Avalanche Forecast
Issued: Mar 15th, 2017 12:12PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Wet Slabs and Glide Cracks.
NWAC,
Northwest Avalanche Center
Cooling by Thursday will help to lower the wet snow avalanche problem. Glide and wet slab avalanches will be unlikely, but are difficult to predict and remain a threat while water in the snowpack from recent heavy rains drains. Avoid travel on or below steep, unsupported terrain. Shallow fresh wind slabs are possible near ridges in higher terrain.
Summary
Detailed Forecast
Gradual cooling is underway as of Wednesday afternoon as a cooler trough approaches. Light to moderate showers at further lowering snow levels are expected Wednesday night and early Thursday with moderate crest level winds.
Cool air should be over the area by Thursday with only scattered light showers expected as high pressure rebuilds over the area.
This weather will allow the previously wet snow pack to begin re-freezing and strengthening, causing an overall decreasing trend in the avalanche danger.
We have already experienced a region-wide avalanche cycle, such that the overall likelihood of wet snow avalanches has diminished. However, destructive wet slab avalanches are still being reported in isolated areas in the Cascades as of Wednesday. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.
The cooling trend through Thursday will lower the overall danger as surface snow begins to form a crust and liquid water continues to drain.
With that said, there remains a possibility of further wet slab avalanches, especially on unsupported slopes with a rock face bed surface. These types of avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Besides their huge destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected. Watch for evidence of large cracks forming in the snow due to glide, especially on terrain with rocky bed surfaces.
Cornices have been weakened and prone to failure due to rain and mild temperatures. Very large cornices have been reported in many areas and have been involved in recent accidents and close calls. A cornice failure could trigger a large and destructive wet slab avalanche.
If you manage to venture into the above treeline zone Thursday, expect new or previous wind slabs mainly on NW-SE aspects due to recent SW-W winds. Continue to watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
Snowpack Discussion
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.
A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday 3/9. On Thursday night rain pushed up to about 5000 feet in the northwest Cascades and about 6500 feet in the southwest Cascades. This caused a region-wide avalanche cycle with several large to very large wet avalanches impacting mountain roadways.
Moist W-SW flow and a stalled frontal boundary have produced periods of moderate to occasionally heavy rainfall at most locations throughout the west slopes of the Cascades on Monday and Wednesday. Three day precipitation amounts along the west slopes of the WA Cascades ranged from 3 to over 7 inches of water, most all in the form of rain.
Cooling by late Wednesday has begun to add shallow new snow to higher terrain.
Recent Observations
North
The Mt Baker patrol reported natural avalanches last Thursday night. A very large avalanche released during avalanche control work on Shuksan Arm Friday, 3/10 producing a 8 ft x 75-100 yd crown.
Shuksan Arm avalanche crown from Friday, 3/10. Photo Mt Baker Ski Area.
NWAC observer Lee Lazzara was out near the Mt Baker ski area on Sunday 3/12 and reported 8-10 inches of homogeneous moist snow well bonded to the 3/10 crust over deep dense snow. Cornices were huge with ski triggered, loose wet avalanches possible.
On Tuesday, the Mt. Baker pro - patrol reported hearing a very large natural avalanche high on Mt. Shuksan as well as observing fresh debris in Rumble Gully from wet slab avalanches releasing off Shuksan Arm. Due to poor visibility, start zones could not be identified other than placing them in the above treeline zone.
Central
The Stevens Patrol and Stevens and Snoqualmie WSDOT crews reported many wet slab and loose wet avalanches from control work and natural cycles Wednesday 3/8 to Friday 3/10. Several large slab avalanches ran on the south side of Granite Mountain near Snoqualmie Pass during this period.
NWAC observer Dallas Glass was in the Alpental Valley Sunday 3/12 through Tuesday 3/14 and reported tough travel conditions and no natural avalanche activity in the terrain he observed near and below treeline despite the persistent rain.
Stevens Pass Pro Patrol reported 4 natural D3 (very large) wet snow avalanches on the Tye Mill side of Cowboy Ridge that occurred between Monday night and mid-day Tuesday. Two of the avalanches occurred mid-day Tuesday and were classified as wet slabs with 3' crowns and 15' debris piles. A shallow, loose wet cycle was also noted in the Susan Jane area outside the ski area early Tuesday morning.
NWAC observer Ian Nicholson, went for a walk in the rain along the Alpental Valley Wednesday to make observations. Of note, the upper snowpack was draining liquid water very well and was reluctant to produce loose wet avalanches. There was evidence of several recent large wet slab and glide avalanches however, likely releasing over the past 1-2 days. Ian saw over a dozen open glide cracks on rock faces and when encountered, should be given a wide margin of safety as failure is difficult to predict.
South
Dallas was in the Crystal backcountry on Friday, 3/10 and reported that a wet slab cycle had occurred on Thursday night with several releases on the ridge running from 3 Way to Crown Point on W-NW-N slopes in the 5800-6000 range. Natural loose wet avalanches also ran from steep and rocky terrain.
The NPS rangers at Paradise reported that a large natural wet slab or a loose wet avalanche had covered the road to Paradise on Friday 3/10. This occurred on a NW aspect at about 4500 ft.
Just above the Nisqually Bridge to Paradise on Friday morning 3/10. Photo Doug Jones.
The Crystal Mountain Pro Patrol reported an active wet slab cycle Wednesday. Numerous fresh loose wet and wet slab avalanches had released with the past 24 hours. Wet slabs remain one of the most difficult of all avalanche problems to predict with certainty. Wet slab releases can occur well after cooling takes place.
Problems
Wind Slabs
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Glide Cracks
A release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
Predicting the release of Glide avalanches is very challenging. Because Glide avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide avalanches.
This Glide avalanche broke to the ground on a smooth, grassy slope. From all the mud on the bed surface, water pooling at the base of the snowpack likely caused the failure.
Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 16th, 2017 12:12PM
