Avalanche Forecast
Regions: Mt Hood.
A weak layer 2-3 feet deep near and below treeline can still produce very large and destructive slabs that break widely across terrain features. Recently formed wind slabs or surface snow avalanches formed on Saturday afternoon and evening and you can still trigger them on Monday. Choose simple terrain to safely navigate this tricky snowpack.
Discussion
Snow and Avalanche Discussion
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
NWAC has not received observations from the Mt. Hood Backcountry, making it difficult to refine the forecast. Please help your forecast by submitting an observation.
The epic storm cycle that wrapped up Wednesday produced several deep avalanches averaging around 4 feet that were triggered Wednesday and Thursday by both skiers and explosives near and below treeline.
On Thursday and Friday, Mt. Hood Meadows Patrol identified a layer of weak snow (1-2 mm facets) above a hard crust as the weak layer/bed surface combo on northerly aspects below treeline. This weak layer/crust combination is likely most reactive near treeline and in the upper treeline elevations, but information is still limited. Stay safe by keeping your terrain selection simple.
By Saturday evening, 7†of storm snow had accumulated during the day and was being blown around by moderate to strong winds with more snow and transport anticipated.
The risk of tree well and snow immersion suffocation continues in areas with deep snow. Don't travel alone and always keep in contact with your partner. You can learn more about deep snow safety here.
If the sun comes out on Sunday, expect roller balls and loose wet activity that will entrain the surface snow and shed off rocks and steep slopes.
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 Equivilent (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
Saturday afternoon and evening, moderate to strong winds are anticipated to have created fresh wind slabs while older slabs linger from moderate winds Thursday and Friday on steep, leeward slopes. These slabs were reactive, generally soft, and small to large. Recent cornice formation and textured snow surfaces indicate that wind slabs are present in your area.
Where wind slabs linger near treeline and overlap with the persistent slab potential, don't thread the needle between these avalanche problems and stick to slopes 30 degrees or lower. A triggered avalanche may step down into deeper weak layers to create deadly avalanches, so think seriously before testing larger slopes.
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: Likely
Expected Size: 1 - 1
Persistent Slabs
A weak facet/crust interface 2-3’ deep was triggered by Mt. Hood Meadows Pro Patrol on Sunday. The large persistent slab avalanche near Stevens Pass should give you pause before venturing onto large or unsupported open slopes, steep chutes, or large openings in the trees. While we think it is becoming more difficult to trigger a persistent slab avalanche by Monday, the consequences remain severe. These avalanches are difficult to predict and can break wide and in surprising ways by wrapping around terrain features. Be especially suspicious near and below treeline where weak snow over a firm crust was most likely buried intact and recent avalanches have occurred.
You are less likely to experience the obvious indicators of instability such as whumpfs, sudden collapses in the snowpack, or shooting cracks in the snow on Monday. But that doesn’t mean that you are safe.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 2