Avalanche Forecast
Regions: Cascades - North West.
Keep your risk tolerance to a minimum, avoiding recently formed wind slabs that may be larger and more reactive on polar aspects. Carefully evaluate how the storm snow has bonded to variable old snow surfaces. If the sun comes out for an extended period, adjust your aspect to avoid loose wet avalanches that will be large enough to injure you.
Discussion
A weak frontal system brought 5" of snow that came in with moderate S winds, gusting strong on Monday. The cold upper low and associated convective snow showers should bring an additional 4-6" of snow Monday night through Tuesday at 1500-2500 ft snow levels. Light to moderate winds Monday night may continue to wind-load northerly aspects before winds ease on Tuesday.
Plan for dynamic weather if you are out, with rapidly changing conditions and difficult navigation at times above treeline, especially as convective snow showers increase in the afternoon.Â
During periods of sustained sunshine greater than 20 minutes, expect rollerball activity off trees or rocks to transition into loose wet avalanches that will be large enough to hurt you. Avoid this problem by changing your aspect and reducing your slope angle before the sun starts heating up the slope.
Remember, avalanche mitigation is no longer occurring in closed ski areas. Read more on local uphill ski area policies and the risks associated with closed ski areas: https://support.nwac.us/skiareapolicies/
Snowpack Discussion
March 19, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
This week brought a whirlwind of change to our community, the mountains, and the snowpack. It’s mind-boggling to think of how everything changed in just a few short days. Last weekend, a strong winter storm brought frigid temperatures and fluffy powder to many mountain locations. While the calendar read March, conditions felt more like mid-winter. The hands-down winner during this storm was Mission Ridge and the surrounding mountains. As the skies cleared, sunshine and warm temperatures settled into the northwest. Now we’re in the midst of a prolonged spring-like pattern as the snowpack slowly transitions. The biggest change this week may be the impact of the Coronavirus on our community, local mountain operations, and backcountry travel. We continue to work through and adapt to all of these changes in our daily lives as we mark the first official day of spring. Read more about NWAC Operations under Covid-19.Â
Wenatchees for the Win
We all know the Wenatchee mountains can receive serious dumps of snow, but this season conditions had been fairly dry. Prior to this past week, Snotel sites and weather stations in the Wenatchees reported only about 65% of average snow depth for the season. A strong low-pressure system began impacting the Pacific Northwest last Friday. However this storm did not follow the usual storm track, it meandered south off the coast, spinning moisture around its center, and sending snow into the Cascades from the SE. When moisture wraps around and approaches our mountains from this direction, it can pack quite a punch along the eastern slopes of the Cascades and in particular the Wenatchees. Early Friday morning, Mission Ridge entered a near 48hr stretch of continuous snowfall. A secondary impact of this low-pressure system was an influx of cold Canadian air which dropped snow levels to near sea level for nearly all locations. The cold air mass combined with the unusual storm track led to substantial accumulations of light fluffy snow; Mission Ridge quickly stacked up over 2ft of very light powder. This new snow and strong winds produced some of the spiciest avalanche conditions for this season in the East Central zone. Numerous natural and human triggered avalanches occurred during and following the storm, including some remotely triggered slides.
Skier triggered avalanche on the Diamond Head in the Wenatchee Mountains. NW, 5200’ 3/14 Photo: Adam Butterfield.Â
Other locations in the region only added a few inches to their snow totals for the season. The position of the storm only brought dribs and drabs to the mountains from about Hwy 2 and south. A few standouts further south managed to ring out around a foot (Mt St Helens, White Pass, and Paradise). Strong easterly winds in most areas significantly redistributed the new snow and resulted in reports of numerous natural and human triggered slab avalanches Saturday and Sunday.Â
Natural wind slab avalanche on Mt St Helens. W 5500’ 3/14: Photo Nate BerryÂ
Spring has Sprung
Thursday the 19th ushered in the first official day of spring. All winter, we’ve seen storms creating sometimes unique and occasionally similar snowpacks and avalanche conditions for each of our forecast zones. Following the storm last weekend, a sunny and warm springtime pattern took hold of the region. As it did so, it brought with it a prolonged gradual warm-up and loose wet avalanche cycle. Consequently, differences formerly found in each region slowly resolved as the mountains transitioned into spring.Â
A natural loose wet avalanche near Washington Pass, Cutthroat Peak. 3/17. Photo: Gus Goldman
Two items still stand out about this spring transition. 1: the snowpack has not fully moved into a spring-like state. You can still find pronounced cold dry layers and firm icy crust in many areas. 2: We have not seen a spring “shed†cycle yet, where several large natural avalanches occur as the snowpack adjusts to percolating water and warmer temperatures. Forecasting spring shed cycles can be difficult, and it's still unclear when or even if a larger natural wet cycle will occur.Â
You can continue to support your community-based avalanche center by submitting observations.Â
Stay safe, stay healthy, and thank you for all your support.Â
Dallas Glass
Avalanche Problems
Wind Slabs
Southerly winds transported up to 10" of recent storm snow, loading polar aspects where you are most likely to trigger a wind slab on Tuesday. Expect to encounter thicker and more reactive slabs in wind exposed terrain. Lighter winds and additional snowfall Tuesday may mask the slabs with less wind-affected snow. Find test slopes to identify firmer or punchy snow layers within the recent storm snow where you can more easily trigger an avalanche. Avoid steep potentially wind-loaded slopes greater than 35 degrees, deferring to lower-angle terrain when there is any doubt.
We are forecasting without field observations on Monday in the West North zone. Gather information as you travel by digging hand pits and using small slopes to test the snow. Prior to the storm, surface hoar was widespread on polar aspects and these are the same aspects we expect to be wind-loaded, but we don't know whether any surface hoar survived strong winds Sunday night into Monday. Watch for shooting cracks, whumphing collapses, and recent avalanches as obvious signs of unstable snow. In steep terrain, the surface snow may run as Loose Dry avalanches which also may trigger be able to trigger a wind slab.
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: Possible
Expected Size: 1 - 1