Avalanche Forecast
Regions: Cascades - West.
Stormy weather focused on the northern half of this zone, creating wind loaded areas at mid and upper elevations that may be more extensive in the Mountain Loop region. As you reach more exposed terrain, take time to identify and avoid smooth pillow-like drifts and slopes below sharp cornices. If the sun comes out for longer periods of time, look for the snow to change and react differently, including isolated natural loose avalanches.
Discussion
On Wednesday avalanche professionals in several locations around the adjacent West-North region reported both natural and human triggered wind slabs 8-12†deep. While wind affected snow was found on all aspects, many of these avalanches occurred on NE-E facing slopes. Storm totals from the area ranged from 8-14†for most locations. The adjacent Stevens Pass area picked up several inches of fresh snow by Wednesday morning.
Northern Parts of the West Central zone likely picked up 4-8†of snow, ending early Wednesday, with conditions only slightly less reactive than in the West North. Meanwhile, southern parts of the zone likely behaved more like Stevens with 1-4†of snow exhibiting wind-affect, redistribution, and more limited slab formation near and above treeline.
Winds, clouds and cold temperatures kept the March sun at bay on Wednesday, but with lighter winds and the potential for some extended periods of sunshine, the sun may end up winning the battle in some areas on Thursday. The resultant loose wet avalanches should be isolated and small in most parts of the zone, but may be slightly larger and more dangerous in the north. During extended sun breaks you could see surface snow become wet and sticky, producing rollerballs, and ultimately loose wet avalanches. If you see fan-shaped debris, steer clear of similar slopes and adjust your aspect.Â
If you encounter natural loose wet avalanches, that means the sun is winning the battle. This could also indicate it could be weakening large cornices. Don’t linger below these ticking timebombs.
Snowpack Discussion
March 5, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
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Ridgetop wind transport near Washington Pass. 3/4/20 Photo: Josh Hirshberg
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The beginning of March brought the first spring-like weather to the region. The days are getting longer and the sun (when it shines) has a noticeable effect on the snow. Alternating sun breaks and snowstorms is the theme of the week and the avalanche danger fluctuated just as fast as the weather. The southern zones and the east slopes have seen periods of light snowfall and significant sunshine while the northern zones and west slopes have had significant snowfall and occasional sun breaks. Above freezing temperatures were common at lower elevations throughout the region.Â
Last week in review: Temperatures were above freezing at most trailhead locations and concerns about the previous week’s buried surface hoar and weak layers were fading. Saturday temperatures plummeted as strong winds and new snow built wind slabs in most areas and drove the avalanche danger to considerable in almost every zone. Sunday’s calm weather and clearing in most areas helped stabilize wind slabs, but our attention turned to the strength of the sun and daytime warming. Lingering wind slabs and loose wet avalanches were forecast in every zone. For the remainder of the week, the avalanche problems in every zone were confined to the recent snow and each brief storm cycle had us thinking about wind slabs at upper elevations and loose wet avalanches from daytime warming and sunshine.
Spring isn’t actually here -- another round of cold temps and snow showers are on the way this weekend, but this past week’s oscillating weather is a good reminder to begin to shift our mindset. The sun is gaining strength, the days are getting longer, and we begin daylight savings this week. Even a brief period of sunshine can dramatically alter fresh, new snow. Check the mountain weather and avalanche forecasts for the most up to date info, enjoy the backcountry and let us know what you see out there!
--Peter Moore
Wet loose avalanche debris near Snow Lake. 3/5/20 Photo: Dallas Glass
Fresh cornice development from strong winds in the Stevens Pass backcountry. 3/4/20 Photo: Tom Whipple
Avalanche Problems
Wind Slabs
Windy conditions the last few days transported the recent snow and formed slabs on the lee sides of ridges and along isolated exposed mid-slope features.Slabs are more than 24hrs old, you’ll want to give them a wide berth Thursday where you do find them. Take time to identify and avoid any steep slope with smooth pillow-like drifts, below cornices, or where you find firm hollow snow. Expect wind slabs to grow larger and harder to manage as you travel higher or move into more wind prone areas.
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