Avalanche Forecast
Issued: Mar 6th, 2019 10:00AM
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
Wind slabs may be easy to trigger and act in surprising ways at upper elevations. Deeper slabs are possible on steep and shaded slopes if you find the wrong combination of slab over facets, particularly at lower elevations. Loose dry avalanches may be easy to initiate and entrain lots of snow. Travel cautiously and evaluate the bonding of the new snow carefully.
Summary
Discussion
The recent snowfall just buried some fairly weak surfaces. The new snow is not bonding well initially, particularly on southerly aspects where the new snow sits over a melt freeze crust, sometimes with facets above. Small dry loose avalanches were observed on southerly aspects in adjacent zones on Wednesday. Previously, a number of cold days with little new snow or wind allowed the avalanche danger to ease. Folks have been out enjoying the generally good stability and soft snow. Observers triggered small loose avalanches in shallow faceted snow on a north aspect of Tumwater Canyon on Monday. Over the weekend and last week, observers reported signs of instability like collapses on Blewett Pass, Nahahum Canyon, and the northeast side of Wedge Mountain.
This is a good time to get out to check on older buried weak layers, and evaluate how the new snow is bonding to the old surface. Mission Ridge on March 4.
Snowpack Discussion
March 4, 2019
Here we are at the beginning of March. The days are noticeably longer, the sun feels warmer, but the winter is far from over. This may be a good time to take stock of February’s events, where we are now, and what we might expect in the future.
A February to Remember
Many longtime Cascade travelers continue to remark about the long span of high-quality travel conditions in February. Indeed, we’ve experienced three weeks of very cold and snowy conditions. Backcountry travelers frequently reported, “best conditions in years”. February also saw a few rounds of natural and human triggered avalanches. These avalanches were different from our more frequent Cascade storm slabs; here today and gone tomorrow. Most avalanches failed on buried weak persistent layers. As such, most zones spent much of February with Persistent or Deep Persistent Slabs in their forecasts.
Natural avalanche in Icicle Creek Feb 13, 2019. Photo: Matt Primomo
With the notable absence of high elevation rain events, snowpacks around the northwest grew remarkably. On the west slope, most weather stations added 2-4 ft of snow depth. East-side locations added 1-2 ft. Mt Hood locations added 4-5 ft. Even with these impressive snowpack growths, many Cascade Snotel sites are recording near or just below average snow depths for the winter.
During most of February, snow conditions were soft and powdery making for fun recreating in many areas. Photo Matt Primomo.
Where We Are Now
Lack of avalanche activity on the layer of buried facets from Feb 8 allows us to gain confidence in many areas. In many areas, there has been a lack of avalanche activity and snowpack tests have been less reactive. These two pieces combined have allowed us to drop persistent slabs from some forecasts and shift others to “unlikely.” The exception is the East Cascades where a shallow snowpack has preserved buried facets, and persistent slabs will remain possible until the sensitivity drops further. So, we’re out to the woods, right? Well, maybe. The same cold and stormy weather that brought us excellent snow quality, also allowed us to bury new weak layers. So far we haven’t confirmed any avalanches on these shallow weak layers, but they have our attention and we’re tracking them.
Recently our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow. Several wind events, predominantly from the east, placed wind slabs on a variety of aspect. With the colder than normal temperatures, wind slabs may not heal as quickly as normal.
A small natural wind slab avalanche in the Crystal Mt backcountry. Photo; Dallas Glass
What Might We Expect
As we move into March, it’s anyone’s guess what specific weather patterns lay in store of us. However, there are two items that stick out in our minds.
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The strength of the March sun: You can already feel it just walking around town. As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
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High elevation travel: Frequently March begins to usher in a period where we push higher and deeper into the mountains. Remember, we don’t have as much information about these far-flung locations. If you use the longer days of March to travel to bigger objectives, keep your eyes open. When observations don’t line-up with the avalanche advisory, you may be experiencing a different snowpack. It’s times like these we need to stop and reevaluate.
Mt Baker on a nice day in early February. Photo: Lee Lazzara
February was amazing! but March is here… there’s still plenty of winter left. Stay safe out there.
Problems
Wind Slabs
With a shift to southwesterly and stronger winds, the new low density snow will be easily transported at upper elevations. Any fresh drifts may become quite sensitive and turn into wide avalanches as they may form over weak, faceted snow. Be careful to consider these slabs and how even a small slab may be dangerous in consequential terrain. Look for cracking with these drifts. Ridgetop terrain and exposed gullies are where you are most likely to run into these slabs. Avoid them if they appear to be thick enough and cohesive enough to crack.
More snow has fallen, and is in the forecast for the western edge of the zone. Places like Salmon la Sac and Icicle Creek have more snow, therefore you may find larger and more dangerous wind slabs than in the mountains east of Hwy 97.
Out of the wind effected areas, loose dry avalanches may become common. These may be pack a punch and can run far and fast, particularly on southerly aspects where the new snow is sitting on a thin melt freeze crust. Sun may initiate a round of natural loose avalanches on south facing slopes as well. Avoid lingering beneath steep slopes if you see signs of recent loose slides such as fan shaped avalanches and small, fresh debris piles. Even though these slides have been small, they could knock you off your feet and entrain more snow than you may expect.
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
A crust from early February associated with facets above and below is main layer of concern. Contrary to what is more conventionally found in the mountains, the most likely place for triggering one of these are at lower elevations. Here the snowpack is shallow, and weak on shaded aspects. In the foothills, these facets can be found near the ground beneath the snow from February and the crust has completely decomposed. On these isolated, shaded slopes where stronger snow may rest over facets, avalanches remain possible. This structure is much more concerning than at upper elevations, where the snowpack is generally deeper and stronger. At upper elevations, persistent slabs are unlikely.
If you find obvious signs of instability such as cracking and collapsing, avoid steep slopes nearby. Terrain management becomes key. Stick to well-supported slopes and avoid places where firm slabs look to exist near shallow rocky zones. Travel one at a time from well defined safe zones when moving through avalanche terrain.
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: Mar 7th, 2019 10:00AM
