Avalanche Forecast
Regions: Cascades - North West.
Expect rising avalanche danger throughout the day Monday as new snow accumulates. Carefully evaluate how new snow bonds to variable old snow surfaces. Fresh slabs may become reactive to human triggers on slopes 35 degrees and steeper by the afternoon.
Discussion
An incoming storm looks to bring 8-12in of new snow (2000-3000ft snow levels) to the West North zone over the next 24hrs, ending a weeklong stretch of warm sunny weather. Snow levels will gradually lower overnight Sunday, and the bulk of precipitation will arrive Monday. Incoming snow may not bond well initially to variable old snow surfaces. Carefully monitor changing conditions throughout the day.
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. Itâs 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 weâre 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, 5200â 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 5500â 3/14: Photo Nate BerryÂ
Spring has Sprung
Thursday the 19th ushered in the first official day of spring. All winter, weâve 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 âshedâ 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
Storm Slabs
Only a few inches of snow are expected overnight, but storm slabs will quickly build throughout the day Monday as precipitation intensifies. Expect the most dangerous conditions in the afternoon. Storm slabs will be possible to trigger on slopes 35 degrees and steeper by the time 6in of new snow accumulates. Gather information as you travel by digging hand pits and using small slopes to test the snow. Expect to encounter thicker and more reactive slabs in wind exposed terrain. Watch for shooting cracks, whumphing collapses, and recent avalanches as obvious signs of unstable snow. Loose dry avalanches may occur as well, and could run far and fast on their own, or even trigger slab avalanches.
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: All elevations.
Likelihood: Possible
Expected Size: 1 - 1