Avalanche Forecast
Regions: Cascades - South West.
Recent wind slabs may be hiding under some new soft snow. You’ll have to look for larger clues such as cornices or big drifts to highlight where the wind affected the snow. If you suspect wind loading occurred, stay off of nearby slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
We just received a report of a very large persistent slab avalanche near Crystal Lakes. This is our first confirmed report of a persistent avalanche in the West-South zone. The avalanche likely occurred on Thursday, as wind drifting as obscured some features of the avalanche. Details are sparse, but we know it failed low on a slope, crossed several terrain features, and was 4-5 ft deep. This screams persistent slab. (NE aspect 6200’)
Observations in other areas of the West South have been very limited. Do not equate this lack of information with a safe snowpack. The lack of informations leads us to a higher degree of uncertainty. If you travel into some of these areas, select trips that give you plenty of choices to avoid steep, open slopes and manage your risk.
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
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
Wind Slabs
Wind Slabs formed late this week may now be hidden under recent soft snow. You’ll need to do a bit of detective work to find where the wind loaded slopes. Look for cornices, large snow drifts, and uneven snow surfaces to clue you into where wind slabs may be hiding. If you suspect wind loading, avoid open 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: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Even with the new avalanche observation, we don’t have substantial supporting evidence of persistent slabs in the West-South zone. However, we have a high degree of suspicion. Persistent slab avalanches have been confirmed in all our forecast zones, including Mt Hood. This is making us leery of areas near Mt Rainier, Mt Adams, Mt St Helens, and the Columbia River. If you dig, you may find a layer of weak, sugary facets or feathery surface hoar 3-5 ft below the snow surface and just above a very obvious, firm crust. While snowpack test can confirm the presence of a buried persistent weak layer, they can not prove its absence. If you suspect persistent slabs exist in the area you are traveling, give all open slopes, greater than 30 degrees a wide berth.
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