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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 18th, 2024 - 7:00AM
Expires
Tue, March 19th, 2024 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations, and there are several avalanche concerns today. The biggest issue is the lingering possibility of triggering a large avalanche on a weak layer of snow buried 2-3 feet deep. There is also a chance of triggering an avalanche up to a foot deep where yesterday’s winds drifted snow into slabs that may remain reactive today. Finally, saturated snow at the lower elevations is likely to produce loose wet avalanches through today. The avalanche problem is complex today, which means safe travel will require a careful snowpack assessment. When in doubt, you can avoid all of these problems by sticking to lower-angle terrain.

Roof Avalanches remain a concern as light rain continues to trickle in at least for the first part of today. Keep an eye on kids and pets, be mindful of where you park your vehicles, and be careful entering and exiting buildings.

Special Announcements

Turnagain Pass Avalanche Awareness Day – Saturday, March 23!
Swing by the Turnagain motorized parking lot between noon and 4pm to grab a hotdog, practice your beacons skills, chat with the forecast team, and possibly test out a demo snowmachine provided by local dealers.

Mon, March 18th, 2024
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
Tue, March 19th, 2024
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
Tue, March 19th, 2024
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

There were no new avalanches reported yesterday, which is likely partly due to the fact that there was very little traffic in the backcountry. The last known avalanche was a skier-triggered avalanche seen on Notch Mtn. in Girdwood on Friday.

Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

With the weather quieting down today our main focus switches back to the weak layer of snow buried by last week’s storms, now 2 to 3′ deep. Prior to the storm, we observed a well-connected layer of surface hoar from the road up to ridgetops. This was sitting on top of a layer of faceted snow on shaded aspects, and on top of a sun crust on solar aspects. We saw avalanches failing on that interface four days in a row last week, and there is still a chance a person will be able to trigger a big avalanche on that layer today.

These persistent weak layers can be a real challenge to assess. With a depth of 2 feet or deeper, it is unlikely you will be able to gain a lot of info digging a quick hand pit and it is dangerous and unreliable to try to use test slopes to see how the layer reacts. That leaves us with full snow pits and stability tests to try to gain some information on how reactive the layer remains. We saw this layer producing reactive results in pits on Seattle Ridge on Saturday, and expect it to be very slow to change.

For now, we are still treating this layer with caution. With quiet weather on the way it is unlikely we will see natural avalanches, but we may still be able to trigger a large avalanche. The easiest way to manage the problem is to avoid steep terrain while the snowpack gains strength. If you are trying to get into steeper terrain, it will be important to take the time to carefully evaluate the layer of concern. This includes things like looking for fresh avalanche activity, paying attention to clear signs like shooting cracks or collapsing (which you might not always see, even when conditions are unstable), and digging quick test pits. Because of the higher uncertainty associated with persistent weak layers like the one we are dealing with now, it is a good idea to give yourself wider safety margins and avoid more consequential terrain.

Wet snow avalanches will remain likely at lower elevations that have seen rain on snow for the past two days. Snow surfaces are now saturated below treeline, making natural and human-triggered wet loose avalanches likely below treeline. There is a chance that a wet loose avalanche may pull out a larger slab as it travels downslope.

This is the challenging setup we are currently faced with. We got a very unstable test result in this pit on Saturday, and expect similar behavior today. 03.16.2024

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Winds are expected to taper off quickly this morning, but after 24 hours of sustained easterly winds averaging 10 to 35 mph, it will remain possible to trigger an avalanche up to a foot deep on wind-loaded slopes. Be on the lookout for reactive wind slabs at upper elevations below ridgelines, steep rollovers, or loaded gullies. Be skeptical of stiff snow sitting on top of softer snow, and watch for clear signs of unstable snow like shooting cracks or fresh avalanches. Unlike the persistent problem mentioned in Problem 1 above, these wind slabs may be assessed quickly using travel tests like hand pits and test slopes.

Although the fresh wind slabs that have formed over the past 24 hours may not be huge on their own, they have the potential to step down to the weak interface mentioned above, making a much bigger avalanche. With challenging avalanche conditions like we have today, it is usually best to keep your terrain objectives simple so you don’t get caught by surprise.

Strong winds were building fresh wind slabs through last night. They are expected to calm down today, but those fresh wind slabs will remain reactive through the day. 03.18.2024

Weather
Mon, March 18th, 2024

Yesterday: Precipitation continued to trickle in under cloudy skies yesterday. Stations picked up 0.4-0.8” SWE in Girdwood, 1.5” in Portage, and 0.1” or less from Turnagain to Seward. This fell as rain up to around 1800’, and likely amounted to around 6-8” snow in Girdwood and over a foot in Portage at higher elevations. Winds were strong out of the east, averaging 10 to 35 mph with gusts of 45 to 50 mph. High temperatures ranged from the high 20s F at ridgelines to the low 40s F at lower elevations closer to the coast.

Today: Precipitation is expected to taper off today, with a trace to 2” possible for Girdwood, Turnagain Pass, Summit, and Seward, and 4-6” for Portage and Placer. The rain line is expected to drop to around 500-700’ as the storm finishes. Easterly winds are also expected to calm during the day, with average speeds dropping from around 20 mph this morning to 5 to 10 mph by mid-day. High temperatures should be in the upper 20s to low 30s F, with lows in the mid 20s F.

Tomorrow: High pressure is headed our way for the rest of the week starting tomorrow, and things are really going to heat up. Skies should be partly to mostly sunny with light variable winds. We may see some lingering valley clouds as the skies start to clear. High temperatures are expected to reach the mid 30s to low 40s F, with lows in the md 20s to 30 F. No precipitation is expected.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34 tr 0.1 102
Summit Lake (1400′) 36 0 0 46
Alyeska Mid (1700′) 34 tr 0.39 105
Bear Valley – Portage (132′) 38 0 1.57
Grouse Ck (700′) 36 0 0.1 71

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 ENE 17 51
Seattle Ridge (2400′) 28 SE 13 26
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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.