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Issued
Sat, December 19th, 2020 - 7:00AM
Expires
Sun, December 20th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A MODERATE avalanche danger is expected today in the Alpine (above 2,500′) as 4-6″ of snow with a bump in wind is forecast. Fresh wind slab avalanches, up to a foot thick, should be forming along ridgelines and in cross-loaded gullies. Watch for changing conditions as these wind slabs, although on the smaller side, should be easy to trigger. A LOW danger exists below 2,500′ and in areas that do not see wind effect.

PLACER VALLEY/SKOOKUM:  We have little information on the snowpack in this area. Use extra caution, watch for red flags, and ease into big terrain.

South of Turnagain Pass to SUMMIT LAKE:  A thinner and weaker snowpack exists in these areas. On steep rocky terrain above 2500′, there is still a chance a person could trigger a slab avalanche failing near the ground.

Special Announcements
  • A Special Weather Statement has been issued by the National Weather Service for much of Southcentral for today. While snow amounts are not expected to be very high (4-6″), reduced roadway visibility is expected.
  • Headed to Hatcher Pass? Check out HPAC’s Saturday Forecast!!
  • Placer Valley snowmachiners: Please do not ride on the railroad tracks and cross them at a 90 degree angle. A rider was severely injured yesterday riding on the tracks.
Sat, December 19th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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.

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 unique weather system is just moving in this morning that should bring a fairly widespread snowfall event region-wide. In our area of the Eastern Turnagain Arm, between 4-6″ of snow, with more in favored locations, is expected to fall by this evening. Although this isn’t a lot to write home about, the flow direction (northwesterly) is such that we could see some bands of snow produce much more than 6″. As far as new avalanche issues go, the winds are really the game changer. A northwest wind is expected to develop today and blow in the 15-20 mph range by noon(ish) before increasing to the 25mph range tonight. These are prime wind slab development speeds in areas seeing the winds. With not a whole lot of new snow to work with, there is plenty of loose existing surface snow to move around; especially with the flow from the northwest. Hence, watching for the winds and any wind deposited snow will be the key today.

Any new wind slab should be touchy, yet relatively shallow and many could not pose much of a problem. However, watching for those larger slabs up to a foot thick or more is where things could become unmanageable. Stiff snow over softer snow and cracks that shoot out in the snow around you are clues you’ve found a wind slab.

Areas out of the winds in the core of Turnagain Pass may see some shallow sluffs and a generally LOW avalanche danger. If venturing to areas with a thinner snowpack, know there is some funny business at the base of the pack – discussed below.

You may have noticed the NWS has a new radar product that has replaced the older radar loops. Green means snowfall, note the bands of snow this morning. It takes a bit of patience to load, but it will be interesting to see how these bands move around today! 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

In areas with a much shallower snowpack than Turnagain Pass, such as Crow Pass outside of Girdwood and the southern end of Turnagain Pass (Lynx Ck, Silvertip) to Summit Lake, that old faceted snow from October at the bottom of the snowpack remains a concern. This is at elevations above 2500’ where there are no mid-pack crusts and the slab on the facets is composed of very hard snow. Snow pit tests and a lack of avalanche activity on this layer for the past two weeks is pointing to it becoming less and less likely to trigger. However, it is still a tricky problem as this structure can produce a dangerous avalanche if someone hits just the wrong thin spot. This is becoming more of an outlier situation, but something we can’t quite forget about, especially in shallow regions. If choosing to totally avoid this situation, simply keep slope angles less than 35 degrees.

Weather
Sat, December 19th, 2020

Yesterday:  Most cloudy skies with a bit of clearing were over the region yesterday (no precip). Ridgetop winds were westerly in the 5-10mph during the day before shifting back easterly overnight and picked up into the 10-15mph range. Temperatures were generally in the teens yesterday at the mid and upper elevations and warmed overnight, approaching 20F along ridgelines this morning.

Today:  A low-pressure system in the eastern Gulf is sending moisture over us from first an easterly direction then a northerly direction of all things later today. This unusual pattern is what will bring snow to such a large part of Southcentral. We can expect anywhere between 4-6″ of snow at mid-elevations and even this same amount for the central Kenai and Summit lake zone. Temperatures should remain in the 20’sF at most elevations bring this low density snow to sea level. Ridgetop winds that are 5-15mph from the east this morning should wrap around to the northwest and increase this evening to the 15-25mph range.

Tomorrow:  A brief break in weather is expected tomorrow before a strong bearing low is headed in Sunday night/Monday bringing  a warm and wet storm. Although no precipitation is expected, the northwest winds look like they could persist in the 20-25mph range into Sunday evening. Stay tuned – active weather is headed out way!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 16 0 0 60
Summit Lake (1400′) 15 0 0 25
Alyeska Mid (1700′) 18 0 0 60

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 W 12 27
Seattle Ridge (2400′) 14 *N/A *N/A *N/A

*Seattle Ridge anemometer (wind sensor) is rimed over and unable to spin.

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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.