Avalanche Forecast

Issued: Apr 9th, 2016 11:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Loose Wet and Cornices.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger should lower slightly on Sunday due to a slight cooling trend, but still remain Moderate. Larger avalanches caused by glide releases or cornice fall are possible in isolated areas. Get off steep slopes early in the day and be aware of the overhead hazard to minimize your exposure. 

Summary

Detailed Forecast

A shift to onshore flow will maintain fair and mild conditions Sunday, but slightly cooler than the previous several days. Winds should remain light Sunday with some night and morning low level clouds possible in the valleys.  Freezing levels and temperatures should remain near 10,000 feet Sunday. Stay aware of the overhead hazard to minimize your exposure. 

Surface crusts should form overnight through the early morning hours followed by gradual melting and weakening. Pay attention to the integrity of surface crusts formed overnight. Loose wet avalanches will be possible on any steeper slope due to warming Sunday. Backcountry travel early in the day is encouraged since conditions can change rapidly due to daytime warming. Watch for wet snow deeper than your boot tops, especially on steep solar slopes during the late morning to afternoon hours.

The potential for cornice releases will continue on Sunday. Cornice releases can be unpredictable during the spring so avoid areas below cornices and remember that cornices can break much further back than expected along ridges.

Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.

Avalanche activity on Sunday should be less extensive as during the very warm weather last week which more closely followed a period of heavier snowfall and caused consolidation and some stabilizing. Continue to evaluate avalanche conditions on specific terrain features and change your plans if snow conditions are more dangerous than expected.

Snowpack Discussion

Weather and Snowpack

A large upper ridge dominated the weather last week through early Sunday, April 3rd, resulting in very warm and dry weather. A spring avalanche cycle occurred in most areas over this stretch, likely peaking a week ago, Thursday and Friday. After a stormy March, the snowpack has undergone significant settlement and multiple melt-freeze cycles over the last two weeks.

The last precipitation occurred early Monday morning with showers Tuesday, depositing storm snow from 3-12 inches, now indistinguishable following several melt freeze cycles during the very warm weather. 

Another upper ridge is currently over the Northwest, causing sunny weather and warm temperatures. High temperatures at many NWAC stations throughout the Cascades pushed into 50s, 60s on Thursday with even a few 70s recorded Friday afternoon!  

Frequent March storms have built unusually large cornices along many ridges.

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Numerous professional and recreational observations focused on the spring avalanche cycle that occurred last week throughout the west slopes of the Cascades.  

NWAC observer Lee Lazzara was in the Mt Baker backcountry Tuesday 4/5 and reported about 4 inches of storm snow, well bonded to the moist old melt-freeze layer. There were some specific features that had up to a foot of wind loaded snow, but good bonding and settlement kept that snow from moving. He was out again in the Swamp Creek drainage on Wednesday 4/6 and found similar conditions. Debris from a large heat related avalanche was seen from a steep, complex northwest facing slope. The main take away currently in this area is to continue to be aware of the potential for low probability/high consequence encounters, such as cornice failures and glide avalanches from steep unsupported slopes and smooth rock faces.

On Thursday 4/7, WSDOT avalanche professionals in the Chinook Pass area reported a natural loose wet cycle occurring in the early afternoon on solar aspects. Most loose wet avalanches were small and not entraining much surface snow but were widespread.  

Crystal mountain pro-patrol on Friday, 4/8 saw little in the way of loose wet activity, but reported isolated cornice failures triggering wet slab avalanches of about 2-3 ft deep and 50' - 75' wide.   

Dallas Glass was in the Stevens Pass backcountry Friday, 4/8 and found a supportable melt-freeze crust breaking down by late morning on the east through south aspects. Most natural loose wet activity was small and confined to steep slopes below rock bands. Debris from one larger loose wet avalanche was observed on an east aspect around 5500' that likely occurred Thursday afternoon. 

Problems

Loose Wet

An icon showing 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

Possible

Expected Size

1 - 1

Cornices

An icon showing 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, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Valid until: Apr 10th, 2016 11:00AM