Avalanche Forecast
Regions: Snoqualmie Pass.
The same persistent weak layer responsible for so many avalanches this week still has our attention and deserves our respect. Give any open slope greater than 30 degrees a wide berth.
Discussion
Snow and Avalanche Discussion
We aren’t seeing the same obvious signs of unstable snow that were found earlier in the week around Snoqualmie Pass, but there is plenty of evidence that our persistent slab problem is still alive. Alpental patrol triggered a few larger persistent slabs with explosives on Saturday. Some of these caught our attention because they occurred at lower elevations (4000’) and on more stubborn slope angles (30-35 degrees). Snow profiles and snowpack tests from Snoqualmie Mountain and Roaring Ridge continued to highlight the Feb 8 facet and/or surface hoar layer. It's this layer of unstable snow that has our attention and continues to shape how we are traveling in the mountains.
As the weather continues to improve, if you travel to higher elevations or more remote trailheads, know that we have a higher degree of uncertainty in these areas. Proceed cautiously, and ease into the terrain.
There are two other hazards to consider Sunday.
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When the sun comes out, you may see rollerballs and small loose avalanches from steep, rocky slopes. Don’t let these easy to predict avalanches catch you off guard.
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The snow out there is deep, especially around tree wells. Be careful when traveling near small trees. Keep your partners in sight, and maintain voice contact.
Snowpack Discussion
February 15, 2019
Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experience cold and very storm weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th
5 day totals ending morning of Feb 13th
Water Equivalent (inches)
24hr storm totals
(inches)
Difference in Height of Snow (inches)
Hurricane Ridge
1.97
N/A
+ 30
Mt. Baker
1.94
44
Â
Washington Pass
1.66
NA
+ 16
Stevens Pass
2.71
49
Â
Snoqualmie Pass
3.91
80
Â
Mission Ridge
1.86
38
Â
Crystal
2.91
59
Â
Paradise
4.55
N/A
Â
White Pass
N/A
57 (4400ft)
+ 26 (5800ft)
Mt. Hood Meadows
4.70
43
Â
Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).
A natural persistent slab avalanche (D2.5) on a southeast aspect at 6,600ft. Grindstone Mtn in Icicle Canyon. Likely ran 2/12. Photo: Matt Primomo
The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.
Large surface hoar near Snow Lake Divide on February 7, 2019 just before it was buried on the 8th. Photo: Jeremy Allyn
Avalanche Problems
Persistent Slabs
Around Snoqualmie Pass, the buried persistent weak layer is deep. Three to five feet in some locations. You may not see obvious signs of instability as you travel. That doesn’t mean it’s safe. The recent avalanches and observed snowpack structure provide ample proof that you may trigger a persistent slab avalanche on open slopes greater than 30 degrees. Several lower elevation (3000-4000’) avalanches have been reported this week. This should cause us to pause, and make sure that we aren’t falling into our normal travel patterns. Yes, this layer is gaining strength, but it is not gone. Continue, to be cautious and select trips that give you plenty of choices to avoid steep, open slopes and manage your risk.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 2