Avalanche Forecast

Issued: Apr 16th, 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

Surface snow could get a better start on being available for loose wet snow avalanches on Sunday. Continue to avoid cornices and areas below where glide avalanches could be a problem.

Summary

Detailed Forecast

The upper ridge and warm air mass should further build over the US west coast on Sunday. This should cause light winds and mostly sunny weather in the Olympics and Cascades with further warming temperatures.

Sun and heat effects should be the main factors to consider on Sunday. From a couple of observations it seems like surface snow was probably pretty solidly refrozen in most areas on Saturday morning. But the refreeze may more limited Sunday morning due to warmer weather. If so then surface snow could get a better start on being available for loose wet snow avalanches.

Loose wet avalanches should remain possible Sunday on steeper slopes involving the recent snow. Watch for wet surface snow getting deeper than your boot tops. Triggered loose wet avalanches should be most likely on steep solar slopes but could be seen on any steep aspect in this warm weather. Avoid areas below steep gullies and run out zones that might be prone to natural loose wet avalanches.

The potential for cornice releases may increase through the warm weekend. Cornice releases are basically unpredictable 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, powerful glide avalanches can release unexpectedly so also avoid areas below steep unsupported slopes or rock faces especially if they are showing glide cracks. Glide cracks may be hidden by the recent snowfall.

Storm and winds slabs will not be listed as avalanche problems along the west slopes due to reports of good bonds of the recent snow to the 4/12 crust and fast stabilizing rates seen at this time of year. However, be aware of the potential for isolated slab avalanche layers involving the recent snow mainly on non-solar aspects of very high terrain.

 

 

Snowpack Discussion

Weather and Snowpack

We've had two big warm-ups and spring shed cycles since the end of March. Over this time period the snowpack has undergone significant settlement with lower elevation snow beginning to fade away and with an overall transition to a spring snowpack.

A front crossed the Northwest Tuesday afternoon and night. Then a cool upper and surface low pressure system moved east along the Washington-Oregon border on Wednesday and Thursday. NWAC sites along the west slopes above the pass levels had 4-11 inches of late season snow in the 3 days ending Friday morning.

Storms in March continued to build unusually large cornices along many ridges. The potential for low probability/high consequence encounters, such as cornice failures and glide avalanches from steep unsupported slopes and smooth rock faces, should continue for awhile this spring. 

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

An observation from the NWAC observation page Wednesday 4/13 from the Mt. Baker area showed a natural loose wet avalanche involving the new storm snow and running well.

DOT professionals in the Chinook Pass area reported small natural avalanches on Wednesday 4/13 involving the new storm snow. Below 5000 feet on solar aspects, the snowpack had not refrozen and loose wet avalanches could easily gouge down to deeper wet grains on steeper slopes.

NWAC observer Dallas Glass was out on Thursday 4/14. Dallas was in the Skyline area of Stevens Pass area and found 10-15 cm of new snow well bonded to the 4/12 crust.  Dallas was able to ski trigger small loose wet avalanches on steep slopes on all aspects.

NWAC observer Lee Lazzara was also out on Thursday 4/14 in the Bagley and Anderson Creek drainages in the Mt. Baker area and found 10-15 cm of new snow on the 4/12 crust. He found 10 cm of wind slab over 20 cm of lower density snow on lee N-E aspects near and above tree line but these layers were not reactive. Several loose wet small to large loose wet avalanches from 4/13 that packed a punch were seen on all steep aspects.

Although Alpental was closed Friday 4/15 but a couple of folks from the pro-patrol were on the hill and reported 4-5 inches of storm snow on the upper mountain. Small natural loose wet avalanches were seen on most steep slopes in misty weather with low visibility.

Alpental was open on Saturday 4/16 and the pro-patrol reported shallow softening surface snow and no significant avalanches. A natural cornice release Friday caused an 8 foot deep area of snow to release from a rock slab below.

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

Unlikely

Expected Size

1 - 2

Valid until: Apr 17th, 2016 11:00AM