Avalanche Forecast
Issued: Feb 16th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Even though the unstable snow is slowly gaining strength, you may still trigger very large avalanches on open slopes greater than 30 degrees. Persistent slab problems deserve our respect. Continue to be cautious as you travel by selecting less consequential slopes.
Summary
Discussion
Late on Saturday, we received a report of a natural avalanche on the N side of Mt. Ann in the Baker backcountry. Details are limited, but the avalanche appears new. It is difficult to say if this is a persistent slab avalanche or a wind slab. The cornice on the ridgeline suggests that may be a cornice collapse triggered the slide.
Natural avalanche from Mt Ann, February 16, 2019: Photo Jason Speer
Snowpack observations and small column tests from the Heather Meadows area continued to highlight a layer of weak, sugar facets buried on Feb 8. This tells us the unstable snowpack structure is still present in some locations. It’s this layer of facets, that has our attention, and is shaping how we travel in the mountains.
As the weather continues to improve, if you travel to higher elevations or more remote trailheads, know that we have a higher degree of uncertainty in these areas. Proceed cautiously, and ease into the terrain.
There are two other hazards to consider Sunday.
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When the sun comes out, you may see rollerballs and small loose avalanches from steep, rocky slopes. Don’t let these easy to predict avalanches catch you off guard.
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The snow out there is deep, especially around tree wells. Be careful when traveling near small trees. Keep your partners in sight, and maintain voice contact.
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
Persistent Slabs
The slab-weaklayer combination required to produce very large persistent slab avalanches does not exist in all locations. However, avalanches triggered on this layer from the past week, combined with recent snowpack tests, show this avalanche problem is not fully put to bed. You may not see obvious signs of instability as you travel. This doesn’t mean it’s safe. Give open slopes greater than 30 degrees a wide berth.
While we have reports of persistent slab avalanches on all aspects, most reported events occurred on SE-SW slopes where the facets sit atop a thin sun crust. In any location, you may find a layer of weak sugary snow 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
Wind Slabs
The wind slabs in the West-North are now a few days old. As they get older, they can become more stubborn and harder to trigger. However, these wind slabs may live in close proximity to the larger and deeper persistent slabs. This may not be the best time to try to slip around wind loaded features, especially if they are part of larger more opened terrain. Avoid all wind loaded sloped greater than 30 degrees. Triggering even a small wind slab, could step down into the older persistent weak layers below.
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
Valid until: Feb 17th, 2019 10:00AM
