Avalanche Forecast
Issued: Feb 24th, 2019 10:18AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Shifting wind directions may create fresh wind slabs on a variety of aspects on Monday. You are most likely to encounter the problem at higher elevations and in more complex terrain. Steer around recently wind-loaded slopes and terrain features steeper than 35 degrees.
Summary
Discussion
A skier was caught and carried a short distance in a wet loose avalanche on a steep south facing aspect on Mt. Herman on Sunday. The skier was not buried and was uninjured. A few small human-triggered wind slabs (D1) were also reported on Sunday on east facing slopes above 4500ft.
Incremental loads of low-density snow over the past week have made for deep travel conditions. Impressive snow totals have been squeezed out of small water amounts. The low elevation snowpack is finally developing following this series of cold February storms. Plenty of recent snow is available for transport. Watch the winds closely - it will not take much wind to drift the light snow into fresh slabs. On steep sun-exposed slopes, wet loose avalanches may continue on Monday. Be careful as these slides can entrain quite a bit of snow.
Two buried surface hoar layers can be found in the upper snowpack down about 12in and 18in - these were the old snow surfaces buried on February 22nd and 19th respectively. You are most likely to find them preserved on shaded and sheltered slopes. In other locations, they have been removed/destroyed by sun or wind making their distribution spotty. So far, no avalanches have been reported on these layers, but that may be due to a lack of a slab. Continue to dig down and investigate these layers as the recent storm snow settles.
Snowpack Discussion
February 24th, 2019
The Status Quo
As we look at the avalanche conditions the phrase “Status Quo” keeps coming to mind. We use this term in the avalanche industry to note periods of no substantial changes, where observations continue to support the current avalanche danger, and our terrain use doesn’t appreciably change. We’ll look at recent avalanche conditions with this lens.
Avalanches
Following several large winter storms just prior to Valentine's Day, we saw very active avalanche conditions. With a few notable exceptions, most of the avalanche activity occurred more than a week ago. This lack of activity has allowed us to gain some confidence with the persistent weak layer buried on February 8th. You can see this reflected in the progression in the likelihood of persistent slab avalanches from “Likely,” to “Possible,” to “Unlikely.” The timing of this progression has been different depending on the zone.
The recent exceptions were two very large avalanches in the Crystal Mountain area on February 20th. During avalanche mitigation work, Crystal Mountain Ski Patrol triggered a slide on a SE aspect at 6300 ft in a less often skied portion of their ski area. On the same day, a skier triggered a persistent slab on a W aspect at 6300 ft in the backcountry adjacent to the ski area. Both were 3-4 ft deep. It’s hard to ignore these two obvious signs of instability.
Feb 20, 2019: SE 6300’: SS-AE-D3-R3-O. Photo: Crystal Mt Ski Patrol
Snowpack
Snow profiles and snowpack tests have been coming in from nearly every region. This is great! It helps us put together a more complete picture of the snowpack structure. Observers continue to report a layer of facets 2-5 feet below the snow surface and just above a crust. These facets are showing signs of rounding (gaining strength). In the past week, snowpack tests have become more variable. Some tests are indicating triggering an avalanche and crack propagation (necessary for a slab avalanche) are becoming less likely. That said, we are still seeing other tests that indicate that triggering deep persistent slabs remains a possibility. More tracking of this trend is needed to concretely illustrate the trend.
The Feb 8 facet layer is 3-5ft deep in the Snoqualmie Pass area. Photo: Susie Glass
Weather
Our recent weather systems have not been big water producers, especially by Cascade standards. However, the colder than normal temperatures have produced light, low-density snowfall. Wind transported snow have been the main driver of slab avalanches in the new snow. These storms have been large enough to keep the avalanche danger elevated, but not enough for major spikes in danger.
Recent low-density snow drifted by the wind to form fresh slabs Photo: Dallas Glass
Looking Ahead
At this point avalanches on persistent weak layers can’t be fully ruled out on specific aspects or elevations in most zones. Your chances of triggering an avalanche on a persistent weak layer are lower than they were on February 13th, but the change from day-to-day will remain slow and incremental. As a result, you’ve seen slow changes in the avalanche danger in most zones. Due to the serious consequences and the uncertainty that these avalanches present, we will continue to operate with a “Status Quo” mindset. Don’t let this lack of change lull you to sleep.
When will the conditions change? Not tomorrow, but possibly within the next week or two. Keep checking the forecast. Make sure to read the Snow and Avalanche Discussion and the Avalanche Problem text so you can stay up to date with any changes as they arise.
Problems
Wind Slabs
As the wind direction shifts and winds blow from the east and north, we may see fresh wind slab development on Monday. Older wind slabs that formed over the weekend still exist. You are most likely to encounter wind drifted snow at higher elevations and in more complex terrain. In isolated areas, you may find wind slabs resting on layers of buried surface hoar. Look for signs of wind transported snow such as uneven snow surfaces, fresh cornices, and snow drifts. Steer around wind loaded slopes greater than 35 degrees where you may trigger a wind slab avalanche.
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
The last confirmed persistent slab avalanche in the West North zone occurred over one week ago. We continue to gather more and more evidence showing us that this layer is gaining strength, has an isolated distribution, and is becoming unreactive to triggers. However, critical observations from other zones such as skier triggered avalanches and collapses, are keeping this potentially dangerous layer on our mind. Remember, you may not find obvious signs of unstable snow. The only way to observe this persistent layer is to dig. Snow profiles and snowpack tests may reveal a layer of buried surface hoar and/or facets 3-4 feet below the snow surface. If you suspect this layer is present, its best to simply avoid large open slopes greater than 35 degrees.
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 25th, 2019 10:18AM
