet weather before things get exciting again this weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 0 0 72
Summit Lake (1400′) 17 0 0 37
Alyeska Mid (1700′) 25 2 0.18 67
Bear Valley – Portage (132′) 26 2 0.2
Grouse Ck – Seward (700′) 23 4 0.3 40

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 E 6 24
Seattle Ridge (2400′) 20 SE 3 10
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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.