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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 23rd, 2020 - 7:00AM
Expires
Tue, March 24th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today. The outflow (northwest) wind event that started Saturday night will continue through the day. Human triggered wind slabs 1-2′ thick are possible in steep wind loaded terrain. Watch for blowing snow and wind effect. Late in the the day there is a chance of small wet loose avalanches on steep solar aspects at lower elevations. Give cornices a wide berth. Evaluate snow and terrain carefully.

SUMMIT LAKE:  Natural avalanches were observed in Summit Lake yesterday. Continued active loading could trigger avalanches today and human triggered wind slabs are likely. A shallower snowpack with weak snow in the mid and base of the pack exists from the southern end of Turnagain Pass south to Summit Lake.  Extra caution is advised.

PORTAGE VALLEY/Byron Glacier Trail: Strong wind will increase the avalanche danger in this zone. Avoid avalanche runout zones such as the Byron Glacier hiking trail up to the ice caves

***Roof Avalanches:  Roofs may still shed remaining snow!

Special Announcements
  • There were close calls in both Hatcher Pass and Chugach State Park yesterday. Please check out the Hatcher Pass Avalanche Center’s observation page HERE and the Chugach State Park observations HERE.
  • The CNFAIC is grateful for all those who have been submitting observations! Let us know what you see if you’re getting out in the mountains. You can submit on our website HERE or shoot us an email at staff@chugachavalanche.org. Thank you!!
Mon, March 23rd, 2020
Alpine
Above 2,500'
2 - Moderate
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.
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

The northwest winds (outflow) started ramping up Saturday night, were sustained throughout the day yesterday, blowing 15-25 mph with gusts into the 40s and are forecast to continue today.  With the 2-8″ of snow from Saturday available for transport, blowing snow was observed throughout the forecast area and wind effect was notable. A few small naturals were observed through Turnagain and Portage. Today with continuing winds it will be important to pay attention to active loading, wind loading patterns in terrain and wind hardened surface conditions. Triggering a wind slab in steep loaded terrain will be possible. Look for cracking and areas with hard snow over soft snow. The St. Patrick’s Day surface hoar may make these even more touchy. Remember the northwest winds create some funky loading patterns as they are visibly channeled through Turnagain Pass from the south but are picked up on the Sunburst weather station blowing from the northwest. Areas like Crow Pass, upper elevation terrain in both Portage and Placer valleys and the south end of Turnagain Pass to Summit Lake tend to be impacted even more by this wind direction. As noted below in additional concerns Summit Lake had some large natural wind triggered avalanches yesterday.

Wind transport observed over Goat mountain from Girdwood Valley. 3.22.20 

Wind loading from the south on Tincan. 3.22.20. Photo: Heather Thamm

Surface conditions on skin track up East aspect of SE shoulder of Fresno. 3.22.20. Photo: Heather Thamm

Avalanche Problem 2
  • 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

Low elevation solar aspects could heat up enough to soften surface crusts by the late afternoon. If and when this happens, triggering small wet sluffs on steep sunny slopes will be possible, especially near rocky areas.

Additional Concern
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.
More info at Avalanche.org

In areas with a thinner snowpack, from the southern end of the forecast zone to Summit lake, weak snow (both surface hoar and facets) is buried 1-3′ below the surface.  The northwest winds impact this region more and can rapidly load slopes. Yesterday natural wind triggered avalanches were observed in Summit Lake and some looked to have stepped down in places to the facets. The continuing outflow winds today could trigger more avalanches and make human triggered avalanches likely. Watch for blowing snow and choose terrain carefully.

Wind triggered slab on Fresno. 3.22.20

 

Weather
Mon, March 23rd, 2020

Yesterday: Skies were clear with temperatures in the low to mid 30°Fs at lower elevations and teens to mid 20°Fs  at upper elevations. Winds were northwesterly 15-25 mph with gusts into the 40s and higher at some locations. Overnight skies remained clear and temperatures were in the high teens and mid 20°Fs in the Alpine and mid 30°Fs to high 20°Fs at lower elevations. The northwest winds continued to be moderate and gusty.

Today: Sunshine and clear skies are in the forecast with increasing clouds later in the day. Temperatures will be in high 30°Fs at lower elevations and mid to high 20°Fs at upper elevations. The northwest winds will continue during the day easing off in the evening. Overnight skies will be partly to mostly cloudy with temperatures in the 20°Fs.

Tomorrow: Partly sunny skies with temperatures in the high 20°Fs to high 30°Fs. Winds will be mostly light and westerly. Skies will become mostly cloudy in overnight and temperatures will be in the high 20°Fs.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 0 0 68
Summit Lake (1400′) 27 0 0 32
Alyeska Mid (1700′) 26 0 0 78

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 NW 12 36
Seattle Ridge (2400′) 24 NW 14 39
Observations
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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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