Avalanche Forecast
Issued: Feb 17th, 2019 10:02AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Even though the avalanche danger is dropping, and the weather seems nice, you may still trigger avalanches West-North zone. Watch out for wind loaded slopes at higher elevations where new wind slabs formed Sunday. There is also a lingering persistent slab problem in some areas that may warrant avoiding large, steep, open slopes.
Summary
Discussion
The most recent major avalanche from the West-North zone was reported on Saturday. It occurred on a N-aspect of Mt Ann near the Mt Baker ski area. Snow profiles and snowpack tests from Heather Meadows, Bagley Lakes, and Heliotrope Ridge confirmed the same snowpack structure we’ve seen for the past week.
Natural avalanche from Mt Ann, February 16, 2019: Photo Jason Speer
Nearly every zone in the Cascades contains a similar potentially dangerous snowpack structure. Each area has subtle differences, but they appear more alike than different. 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 in the event. We will post more information as it becomes available.
Human triggered persistent slab avalanche form Tunnel Creek: Photo: Stevens Pass Ski Patrol
If you are using the nice weather to travel to higher elevations or more remote trailheads, know that we have a higher degree of uncertainty in these locations. Consider trips with plenty of options to reduce your risk, and ease into the terrain.
When the sun comes out, you may see new rollerballs and small loose avalanches on steep rocky slopes. Don’t get surprised by one of these easy to predict and manage avalanches.
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
Wind Slabs
The low-density snow doesn’t take very strong winds to drift into new wind slabs. On Sunday, new and reactive wind slabs formed at higher elevations. You will still be able to trigger these slabs on Monday. 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 steering around wind loaded slopes greater than 35 degrees.
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
Expected Size
Persistent Slabs
The slab/weaklayer combination required for these persistent slabs doesn’t exist everywhere in the West-North zone. It’s this spotty distribution that makes it so tricky. Here’s what we know. We have reports of observed avalanches and/or snow profiles highlighting this problematic layer on all aspects and in each elevation band. This layer is buried 2-3 feet below the snow surface. On colder shaded slopes, the culprit seems to be buried surface hoar. On sunnier aspects, we are finding small facets over a crust. Either way, this is a weak snowpack setup.
You aren’t likely to find obvious signs of instability such as whumphs or shooting cracks. Only 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.
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 10:02AM
