Avalanche Forecast

Clouds and mild temperatures Monday night will maintain shallow wet surface snow and limit a hard re-freeze. A weak front should spread light rain or wet snow over the area Tuesday but amounts should be light. The precipitation should come with cooling through the day. Mild temperatures overnight and early Tuesday should continue to allow for older wind slabs to settle and stabilize. 

Older wind slabs have been stabilizing but you are most likely to find and trigger lingering wind slabs on shaded slopes above treeline where wind deposited snow Thursday and Friday. Use visual clues such as fresh cornices and snow drifts to identify and avoid wind loaded slopes.

Persistent slabs continue to exist along the East slopes of the Cascades. The exact depth and type of persistent weak layer depends on location. These present a low likelihood-high consequence scenario that resulted in three fatalities last weekend (3/3-3/4). Just because you see tracks on a slope does not mean it is safe. To avoid persistent slabs, stay off of large open slopes where these larger avalanches may occur. While these avalanches may be difficult for a person to trigger, a small avalanche such as a loose wet or cornice fall may trigger these deeper layers. 

Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime heating and the sun.

Sunshine and temperatures in the 40's to 50's over the past few days has allowed for significant snowpack settlement and for a variety of surface conditions to develop. Wet surface snow in the daytime and firm melt-freeze crusts are forming overnight on all but true north facing slopes which are barely hanging on to settled old snow that fell Thursday.

Clear weather over the past few nights has allowed for surface hoar or near surface faceted snow to develop on many slopes. These surfaces are easily destroyed by wind or sun and warming, so may end up being widely variable when future snowfall occurs. 

Generally 6-14" of new snow fell along the east slopes of the Cascades Thursday through Thursday night, with lesser amounts further east. Strong winds transported snow forming wind slabs and other wind-affected snow surfaces. This resulted in several wind slab avalanches being reported from the Washington Pass area Friday and Saturday. 

Weak persistent layers exist within the snowpack. In the past two weeks three avalanche fatalities have occurred because of persistent slabs. The exact persistent weak layer and depth is dependent on location.

Three commonly seen persistent weak layers are:

• Surface hoar and facets buried on 3/8. This layer is generally found near and below treeline just below the recent strom snow.
• On slopes that received direct sun, a thin facet-sun crust can be found. This layer is generally 1-2 feet below the snow surface.
• A more wide spread persistent weak layer is found just above a firm crust layer buried on 2/5. This layer of weak sugary facets (2/13) can be found 2-4 feet below the snow surface.
• Other shallow and very weak snowpacks may be found further east of the crest or in areas with less snow.

Observations

North

On Monday, North Cascade Heli reported two very small wind slabs, one triggered and one natural on steep shaded slopes at 6000 and 7000 feet.

On Friday and Saturday, North Cascade Heli reported several slab avalanches near and above treeline. These avalanches occurred on a variety of aspects and were up to size D2.

On Thursday during the storm, NCMG generally found the 2/5 crust down 30-35" in the Hairpin valley with a 0.5 cm layer of 1.5 mm rounding facets. They did not see the Feb 23 facets, but found a layer of 5 mm surface hoar buried on March 8th down 3-12" due to variable wind transport. Wind slabs were reactive on N-E-S aspects. 

Mid-week, observers reported a mix of sun crusts and surface hoar/near surface facets prior to snow on 3/8.

Central

On Friday, Mission Ridge Pro Patrol reported 2" of new snow with extremely high winds forming slabs that were not reactive on test slopes or column tests. The recent 2" of snow sits on a layer of 3-4 mm surface hoar buried 3/8. The 2/5 crust was down 20" and was still reactive in snowpack tests.