Avalanche Forecast
Issued: Feb 18th, 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. Light snow and wind will add to the unstable snowpack over the next 48 hours. Avoid big features and steep complex terrain. Allow for lots of room between where your travel and any steep slopes.
Summary
Discussion
On Saturday afternoon, a party of two snowboarders triggered a D2 avalanche at 5,400 ft on a southeast aspect in Tunnel Creek. Both members were partially buried. They were able to self-rescue without injuries and hiked up the bed surface where they re-entered the ski area boundary. This is as a very relevant near miss.
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. On shaded aspects 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. On the 18th an observer reported large collapses on a northeast aspect at 2800ft in Olalla Canyon. A very weak basal structure with large faceted grains near the ground can be expected on shaded aspects in the Wenatchee foothills.
In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight.
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 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
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
Continue to use caution when traveling in the mountains and minimize your exposure to avalanche terrain. In addition to obvious avalanche paths, you may be able to trigger avalanches in steep open trees. Avoid large, open slopes 35 degrees and steeper. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Professional guides and astute backcountry travelers are still avoiding big terrain and unsupported slopes. When dealing with this tricky snowpack, prioritize signs of instability and keep you decisions conservative and simple.
You’ll find the February 8th facets buried 2-3 feet below the surface with an underlying crust. The snowpack structure is widespread on all aspects and elevations. Another round of snow and wind in the next 48 hours will build more fresh slabs. These avalanches should be considered guilty until obviously innocent. Watch for signs like collapses (whumphs) or tests indicating propagation/triggering. Prioritize information that indicates a potential for triggering avalanches over observations that point at relative stability. The reactivity may be decreasing, though not everywhere. 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.
As we get further from the last avalanche cycle, signs of instability are becoming less frequent and sometimes less obvious. But they are there. On Sunday on the east end of the Stevens Pass zone, I poured over the snowpack identifying grains and performing many tests. After emerging with contradictory test results, we experienced a large rumbling collapse 2000ft from the profile site. The collapse was all the information I needed to avoid avalanche terrain.
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 19th, 2019 10:00AM
