Avalanche Forecast
Regions: Cascades - East.
A warm and windy storm will create dense new slabs at upper elevations. Use caution as you gain elevation, and find wind affected snow. Steer around these drifts on steep slopes near ridgelines, and in open alpine terrain.
Discussion
On Tuesday expect to find wet snow surfaces at lower elevation with a rain line near 5,000ft overnight. Above this, the new snow may get drifted into dense slabs. More snow near the Cascade Crest in areas such as Salmon la Sac and upper Icicle Creek may create more dangerous conditions. The snow will be falling on a range of surfaces including dry settled snow, old wind slab, crusts, and wet snow below 5,000ft. On Sunday observers in the Mount Mastiff area near the western zone boundary found about 8" of new snow, and surface hoar growth in sheltered terrain from 4,000ft to 6,000ft. In the Eightmile Lake area, observers found about 6" of new snow on upper elevation slopes, with minimal snow below.Â
The last high elevation rain event was in early February. You may still find the crust from this event near the surface in isolated areas, with weak snow above it. There is some remaining potential to trigger deeper slabs with very isolated areas of buried surface hoar or near surface facets above this early February crust.Â
Snowpack Discussion
February 27, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
In the lead up to last weekend, dry, clear and cold conditions produced surface hoar in many areas as well as faceted snow (check out the National Avalanche Centerâs encyclopedia for more details). These weak grain types were not difficult to find and recreationists and professionals across the region began to wonder what role these weak snow surfaces would play when the next storm arrived.
Surface Hoar was not difficult to observe during this period of high pressure. This picture is from 2/21 on an SSE aspect, 5,000â at Stevens Pass just a day before it may have been buried. Photo: Josh Hirshberg
We didnât have to wait too long, as a major storm arrived on Sunday 2/23, creating dangerous avalanche conditions for most zones. This system dropped close to 2â of snow at Mt. Baker while areas to the south along the west slopes of the Cascades saw closer to 1 foot. Areas along the east slopes of the Cascades also saw significant new snow amounts with Blewett Pass and Mission Ridge receiving around 6-8â of fresh snow.Â
The wind was also a major factor with this storm from about Stevens Pass southward. Multiple stations recorded winds over 80mph and some went over 100mph. This wind was enough to strip surfaces down to old crusts on windward aspects, leaving leeward aspects with wind slabs and touchy cornices. In the case of Mt. Hood, relentless winds this season have stripped the snow down to blue ice from one of our previous atmospheric river events, creating slide for life conditions on certain slopes.Â
The old snow interface, termed 2/22 for the day it was buried, has us thinking hard about the snowpack moving forward. In the northern part of the region, less wind allowed for more storm than wind slabs, with avalanches that released on both interstorm layers and on the buried 2/22 interface for a day or two following the storm. As you moved further southward or eastward, reactivity on the 2/22 interface was trending downward or non-existent after a natural avalanche cycle that immediately followed the storm. Was the storm potent enough to wipe out the surface hoar and facets? There was a surprising natural avalanche on Stevens Pass during the evening of Monday 2/24, which is suspected to have run on small facets. This avalanche and continued snowpack observations around the region proves that in specific areas, reactive weak snow still existed.Â
This natural avalanche failed 2ft deep on the 2/22 interface on a SE aspect near 5700ft on Cowboy Mountain. 02/25/20. Photo: Stevens Pass Ski Patrol
On Tuesday, the sun started to shine in some locations, bringing back the threat of wet avalanches and again making us wonder about how warming would affect the 2/22 interface. We never reached the tipping point for widespread wet avalanche activity, but rollerballs and snow shedding off trees and rocks were certainly present in many zones. Drizzle or light snow made an appearance on Wednesday along many west-side zones. Thursday saw a return to warm temperatures and sunny skies all the way from Mt. Hood to Stevens Pass with cooler and cloudier conditions for the North Cascades. All in all, it was a quiet week avalanche-wise and the 2/22 wasn't activated.Â
We have one more dry mild day before we move into a stormy Saturday. During this stretch, weâll continue to monitor buried weak snow grains for distribution and reactivity. Check your local forecast zone for the latest.Â
Avalanche Problems
Wind Slabs
Up to 5" of new snow may fall overnight near the Cascade Crest, with less snow expected east of Blewett Pass. Winds will be strong at higher elevations, and may create dense slabs over various old surfaces. These slabs may be the most sensitive early in the day, as the storm tapers off. Use caution on leeward slopes near ridgelines, and on open alpine features. Look for signs of wind textured snow, strong over weak layering, or hollow sounding slabs. If you see signs of instability like shooting cracks or hear whumphing, avoid slopes 35 degrees and steeper.
At lower elevations the snow may be wet from overnight rain. If you see rollerballs or recent, fan shaped avalanches, expect that you could trigger other small wet loose avalanches on steep slopes in similar 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