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Avalanche Forecast

Archived

Mar 1st, 2017–Mar 2nd, 2017

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

Regions

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Careful snowpack evaluation and cautious route finding will be essential for travel in back country areas along the the Cascade east slopes on Thursday.

Detailed Forecast

The next front will begin to move over the Northwest Thursday afternoon and night. The east slopes should be spared significant new snow until Thursday evening but increasing SW-W alpine winds should generally be seen in all areas on Thursday.

Existing wind slab should linger and possibly further build on NW-SE aspects along the Cascade east slopes on Thursday.  Remember to watch for wind transported snow on other aspects as well in all areas.

The cornice accident at Washington Pass on Tuesday is a reminder to avoid ridges or mountain tops where there may be a cornice and avoid slopes below cornices.

Remember to change your plans if the weather deteriorates sooner than expected in any area.

Loose wet avalanches also won't be listed as an avalanche problem but it is March so watch for roller balls and loose wet snow deeper than a few inches in you are on solar slopes in sun breaks.

Further increasing alpine winds and snow with a further increasing avalanche danger should be seen in all areas Thursday night and 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 slightly unstable weather pattern last week caused a mix of sun and light snow showers with sun crusts forming on solar aspects.

A period of increasing W winds Saturday afternoon was noted at the Mission Ridge and Dirty Face Peak NWAC weather stations and by guides 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 W winds decreased on Wednesday following the last front. The 4 day storm snow amounts range from 2-16 inches along the Cascade east slopes.

See the information near the top of the Avalanche Forecast tab regarding the cornice accident at Washington Pass on Tuesday.

Recent Observations

North

A pretty extensive report came 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 HST was found on the Valentine's Day crust on some slopes, but some windward slopes were scoured and the crust was not found on some other slopes.

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

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.

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.