This is ARCHIVED information related to an avalanche fatality on Mar 1 in this area.

Backcountry Avalanche Forecast
East Slopes South

Issued: Thursday, February 29, 2024 - 6:00PM
Expires: Friday, March 1, 2024 - 6:00PM
Author: Katie Warren

THE BOTTOM LINE

You could still trigger smaller avalanches in the recent snow, but more concerning is the potential for these avalanches to step down to deeper weak layers and become very large. They may break widely and run long distances. Your best option for avoiding triggering one of these destructive slides is to avoid traveling on or directly under slopes greater than 30 degrees. 

Avalanche Danger
Friday, March 1, 2024
Upper Elevations
7500-6000ft 3 - Considerable
Middle Elevations
6000-5000ft 3 - Considerable
Lower Elevations
5000-3000ft 2 - Moderate

Avalanche Problems (2)

Problem #1: Persistent Slab
Aspect/Elevation: All aspects, middle and upper elevations
Likelihood: Likely
Size: Large (D2)

Weak, faceted snow over the Early February Crust has created a tricky and dangerous persistent slab problem. We have some uncertainty about the extent of this problem in East South. However, in nearby East Central, multiple human-triggered and natural avalanches have failed on this persistent weak layer. After the recent storms, a thicker slab exists over this weak layer, so avalanches could be much larger. A smaller avalanche could also step down to this layer. You can trigger a persistent slab avalanche from low-angle terrain below or adjacent to steep slopes, and they may propagate widely across terrain. You may or may not observe signs of instability, like collapsing, before triggering an avalanche, and it may not be the first person on the slope to trigger it. 

The best way to stay out of harm's way is to avoid traveling on or under slopes over 30 degrees. Give this problem wide safety margins. Stay off of and out from underneath steep slopes at elevations above approximately 4500ft where 1-3ft of new snow has accumulated since 2/24. Heavy rain likely created a thick, slippery crust at lower elevations and will limit avalanche potential. 

Problem #2: Storm Slab
Aspect/Elevation: All elevations, all aspects
Likelihood: Possible
Size: Small (D1) to Large (D2)

The recent snow totals are impressive, with 1-3ft of snow across the zone. Several waves of precipitation, fluctuating freezing levels, and strong winds have resulted in a layered upper snowpack. Check the recent storm snow for weak interfaces and the bond to the old surface by digging. Small test slopes and quick hand pits may not cut it when the snow is this deep. Heavy snow over weak snow, layers of weak grains such as graupel, or wind-drifted slabs hidden below the surface are some potential failure points. Slabs will be larger at higher elevations or in wind-loaded terrain below ridges and the base of cliffs.

Avoid traveling on slopes over 30 degrees, and watch out for likely trigger points like convex rolls or unsupported features. These avalanches might be small to start, but if they step down to the Early February Crust and propagate widely, they could quickly become very large and inescapable.

Forecast Discussion

You're not alone if you feel like we've been in a perpetual pattern of persistent slab problems. Lower-than-average snowfall and warmer-than-average temperatures contribute to crust formation and a shallow snowpack prone to developing these tricky weak layers. The issue is especially apparent in the eastern Cascades this winter. So, if you're venturing into these eastern zones this winter, be prepared to step into what can feel like a very unfamiliar snowpack compared to what we typically experience in the Cascades. 

If the sun comes out to stay for a while on Friday afternoon, you may see some small, wet loose avalanches, rollerballs, and pinwheels. 

Areas that have received the most snowfall during this storm, such as near Bumping Lake, will have larger and potentially more sensitive slabs. On Wednesday, an NWAC forecaster near the West South/East South border reported wind-loaded slopes failing easily under their weight. On Thursday, Crystal Ski Patrol triggered a widely propagating avalanche during control work. Observations were limited by the ferocity of this storm not just in the East South but in neighboring zones as well. However, before this storm, I was traveling near the northern end of East South and found a weak snowpack structure similar to East Central. I identified weak, faceted snow grains over the Early February Crust (EFC). With widespread human-triggered and natural avalanches reported in East Central, we expect a similar problem to exist throughout East South. There are also potential layers of surface hoar and faceted grains from the 14th, 18th, and 24th. Widely propagating avalanches could fail on any of these layers. Approach the mountains cautiously and enjoy some low-angle powder until these weak layers gain strength. But be patient, as it could be a while before we see improved snow stability.