We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches have diminished. However their destructive potential remains high. 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.
Summary
Detailed Forecast
The relatively high snow levels will continue with rain forecast below 6000-7000 feet through Tuesday morning. We should see a relative break in the steady precipitation Tuesday afternoon before another wave of moisture returns Tuesday night. Periods of moderate W-SW winds above treeline will continue to transport new and recent snowfall only for the upper portion of the above treeline band Monday night through Tuesday.
We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches have diminished. However their destructive potential remains high. 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.
Loose wet avalanches are likely on steeper slopes with the additional rainfall. While most loose wet avalanches will be shallow, a few may gouge down to deeper layers and become more powerful than you expect.
Cornices will be 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.
Wet slab avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Glide cracks can indicate areas where wet slab avalanches are more likely to occur. Besides their destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected.
If you manage to venture into the above treeline zone Tuesday, expect new or previous wind slab 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 Cascade west slopes 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. On Thursday night rain pushed up to about 5000 feet in the northwest Cascades and about 6500 feet in the southwest Cascades. By Friday morning NWAC stations along the Cascade west slopes had about 1-2 inches of WE (water equivalent) but only 0-7 inches of snow (most snow at Stevens and Mt Baker). This load gave the snowpack a huge test and caused an avalanche cycle along the Cascade west slopes.
Another front crossed the Cascades on Saturday causing SW-W winds, snow at higher elevations and rain at lower elevations. NWAC stations along the Cascade west slopes had about 1-7 inches of new snow on Sunday morning with the most at Mt Baker, the top of Alpental and at Paradise.
A mild frontal system slowly sagged south across the Cascades on Monday with moderate to locally heavy rainfall. Rainfall totals through 5 PM PST (or 6 PM PDT)
Recent Observations
North
The Mt Baker patrol reported natural avalanches on Thursday night. A very large avalanche released during avalanche control work on Shuksan Arm Friday producing a 8 ft x 75-100 yd crown.
Shuksan Arm avalanche crown from Friday 3/10. Photo Mt Baker Ski Area.
Shuksan Arm debris on Friday 3/10. Photo by Sam Llobet, Mt. Baker Ski Area.
NWAC observer Lee Lazzara was out near the Mt Baker ski area on Sunday and reported 20-25 cm 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.
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 to Friday. Several large slab avalanches ran on the south side of Granite Mountain near Snoqualmie during this period.
The Alpental patrol reported a widespread natural cycle occurred Thursday night.
NWAC observer Tom Curtis made a trip to Rainy Pass just east of Stevens Pass from the Lake Wenatchee side on Saturday. He noted wind effects on nearby ridges and 1-3 days worth of old loose wet and wet slab releases. The upper snowpack consisted of a surface crust over right side up snow and no alarming results in snowpack tests in the area he traveled.
NWAC observer Jeremy Allyn was in the Alpental Valley from 3000-4000 ft on Saturday and reported evidence of the avalanche cycle on Thursday and Friday. A 2 inch thick surface crust was stabilizing underlying moist snow. Travel conditions were extremely difficult.
NWAC observer Dallas Glass was in the Alpental Valley on Sunday and Monday and reported tough travel conditions (boot sucking goo) along with some rollerballs on steeper slopes.
South
Dallas was in the Crystal backcountry on Friday 3/10 and reported a wet slab cycle occurred there 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 Paradise NPS rangers reported a large natural wet slab or loose wet avalanche covered the road Friday morning just up the road from the Nisqually Bridge. This occurred on a NW aspect at about 4500 ft.
Just above the Nisqually Bridge to Paradise on Friday morning. Photo Doug Jones.
Problems
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, 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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 14th, 2017 12:49PM
