Avalanche Forecast
Issued: Mar 9th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
It is still possible to trigger slab avalanches about one foot deep on slopes 35 degrees and steeper. Expect wet loose activity to continue with more sun and warming temperatures on Sunday. Avoid large convex rollovers at upper elevations, and don’t linger below steep sunny slopes.
Summary
Discussion
Saturday brought sunshine, calm winds, and high temperatures in the low-30s at the 5000ft level. The low-density storm snow from Thursday and Friday (15in snow/.85in SWE) has already settled 6 inches. Wind drifting has been minor and appears confined to upper elevation north facing terrain. The new snow has formed a cohesive slab that sits on a variety of old snow surfaces.
Two large skier-triggered avalanches (D2) were reported on Friday on north aspects around 5200ft on Shuksan Arm. These avalanches failed 12-18in deep on the old/new snow interface, propagated 200ft wide, and ran 500-1000ft downslope. Several smaller and shallower human-triggered slab avalanches (D1, 6-8in thick) were also reported over the past few days that occurred on all aspects near and above treeline on convex slopes steeper than 35 degrees. The strong March sun caused small wet loose slides on solar aspects at all elevations on Saturday. Loose dry sluffs were also triggered by skiers and riders and ran far and fast in steep shaded terrain.
Skier-triggered avalanche in Hidden Bowl on Shuksan Arm on a north aspect at 5200ft. 03/08/19 Zack Giffin Photo
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
Storm Slabs
You are most likely to trigger an avalanche on convex or unsupported slopes steeper than 35 degrees near and above treeline. The storm snow appears to be bonding well to old snow surfaces in some areas, while in others, it still needs time to stabilize. Be especially cautious of:
1. North facing, upper elevation, wind loaded slopes
2. Solar aspects where the new snow buried a crust
Use small slopes to test the snow. Dig down and assess the old/new snow interface. Probe with your pole to see if you can feel a crust buried 8-12in below the snow surface. If you see recent avalanches, if your snowpack tests show easy and sudden failures, or if you experience shooting cracks and collapses, stick to supported and lower angled terrain.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Sunshine and warm temperatures will cause snow surfaces to rapidly lose strength. Watch for roller balls and pinwheels as signs that loose avalanches may follow. Loose wet avalanches could run naturally or be human triggered. They have the potential to entrain quite a bit of snow, or even trigger slab avalanches. Avoid steep, rocky, sun-exposed slopes during the warmest hours of the day. Even though these avalanches would likely be small, they could have serious consequences around terrain traps like gullies or cliff bands.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 10th, 2019 11:00AM
