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. The NWAC site at Washington Pass and NRCS sites at Harts Pass and Lyman Lake indicated 4-10 inches of late season snow in the 24 hours ending Friday morning. NWAC sites further south along the east slopes above the pass levels had 4-5 inches the previous couple days but little if any in the 24 hours 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. 

Recent Observations

A report was received via the NWAC observations page of a large avalanche running across closed SR 20 which was found on April 10th. This large avalanche may have been triggered by a natural cornice fall during the end of the warm spell and may have stepped down to older weak layers or interfaces from late winter.

Large avalanche across State Route 20 just east of Washington Pass. Photo taken by the DOT 4/11.

DOT avalanche professionals working at Washington Pass area during the week reported a quiet week regarding natural avalanche activity. Colder snow remained intact on non solar aspects around 6000 feet. Glide cracks and moats near rocks were reported as large for this time of year, likely due to the unusual warmth seen earlier this month.

A report on Turns All Year for Earl Peak in the Teanaway on Friday 4/15 indicated 3-5 inches of wet surface snow over firm snow.

NWAC observer Lee Lazzara began a trip near Washington Pass on Friday 4/16 and found 5-10 cm of recent snow over strong firm crusts and layers of rounded crystals and no recent avalanches. Some roller balls were seen along with some old debris from previous wet snow avalanches.

 

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