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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, November 30th, 2020 - 7:00AM
Expires
Tue, December 1st, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger will remain CONSIDERABLE today. Incoming weather will make natural avalanches possible later in the afternoon and there is the potential for a snowmachiner or skier to trigger a large dangerous avalanche that breaks near the ground. Cautious route-finding and conservative decision-making are essential.

Pay attention to changing conditions and watch for signs of instability. With strong winds, rain and heavy snow in the forecast, expect the avalanche danger to rise to HIGH overnight.

The National Weather Service has issued a Special Weather Statement covering Girdwood south to Seward.

SUMMIT LAKE: The snowpack is generally thinner and weaker in the Summit Lake region. This means it will be even easier to trigger an avalanche near the ground.

Special Announcements

• Want to learn more about the state of the snowpack and forecasting mountain winter weather?? Join CNFAIC Director Wendy Wagner and guest meteorologist Kyle Van Peursem TOMORROW night, Dec 1st from 7-830pm on Zoom for our first FREE Forecaster Chat of the season!!! Register HERE.

• Avalanche Education Scholarships are available and the deadline is TOMORROW.
Please see the ‘Scholarships’ page for more details and get your application in ASAP!

Mon, November 30th, 2020
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • Storm Slabs
    Storm Slabs
  • 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.
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

Pay attention to changing conditions today! After a quick pause in the parade of storms yesterday, the next one is upon us and it looks to be a doozy. It should start as snow today with winds ramping up mid-day and then as temperatures rise become rain and heavy snow overnight through tomorrow. Today watch for blowing snow and slab formation as there is plenty of soft snow for transport. Observers yesterday noted a few small winds slabs. In addition, heavier snow falling on lighter snow as temperatures rise could create an ‘upside down’ situation and touchy storm slabs. The added loading may also stress the buried weak layer at the base of the snowpack and result in a large destructive avalanche (see Avalanche Problem 2). Pay close attention to what terrain is above you as natural avalanches will become possible in the afternoon.

Warm moist air headed our way!

NWS forecasted storm total rain/water weight through Tuesday. This will fall as snow at upper elevations!

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

Surprise, surprise! We are still talking about weak snow still sitting at the base of the snowpack! It is now buried anywhere from 3′ deep in Summit Lake and south end of Turnagain to 5′ deep on the north end of the Pass and in Girdwood. The buried facets (sugar snow) may be as deep as 6-8’ in areas that have been wind loaded. Areas where is the snowpack is shallower it will be easier to impact these layers and trigger an avalanche. As the snowpack gets deeper triggering an avalanche in this layer becomes less likely but more spooky. There may be no signs of instability and this type of avalanche could be triggered remotely, from the side or below. Hitting a shallow spot could the be the tipping point that causes a wide and deep destructive avalanche. It might be the 5th snowmachiner on the slope or the 10th skier that triggers this type of avalanche.

With another heavy loading event upon us with the incoming storm, let Mother Nature do her thing and add the stress to the snowpack. Give steep slopes (above 30°s) some space and some time to see how this all shakes out. We have a long season ahead!

Shallower snowpack with weak snow at the ground on the southern end of Seattle Ridge. 11.29.20. Photo: Peter Wadsworth

Weather
Mon, November 30th, 2020

Yesterday: Skies were cloudy in the morning with scattered snow showers becoming partly sunny in the afternoon. Easterly Winds were light in the morning and increased in the afternoon gusting into the high 20 mphs. Temperatures were in the teens to high 20°Fs. Overnight skies were partly cloudy with light easterly winds and temperatures in the single digits to low 20°Fs.

Today: Skies will become overcast and snow is forecast to start falling mid-morning. Easterly winds will increase during the day and are forecast to blow 20-40 mph with gusts into the 50s in the afternoon and peak overnight with gusting into the 60s. Temperatures today will be in the 20°Fs to low 30°Fs and are forecast to rise with precipitation changing to rain at lower elevations overnight. Rain and snow could be heavy at times.

Tomorrow: Rain and snow. Temperatures in the 40°Fs at sea level and high 20°Fs in the alpine. Winds will be mostly light and easterly. Rain/snowline could go as high as 2500′. Precipitation continues into Wednesday with a shift back to snow at lower elevations in the afternoon. Think cold thoughts!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 1 0.1 59
Summit Lake (1400′) 14 0 0 22
Alyeska Mid (1700′) NA NA NA NA

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 ENE 8 31
Seattle Ridge (2400′) 22* SE* 4* 17*

*Seattle Ridge de-rimed at 1:00 pm

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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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