Avalanche Forecast
Regions: Snoqualmie Pass.
Light snow and wind will add fresh slabs to the surface of the snow over the next 48 hours. The same persistent slab problem we’ve been talking about for more than a week hasn’t gone away. This is a good time to avoid large, open slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
We’ve haven’t had any reports of major avalanches in the Snoqualmie backcountry for several days. Don’t confuse this lack of avalanche activity with safe snow conditions. On Saturday and Sunday, Alpental patrol triggered several persistent slab avalanches in their back bowl terrain. A few of these of these occurred at a slightly lower elevation (4000’) and slope angles (30-35 degrees). Snow profiles and snowpack tests from this zone continue to highlight the weak snow structure.
Nearly every zone in the Cascades contains a similar potentially dangerous snowpack setup. The most recent and compelling piece of information we have concerning the persistent slab is from Stevens Pass. On Saturday, two snowboarders triggered a persistent slab avalanche in Tunnel Creek outside the boundary of Stevens Pass ski area. Both were caught, carried and partially buried, but no injuries were reported.
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.
Human triggered persistent slab avalanche form Tunnel Creek: Photo: Stevens Pass Ski Patrol
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
Snow profiles and snowpack tests show the persistent weak layer is gaining strength. The layer is becoming harder to trigger, but still could result in a large and destructive avalanche. The culprit has been a layer of buried facets and surface hoar 3-5 feet below the snow surface and typically above a 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. In areas where the layer is deeply buried, you may not find obvious signs of instability such as whumphs or shooting cracks. Digging in the snow can help you locate where this layer may be present. This could be a good time to simply stay out of large, open slopes greater than 35 degrees where you could trigger one of these lingering persistent slabs.
These avalanches should be considered guilty until obviously innocent. Prioritize information that indicates a potential for triggering avalanches over observations that point at relative stability. On Sunday at neighboring Stevens Pass, 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: Possible
Expected Size: 1 - 2