Avalanche Forecast
Regions: Cascades - East.
A hefty slab of storm snow rests on a weak layer over a crust. The snowpack remains unstable, even below treeline. If you venture into the mountains, avoid slopes steeper than 30 degrees and put plenty of space between yourself and steep slopes.
Discussion
Snow and Avalanche Discussion
An extended period of instability began on the 11th with natural avalanches reported through the 13th on a wide range of aspects and elevations. Common characteristics of these avalanches are very widely propagating crowns 2 to 3 feet deep. While some avalanches ran in storm layers in the upper snowpack, many are running an old weak faceted layer 2 to 3 feet below the surface. Visibility and dangerous conditions have limited our ability to gather more detailed information from near and above treeline.
Mission Ridge has received over 40 inches of snow since February 8th. Low elevation slopes near towns and cities are now locations of concern for avalanches. On Tuesday, I found a very weak basal structure with a thin snowpack in the Wenatchee foothills. Expect similar conditions near the towns of Peshastin and Leavenworth. In addition to avalanches, all the deep snow is building concern for Snow Immersion Suffocation, or tree well hazards higher in the mountains. Travel with partners and keep them in sight.
A natural avalanche in the Icicle Canyon. Grindstone Mtn, SE, 6,600ft. Photo: Matt Primomo
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 Equivilent (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
It is not worth trying to outsmart this problem. We know enough to know its there, a uniform slab and weak layer over aspects and elevations have created a dangerous setup for slab avalanches. The slab rests on facets over a crust, a weak layer that makes me cringe. As long as there is a slab on top, i'd avoid steep slopes nearby. If you decide to go into avalanche terrain this weekend, be sure of your ability to avoid slopes 30 degrees and steeper. Measure your slope angles. Stay far away from underneath big avalanche paths and don't go near steep slopes that may be connected to where you are. These persistent slabs have been breaking incredibly widely across terrain features. They may be triggered from a distance, may come down on top of you, and may kill you.
Professional guides and avalanche workers are avoiding steep slopes like the plague. Even small, steep slopes below treeline that I rarely think much about may be exactly the type of place to avoid this weekend. Why? Numerous observations of:
1. Lots of avalanches with crowns well over 500ft wide.
2. Widespread collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away.
3. A layer of weak, sugary facets and surface hoar resting on a crust 3-4 feet below the snow surface.
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: Likely
Expected Size: 1 - 2