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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 6th, 2021 - 7:00AM
Expires
Sun, March 7th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at all elevations. A cold northwest wind will be over the region and may form small new wind slabs on leeward slopes and in cross-loaded gullies. Keep an eye out for active wind loading, most likely in the higher terrain. In this case, wind slabs (4-8″ deep) will be possible to trigger along with older lingering wind slabs. In areas out of the wind, watch your sluff and remember there is still a chance a larger avalanche could be triggered due to old buried weak layers.

SUMMIT LAKE/SNUG/LOST LAKE/SEWARD:  These zones just south of our forecast area are expected to see stronger outflow NW winds today and tonight. Visible plumes off the peaks are expected with the possibility of naturally occurring wind slab avalanches. Human triggered new wind slabs will be likely and extra caution is advised.

Special Announcements
Sat, March 6th, 2021
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.
Recent Avalanches

Two wind slab avalanches were seen yesterday on the SE face (road side) of Seattle Ridge. They either occurred on Thursday (3/4) evening or early in the day on Friday, yesterday. They were on the smaller side. One was skier triggered and pictured below. The other was believed to have been triggered by people on the ridge just under the Seattle Headwall. More info HERE.

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

And the winds keep flip-flopping! After some breezy easterly conditions yesterday along the high terrain, the wind has just swung to the NW early this morning and is expected to ramp up through the day (to 15-20mph), bringing with it some cold air. The good news for Girdwood and Turnagain Pass is the main channel for the strongest flow should be to our south, impacting the western Kenai, Summit Lake and Seward areas with winds in the 20-35mph range. That said, we all should be watching out for any active wind loading.

New Wind Slabs:  In general, any new wind slabs formed in our forecast area today should be small and on the shallow side (4-8″). How much snow is available for transport is highly variable. But in short, not a ton. Some areas are already pretty scoured, yet several inches of soft snow does remain in many areas as well. Watching for active wind loading and signs of previous wind loading will be key. Even some older wind slabs could be found out there that are still reactive and could be sitting on a weak layer of snow. The higher in elevation one goes, the more likely to find these. As always, watch for punchy feeling snow (stiff snow over softer snow), cracking in the snow around you and any collapsing/whumpfing sounds. These are all signs that you have found and could trigger a wind slab if the slope is steep enough.

Sluffs:  On steep slopes out of the wind that harbor loose surface snow (yes they are out there), watch your sluff. Sluffs have been reported to be fast running and gaining volume over the past week.

Shallow skier triggered wind slab avalanche on the SE (road side) face of Seattle Ridge. This is believed to have occurred either late in the day on Thursday (3/4) or early yesterday (3/5). Note the glide crack on the right side. Photo: 3.5.21 by Paul Wunnicke.

 

Forecast NW winds at 5,000′ today at 4pm, represented by colors. This is Windy.com’s display of the NAM (North American Mesoscale) weather modeled winds. 

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

What about those pesky persistent weak layers lurking 1-3 feet deep in the snowpack? Specifically, those two old layers of buried surface hoar and facets. Well, they are still there, and they are still showing variable results in pit tests. Sometimes they react and propagate and sometimes they don’t. With this, and as time goes on, they are becoming more stubborn, and that is all good news for decreasing the chance one of us could trigger a deeper avalanche.

The last confirmed avalanches on these layers were two skier triggered slabs a week ago (Sat Feb 27) in upper Winner Creek in Girdwood Valley at low elevations (1100-1900′). There were some larger natural wind slab avalanches last Sunday as well. It’s possible these weak layers could have played a role here too. Nonetheless, we can’t totally forget that they are under our feet, especially as we head into longer days and travel to new places. As always, it is really important to follow safe travel protocol and evaluate terrain and consequences. This means only exposing one person at a time to steep slopes, watching your partners from safe spots, and having escape routes planned if the snow breaks under you.

Weather
Sat, March 6th, 2021

Yesterday:  Partly cloudy skies with a fair amount of sunshine and warm afternoon temperatures (into the low 30’sF at the mid-elevations and upper 20’sF at the higher elevations). No precipitation. Ridgetop winds were moderate from the east in the morning before trending to light by the evening, where they overnight.

Today:  Partly cloudy to mostly clear skies are expected today with cooling temperatures as cold air heads in from the north. Ridgetop winds have shifted to the NW early this morning and will rise through the day with averages 15-20mph by this evening and up to 25mph overnight tonight. Temperatures in the mid and upper elevations will be from 20F to the low teens F, while sea level should hover in the mid 20’sF.

Tomorrow:  Cold, windy and clear. The NW outflow winds are expected to remain through tomorrow midday before easing off by the evening. A lull in weather is expected for Monday before a chance of snow is slated for Tuesday. Cross you fingers!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 0 0 111
Summit Lake (1400′) 18 0 0 45
Alyeska Mid (1700′) 24 0 0 115

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 NE 8 23
Seattle Ridge (2400′) 20 SE 6 16
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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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