Avalanche Forecast
Regions: Cascades - North East.
You can trigger a large avalanche on a buried persistent weak layer or in wind drifted snow. Expect increasing avalanche danger with snowfall and strong winds on Wednesday and Thursday. Avoid steep, complex terrain and put plenty of space between where you travel and open slopes 35 degrees and steeper.
Discussion
Snow and Avalanche Discussion
Professional guides reported two small (D1) skier triggered avalanches on Sunday on a northerly aspect at 5400ft, failing within storm layers about one foot deep. Recent observations from the Washington Pass area indicate a layer of buried surface hoar exists within storm layers at about the same depth on shaded aspects. The last major loading event and subsequent natural avalanche cycle occurred on the 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 places further east of Washington Pass such as Goat Mountain and Twisp River, particularly touchy snowpack conditions have been found recently, even below treeline. Facets over a crust are the culprit. Be especially careful if you venture into these areas.
Snowpack Discussion
February 15, 2019
Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experienced cold and very stormy 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
The snowpack structure is problematic throughout the zone. One to three feet of recent snow rests on weak faceted grains which, in some areas, can be found over a crust. This is a great recipe for avalanches. These persistent slabs have been breaking very widely across terrain features. The reactivity has been reported to be decreasing slightly (but definitely not yet healed) with the calm weather at lower elevations near Washington Pass. It appears that the shallower snowpack zones further east, or closer to the Columbia River, may have a far worse snowpack structure. A large amount of uncertainty remains for upper elevation terrain - be especially cautious of recently wind loaded areas.
Professional guides and avalanche workers continue to avoid steep, unsupported slopes and are seeking out well-supported features. Even small, steep slopes below treeline are worth going out of your way to avoid. Why? Observations of:
1. Recent wide crowns from avalanches likely failing on faceted layers.
2. Continued reports of 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 on or beneath a crust 2 to 3 feet below the snow surface continues to produce propagating fractures in snowpack tests.
4. A large amount of uncertainty.
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 - 1
Wind Slabs
Plenty of recent low-density snow is available for transport. Snowfall and increasing northwest winds are expected over the next 24 hours. Fresh wind slabs may form near and above treeline. Any avalanche triggered in upper snow layers could step down and result in a wider and more dangerous persistent slab avalanche. Look for signs of wind transported snow such as textured snow surfaces, drifting, or fresh cornices. If you see these features, expect wind slabs on nearby slopes. You can avoid triggering a wind slab by sticking to wind-sheltered or scoured terrain.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1