Avalanche Forecast
Issued: Feb 17th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
Recent human triggered avalanches have occurred on adjacent Stevens Pass. A poor snowpack structure is to be expected in the area, even well below treeline. Avoid big features and steep complex terrain. Allow for lots of room between yourself and other steep slopes.
Summary
Discussion
On Saturday afternoon on Stevens Pass a party of two snowboarders dropped south into the backcountry off Cowboy Ridge, riding down towards Tunnel Creek. At 5,400 ft on a Southeast aspect, they triggered a D2 avalanche on a 40 degree slope, 150ft wide with an average crown depth of 26”. It ran about 400 vertical feet on facets over a crust, breaking through the crust, and stepped down to facets over another crust a few inches deeper. Both members were partially buried, one up to his his head, the other up to his waist. They were able to self rescue without injuries, and hiked up the bedsurface where they re-entered the ski area boundary. This is as a very relevant near miss. A good reminder that large and dangerous avalanches may still be triggered on this layer of concern.
Recent avalanches have had widely propagating crowns 2 to 3 feet deep. They've been running on a layer of faceted snow over a stout crust, or underneath this crust where it is thin and decomposed. Mild weather with some sun has created a melt freeze crust on solar aspects over the past few days. Shaded aspects have the best snow quality, but it may be where the snowpack structure is most concerning. Here, and in shallow snowpack areas, the storm snow is settling into a slab and instabilities may actually be increasing. See ob from adjacent East North zone here. Low elevation, shaded slopes near towns and cities are still locations of concern, as the snowpack is shallow and remaining weak. A very weak basal structure with large faceted grains near the ground can be expected on shaded aspects in the Wenatchee foothills.
Recent very widely propagating slab in Icicle Creek. North aspect around 5,400ft. 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 Equivalent (inches)
24hr storm totals
(inches)
Difference in Height of Snow (inches)
Hurricane Ridge
1.97
N/A
+ 30
Mt. Baker
1.94
44
Washington Pass
1.66
NA
+ 16
Stevens Pass
2.71
49
Snoqualmie Pass
3.91
80
Mission Ridge
1.86
38
Crystal
2.91
59
Paradise
4.55
N/A
White Pass
N/A
57 (4400ft)
+ 26 (5800ft)
Mt. Hood Meadows
4.70
43
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
Problems
Persistent Slabs
A problematic snowpack structure can be found throughout the zone. 2 to 3 feet of recent snow rests on facets over a crust, a weak layer that makes me cringe. You won't find me in complex terrain or on steep slopes this weekend. How steep is too steep? It's hard to say, but the steeper the slope, the greater your chances of releasing a slab. Recent avalanches have been measured at 36 degrees in Icicle Creek. Do measure your slope angles. Don't go near steep slopes that may be connected to where you are. These persistent slabs have been breaking very widely across terrain features. The reactivity is has been reported to be decreasing with the calm weather at lower elevations, though not everywhere, and that doesn't mean they are healed up yet. It appears that the shallower snowpack zones closer to the Columbia River may have the worst snowpack structure around. A large amount of uncertainty remains for upper elevation terrain, but i'd be mindful of areas that are recently wind loaded.
Professional guides and avalanche workers continue to avoid steep, and unsupported slopes, and sticking to well supported features. Even small, steep slopes below treeline are worth going out of your way to avoid. Why? Observations of:
1. Recent human triggered avalanches in an adjacent zone.
2. Lots of recent avalanches with crowns extending well over 500ft wide.
3. Continued widespread collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away.
4. A layer of weak, sugary facets on, or beneath a crust 2 to 3 feet below the snow surface, and tests that indicate propagation on it.
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
Expected Size
Valid until: Feb 18th, 2019 10:00AM
