Avalanche Forecast
Issued: Feb 28th, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs.
NWAC,
Northwest Avalanche Center
Avalanche danger is increasing due to the return of snow and wind to our zone, with avalanches more likely as you ascend in elevation or on the volcanoes. On slopes protected from wind and sun, there is potential for surprising avalanches breaking wider or on lower angled slopes than you may be used to. If you notice whumpfing collapses or find that the new snow is not well bonded to the old snow surface, head for slopes less than 30 degrees.
Summary
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 Center’s 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, 5,000’ at Stevens Pass just a day before it may have been buried. Photo: Josh Hirshberg
We didn’t 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 2’ 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 6-8” of fresh snow.
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.
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.
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.
We have one more dry mild day before we move into a stormy Saturday. During this stretch, we’ll continue to monitor buried weak snow grains for distribution and reactivity. Check your local forecast zone for the latest.
Problems
Storm Slabs
Storm slabs should develop in all elevations bands with higher snow totals focused on the volcanoes creating locally thicker slabs. Furthermore, strong winds associated with this storm, particularly closer to the crest, may deposit snow into thicker and more reactive slabs at upper elevations on leeward slopes, on the sides of gullies, or at the base of cliffs. NWAC team members positively identified weak snow grains such as surface hoar and facets on sheltered polar aspects on Friday afternoon. If these grains are preserved before being buried by new snow, we could have a setup for surprising avalanches. You should use small test slopes and hand pits to investigate the bond between the new snow and the existing surface where these weak snow grains may exist. Shooting cracks as you travel, wind stiffened snow, or blocks of cohesive snow in your hand pits can give you clues about slab potential. If you find a poor bond and more than 6 inches of cohesive snow, avoid slopes greater than 30 degrees.
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
Expected Size
Valid until: Feb 29th, 2020 10:00AM
