Avalanche Forecast
Issued: Feb 15th, 2019 10:25AM
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
Don’t let the slight decrease in avalanche danger Saturday give you a false sense of security. The snowpack is still unstable, and you can still trigger avalanches. This is a time to keep our travel conservative and steer around slopes greater than 30 degrees.
Summary
Discussion
It’s been an active week in the West-North zone. The Mt Baker area received around 45” of new snow since last Monday. We have seen several avalanche cycles occur in the recent storm snow around Mt Baker and the Hwy 542 corridor. What has our attention though, are the larger persistent slab avalanches. In the last few days, reports from Glacier Creek, Excelsior Ridge, and Schreiber Meadows all indicate persistent slab avalanches. There is now clearer sign of unstable snow, than recent avalanches.
The snow out there is deep. This is when we worry about tree-well and snow immersion fatalities. During times like this, it’s important to keep solid communication with your travel partners. For more information visit https://www.deepsnowsafety.org/
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 southerly winds on Friday formed fresh wind slabs from the recent storm snow. We expect these slabs to still be reactive on Saturday. This isn’t a time to thread the needle around wind loaded features. With these wind slabs living in close proximity to the larger and deeper persistent slab, give any wind loaded slope greater than 30 degrees a wide berth. While triggering a wind slab would be dangerous, in larger terrain, it could step-down into old, persistent weak layers. Look for textured snow surfaces, drifts behind trees and rocks, and fresh cornices to help you determine where wind loading occurred.
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: All elevations.
Likelihood
Expected Size
Persistent Slabs
The most compelling pieces of evidence for persistent slabs are the recent large avalanches this week. Some of these avalanches were remotely triggered from far away. Others occurred on lower elevation slopes. It’s times like these where slopes we normally don’t consider, should cause us to pause, and think about avalanches. The necessary slab and weak layer combination does not exist everywhere. This makes it particularly difficult to manage. Put plenty of distance between you and any startzone. Consider simply avoiding areas where avalanches can run or stop.
While we have reports of persistent slab avalanches on all aspects, most reported events occurred on SE-SW slopes where the weak sugar facets sit atop a thin sun crust. In any location, you may find a layer of weak sugary facets 3-4 feet below the snow surface. This is the primary layer of concern.
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 16th, 2019 10:25AM
