Avalanche Forecast
Issued: Mar 3rd, 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
You can trigger surprising persistent slab avalanches at lower elevations and in the Wenatchee Mountains if you find a combination of slab over facets. At upper elevations and in wind-exposed terrain you can trigger lingering wind slab avalanches. Seek out sheltered, well-supported terrain and minimize your exposure to steep slopes with weak and variable snow cover.
Summary
Discussion
Recent winds with nearly continuous light snowfall have kept the avalanche danger heightened for both wind slab and persistent slab avalanches where faceted grains are closer to the surface of the pack. In the past week observers have reported signs of instability like large collapses, and cracks on Blewett Pass. Other observations include small wind slabs on the northeast side of Wedge Mountain on Thursday and collapses at 2,000ft on shaded slopes. Another observer reported a few localized collapses near ridgelines in the Gallagher Head Lake area and a large recent avalanche. A similar very large avalanche was reported near Longs Pass. It ran on the early February facet/crust at 6,200ft. The slide ripped out the entire bowl, it was over 1,000ft wide and broke a number of small trees. See the obs page for more details.
Recent very large persistent slab near Longs Pass. Photo: Matt Primomo
Snowpack Discussion
March 1, 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 still recording 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 some areas, snowpack tests are showing less than sudden results. 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 through the weekend. 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 from various directions 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 slab avalanches become easier to trigger, natural loose avalanches originating from sunny slopes, or surface snow become thick and heavy. Remember, the sun frequently brings change.
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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
East wind may have built fresh wind slabs on the most exposed slopes on Sunday and Monday. The more obvious lingering slabs from the 27th are now hard to trigger. Use inconsequential test slopes to check if you're dealing with older, unreactive drifts or more recent slabs that are easier to trigger. If you find that they're easier to trigger, stick to low angled slopes and expose only one person at a time to avalanche terrain. These avalanches can be surprising if they break above you.
Look for uneven snow surfaces, hard drum-like sounding snow, and lens-shaped drifts. The most prone slopes are the most wind-exposed areas that funnel easterly winds. Avalanches within upper snowpack layers have the potential to step down and create much wider persistent slab avalanche. Be especially cautious where you find wind drifted slabs east of Highway 97 and in terrain with a weak and shallow snowpack, less than 5 feet deep.
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
Terrain management is key. Choose low angle and well-supported slopes. Avoid shallow rocky zones, convex rolls, cross-loaded gullies, and very steep, extreme terrain. Travel one at a time from well defined safe zones when moving through avalanche terrain.
Weak facets buried in early February are the layer of concern. The most dangerous areas are where you find 4 feet of snow or less and most of the snowpack is weak and faceted with overlying drifted slabs in the top 1-2 feet of the snowpack. Here you may find obvious signs of instability such as cracking and collapsing.
In much of the zone, especially west of Highway 97, you can find over 5 feet of snow on the ground and the February facets are a thin layer sandwiched between an underlying crust and overlying slabs. In these areas, persistent slab avalanches are generally difficult to trigger. This is especially true where the February 8th facet/crust layer is consistently deeper than 3 feet.
On some wind-stripped slopes at upper elevations and especially south of Blewett Pass, the Feb 8th may be found closer to the surface. On these isolated, shaded slopes where recently drifted wind slab overlies weak facets associated with a crust, very large avalanches may be easily triggered.
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 4th, 2019 10:00AM
