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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 16th, 2020 - 7:00AM
Expires
Tue, March 17th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Today the avalanche danger is LOW this morning and could increase to CONSIDERABLE in the afternoon with sun effect and warm air temperatures. Pay attention to changing surface conditions. In the heat of the day natural moist loose snow avalanches are possible and human triggered sluffs will be likely. On steep slopes protected from the sun and wind, expect dry loose snow to be triggered easily.  There is also a chance, with the warming and direct sun, of triggering a slab 1-2′ deep on buried surface hoar on solar aspects. As always give cornices and glide cracks a wide berth.

SUMMIT LAKE to SEWARD: In addition to watching for warming and solar effect, extra caution is advised. In much of the terrain the overall snowpack is thinner and triggering a large slab avalanche is possible due to a weaker snowpack. This area was also highly impacted by strong outflow winds last week.

Special Announcements
  • There have been a series of close calls in Chugach State Park over the past week. We received a report of ptarmigan hunter in Chugach State Park near Eagle River being fully buried last Wednesday, details are still be gathered.  Please check out the recent observations for this area if you are heading that way.
  • Hatcher Pass has also had a number of human triggered avalanches over the past week and over the weekend. Please check out the recent observations for this area if you are heading that way.
Mon, March 16th, 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.
Recent Avalanches

Yesterday after warm overnight temperatures in the Alpine, sunshine, calm wind and then temperatures going above freezing in the the Alpine there were natural avalanches observed in the late afternoon. These were observed on southerly aspects below steep rocky areas. Most were loose snow avalanches but a few were slabs triggered by the loose snow.

Natural slab avalanche triggered by a loose snow avalanche on Penguin ridge. 3.15.20.

Avalanche Problem 1
  • Wet Loose
    Wet Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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 there were natural loose snow avalanches observed in the heat of the day, large enough to bury a person.  This could happen again today. Springtime avalanche conditions can be described as tricky because of so many subtle weather factors determining surface warming or straightforward depending on how you approach it. Will it get as warm and sunny as yesterday? That remains to seen. It may stay just cool enough to keep the surface snow locked up and crusty on sunny slopes and keep the hazard LOW all day or not… The question is, are you traveling in terrain that is getting impacted by direct sun or not? Playing the aspects is an easy way to avoid avalanche hazard on a day like today.  If the surface of the snow is warming, you start to see small roller balls and/or small loose snow avalanches it’s time to switch to a shadier slope.

Moist/wet sluffs are notorious for being ‘heavier’ and larger than expected and can easily knock someone off their feet or machine and plow down to the valley bottom. They can also trigger a slab avalanche on the slope below resulting in an even larger avalanche.

Natural wet loose avalanche above the bike path. 3.15.20

Glide crack Penguin Ridge. 3.15.20

Cornices:  Avoid travel on cornices and don’t linger below.

Glide cracks: They are back… avoid travel below.

Dry loose snow avalanches:  In shaded areas that were protected from the wind, the snowpack is faceting significantly and dry sluffs are becoming larger.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • 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.
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

As the days go by, we continue to assess a weak layer of snow that sits just 1-2 feet below the surface. This is composed of buried surface hoar and/or small facets. A lack of avalanche activity on this layer for the past several days and unremarkable pit results all point to this layer becoming more stubborn to trigger. However, it can’t be written off, especially with warming temperatures and direct sun in the Alpine. There was at least one natural slab avalanche reported yesterday that was triggered by a loose snow avalanche. The chance of triggering a slab is another good reason to switch to shaded aspects at the heat of the day.

As folks travel to more remote or peripheral areas of the forecast region, on traverses or big rides, remember the snowpack could be quite different. Paying attention to signs of recent avalanches, cracking/whumpfing, wind effect, and sun effect are all key. Also, keep in mind areas with a shallow snowpack are more likely places to find a slab that fails in the weak layer described above or other old weak snow buried in the pack. This includes the south end of Turnagain Pass, such as Twin Peaks and Silvertip to Summit Lake and over toward Palmer Creek and the Crow Pass area near Girdwood.

Weather
Mon, March 16th, 2020

Yesterday: Mostly sunny skies with some thin high clouds and light winds. With an inversion in place temperatures were in the mid 20°Fs to high 30°Fs in the Alpine and teens to mid 20°Fs in the mid elevations. The low elevations had the biggest diurnal swing from below 0°F and single digits into the mid 20°Fs to low 30°Fs. Overnight skies were partly cloudy and winds were light. Temperatures dropped at all elevations but less at lower elevations and a little more in the Alpine than the night before with less of an inversion.

Today: Partly cloudy this morning becoming mostly clear during the day. Temperatures will be in the 20°Fs to mid 30°Fs. Winds will be westerly 5-10 mph with gusts into the teens. Clouds will increase overnight. Winds will be mostly light and easterly. Temperatures will be in the mid 20°Fs to mid 30°Fs.

Tomorrow: Cloudy skies and a chance of snow showers. East winds 5-10 mph and temperatures in the mid 20°Fs to low 30°s. Snow showers continue overnight into Wednesday morning. Winds remain easterly overnight and temperatures remain in the mid 20°Fs to low 30°s. Skies look to clear up later Wednesday morning with a window of clearing into Thursday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 0 0 66
Summit Lake (1400′) 19 0 0 30
Alyeska Mid (1700′) 21 0 0 77

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 29 W 3 14
Seattle Ridge (2400′) 23 S 1 6
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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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