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Avalanche Forecast

Archived

Mar 2nd, 2017–Mar 3rd, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Dangerous avalanche conditions are expected to develop Friday afternoon. Dial your plans back if avalanche conditions deteriorate earlier in the day than forecast. 

Detailed Forecast

Several waves of warm frontal moisture will impact the east slopes of the Cascades Thursday night through Friday afternoon. A strong cold front will sweep through the area late in the afternoon. A warming trend should peak in the afternoon along the east slopes of the Cascades. Rain and snow will be the heaviest in the northeast Cascades with lighter amounts seen further south until the front comes through Friday evening. 

Dangerous avalanche conditions are expected to develop Friday afternoon. Dial your plans back if avalanche conditions deteriorate earlier in the day than forecast. 

Wind slab should build on lee easterly aspects near and above treeline, but expect cross-loading and wind slab formation into the upper portion of the below treeline band due to the strength and duration of the winds.  

Shallow storm slab should build and become more reactive late in the afternoon due to a warming trend and increasing precipitation.  

Generally shallow loose wet avalanches are possible below treeline with a switch to rain at lower elevations on Friday. 

Snowpack Discussion

Weather and Snowpack

The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost strong rain crust in our snowpack in the Central East and Southeast Cascades while only a thin freezing rain crust was observed in the Washington Pass area. 

A series of disturbances in cool NW flow aloft from Saturday through Tuesday caused periods of moderate to strong SW-W alpine winds and have deposited significant snow. Very strong westerly winds decreased on Wednesday following the last front. The 4 day storm snow amounts over this period ranged from 2-16 inches along the Cascade east slopes.

High clouds from a warm frontal system passed over the east slopes of the Cascades on Friday but only very light snowfall was seen near the Cascades crest in the central-east and northeast Cascades. 

Recent Observations

North

A report from NWAC observer Jeff Ward on Tuesday for the Varden and Silverstar areas. He reported windy conditions with some ski tests producing small wind slab releases. 40 cm of storm snow was found on the Valentine's Day crust on non-wind affected slopes while some windward slopes were scoured to the crust.

A party of four skiing at Washington Pass near the highway hairpin on the east side of the pass were hit by a natural cornice released avalanche on Tuesday afternoon. Four people were caught and carried up to 1000 ft downslope. Fortunately there were no fatalities and no full burials but there were apparently some injuries. The release occurred on a very steep slope at the top of a northeast facing bowl at about 7500 ft.

Windy conditions prevented the North Cascades Heli Guides from flying on Wednesday.

Another recent and large cornice failure was observed by the NCH on Thursday in the Cutthroat drainage.   

Central

No recent observations.  

South

No recent observations. 

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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 drifts 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.