Avalanche Forecast

Weak and variable snow surfaces comprised of facets and surface hoar were reported in the Washington Pass area over the past few days. The last known avalanches occurred on Sunday. These triggered avalanches were small (D1) and ran in north facing terrain, possibly on a layer of surface hoar buried about 1ft deep between recent storm layers. The last major loading event and subsequent natural avalanche cycle occurred February 12th and 13th. Avalanches failed on facets over a crust with the most significant avalanche activity, and widest propagation, occurring on east and south aspects.

Significant snowpack variability can be found throughout the East-North zone. In outlying shallow snowpack areas further east of Washington Pass such as Goat Mountain and Twisp River, particularly touchy conditions have been found recently, even below treeline. Facets over a crust are the culprit. Be especially careful if you venture into these areas.

February 19th, 2019

Recap

We’re now over a week out from a major winter storm and avalanche cycle, February 9-13th, that left a string of school cancellations and avalanche near misses in its wake. As with snowfall amounts, the avalanche cycles have been similar, but not identical in all regions. The further we’re getting from the peak of the cycle, the more variation in avalanche conditions we’re seeing between different zones and even within the individual zone. Variable snow totals from storms this week are further adding to the range of conditions you will encounter. In some places, these storms may add stress to existing weak layers.

Since the natural avalanche cycle of the 11-12th quieted down, the main concern for avalanches has focussed on the February 8th facets in regions where the weak layer is problematic. In the days after the natural cycle all observations, including rumbling collapses, remote triggering, and snowpack tests screamed, “avalanche!”

A natural persistent slab (D2) on a north aspect at 4200ft low in Glacier Creek drainage (Hwy 542). 02/13/19 Lee Lazzara Photo

Variability and Mixed Messages

As the facets and surrounding snowpack structure changes, we’re seeing the potential for triggering avalanches change, as well. Now, the likelihood of triggering persistent slab avalanches is decreasing. In some regions, the problem is trending to unlikely. Meanwhile, the consequences (size and destructive potential) remain significant, if not the same.

Time has helped round the February 8th facets. No longer will every clue give a resounding answer as to whether or not you can trigger an avalanche. To complicate things, observations like snowpack tests can be notoriously difficult to interpret, requiring a lot of time practicing good snow-craft. Snowpack tests often don’t give us a clear “go or no-go” answer, if such a thing exists.

Q: How do we interpret observations that are contradictory, when some point at the potential to trigger avalanches and others indicate better stability?

A: Focus on the observations that show the potential to trigger avalanches. Look for obvious clues, like recent avalanches, shooting cracks, or collapses. Prioritize observations that indicate triggering (initiation) and propagation.

A natural persistent slab avalanche (D2), likely occured on 2/12 on southwest through southeast aspects of Windy Mountain at 5,400ft in the Tye River drainage. Photo: Dan Veenhuizen.

Case Study

In a recent profile, east of Stevens Pass, I found the February 8th facets (0.5-1.5mm) rounding and buried 59cm from the surface. The results of the profile were:

CTH (SP)

ECTN28

PST 45/100 (END)

5 yellow flags (structural indicators)

Later that day, about 2000 linear feet away from the profile site at the same elevation and slightly different aspect, we experienced a massive rumbling collapse.

All this crypto snow-speak means that some of the observations pointed towards triggering an avalanche was likely, but some did not. Confusing, right? 

With all of this data in my head, it was the collapse that stuck out. That was enough evidence for me to avoid slopes steeper than 35 degrees. That was a more obvious answer than all the other pieces of data I gathered and it’s the easiest to interpret. Without the collapse, I would have prioritized the test results that indicated I could have triggered a slide. If only snowpack tests would give you the sinking feeling of almost triggering an avalanche that you get from a rumbling collapse...