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Avalanche Forecast

Archived

Mar 4th, 2017–Mar 5th, 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 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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The avalanche danger should decrease on Saturday but the decrease will be a moving target and careful snowpack evaluation and cautious routefinding will be essential.

Detailed Forecast

Decreasing winds and mostly light snow showers should be seen over the Cascade east slopes on Saturday with much lower snow levels and much cooler temperatures. Not a lot of snow is expected following the front Friday but we will see how it goes. The avalanche danger should decrease on Saturday but the decrease will be a moving target and careful snowpack evaluation and cautious routefinding will be essential.

Layers of recent or new wind and storm slab may need a day or more to mostly stabilize. Water may also need a day to drain from the upper snow pack below about 3-4000 feet.

Recent moderate to strong southwest to west winds will make wind slab most likely on northwest to southeast slopes but keep an eye out on all aspects in areas of more complex terrain. Watch for firmer wind transported snow from Friday and possible deeper or covered wind transported layers such as from Tuesday.

New storm slab formed on Friday may need a day to mostly stabilize on Saturday. Storm slab is most likely in area that experienced more than a few hours of rapidly accumulating snow.

Avoid areas on ridges and mountain tops were a cornice might be present and avoid slopes below cornices.

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 2/25 through Tuesday 2/28 deposited 2-16 inches snow along the east slopes of the Cascades. Very strong alpine west winds were seen in most of the Olympics and Cascades on Tuesday.

Southwest flow aloft began to ramp up again on Thursday as the first in a new series of fronts crossed the Northwest. Strong southwest flow is carrying a second stronger front across the Northwest on Friday evening. Along the Cascade east slopes this will be causing strong southwest alpine winds and heavier moist or denser new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft.

Recent Observations

North

A report from the NC Heli Guides indicates a cycle of natural and ski triggered wind slab avalanches during the wind event on Tuesday and Wednesday 2/28-3/1. About 50-80 cm of storm snow was found with a good bond to the the Valentine's Day crust. About 20-30 cm of recent snow was covering about 10 cm of wind slab in some areas.

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 6600 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. Plenty of other unreleased large cornices were noted.

A NC Mountain Guides report for Friday for the Delancey area indicates heavy snow in the near and below treeline and 35-60 cm of storm snow on the Valentine's Day crust. Moderate planar shears were found in recent storm snow layers. New storm slab was forming and ski tests were giving storm slab and loose dry releases. Whoomping and cracking was seen above 6600 ft.

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.