Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - East Avalanche Forecast

Mar 25th, 2019 11:00AM

The alpine rating is low, the treeline rating is low, and the below treeline rating is moderate. Known problems include Wet Slabs.

Large wet slab avalanches are possible on shaded aspects below 5,000ft. The new snow may create some small wind slabs in isolated locations in the alpine, and small wet loose avalanches are possible on sunny slopes where there is more than a few inches of snow.  

Summary

Discussion

Snow and Avalanche Discussion:

The great thaw continues. Along with this thaw, 0.2" to 0.4" of rain fell up to about 5,000ft, with up to 4" of snow accumulating above that elevation. The rain has increased the amount of water making its way through the snowpack on all aspects, and rivers have been on the rise. On Monday, a very large wet slab avalanche was observed off the second gully on Wedge Mountain. This likely ran on the early February facets near the ground. A recent glide release was observed in Squilchuck State Park on Sunday that pulled out a wet slab below it. Other glide cracks were opening up as well. These are not problems to mess with. The term "Scary Moderate" has been used to describe the danger in similar situations. 

In addition to wet slabs, a few inches of new snow with respectable winds may create small wind slabs at exposed locations at upper elevations. Small loose wet slides may become possible by the afternoon on steep, sunny slopes with more than a few inches of new snow. Keep normal springtime precautions in mind. Creeks are beginning to open up, and glide slabs may occur in isolated locations. 

Large, dirty, wet slab debris from the second gully on Wedge Mountain near Leavenworth. A number of logs were taken with the churning debris flow. Northeast at 3,900ft. 

Snowpack Discussion

March 22nd, 2019

Enter Spring

If you’ve been in the snow recently, the wintery conditions of early March may seem worlds away. You may be in for a surprise if it’s been a while since you were in the mountains. The weather has taken a turn towards spring in the last couple weeks and the Cascade snowpack the has undergone major changes. Unseasonably warm temperatures and strong sun followed a month-and-a-half of cold, winter storms. Mid-elevation weather stations stayed above freezing from March 15th-22nd with high temperatures reaching the upper 50’s to low 60’s. For an in-depth survey of the regional snowpack, we’ll divide the terrain up by aspect and elevation.

A graph showing temperatures between 4,000-5,000ft around the Cascades from the 16th-21st.

Aspects

Northerlies

Along with the warm temperatures, the spring sun has played a major role in warming snow surfaces. The result is a snowpack that varies by aspect. In most regions, shaded and northerly slopes remain relatively unchanged. Aside from some settlement and firmer or moist surfaces, the snow on north aspects is almost entirely dry. Even some low elevation north slopes are still holding snow.

Sunny slopes

The snowpack on east through south through west aspects is a different story. The strong March sun melted snow surfaces and drove melt-water into the snowpack. This is most dramatic on steep (over 35 degrees) southeast through southwest slopes below 5,000ft.  In some areas, you can find meltwater up to 3 feet below the snow surface with drainage channels well established. Between this warm period and rain events in the first half of the winter, the entire snowpack has transformed to melt forms. An important point to note is that as of the 22nd, these solar aspects remain unfrozen and weak. Cooler weather ahead may help strengthen moist to wet layers.

A glide avalanche (D2) released from a rock slab late on the 20th. Lichtenberg Mtn, 5,100ft, SE aspect. Other glide avalanches occurred on the 20th at Snoqualmie Pass and in Tumwater Canyon. Photo: Josh Hirshberg

Elevation

Low elevations

As you travel from low valleys to higher peaks, you’ll notice a major difference in the snowpack based on elevation. With all the low-elevation snow this winter, there are still some cold, shaded slopes holding pockets of snow down to 1,000ft, especially east of the Cascade Crest. However, most slopes below 3,000ft have lost much of their snow cover. Many low elevation, sun-exposed slopes are bare, especially in areas that previously held less than 3 feet of snow. The low elevation snowpack is no longer substantial enough to allow for easy travel over snow or widespread avalanches.

Loose wet avalanches on the south side of Table Mtn, near Mt Baker. 3/17. Photo: Pete Durr

Mid-elevations

At mid-elevations, around 3,000-5,000ft, the snowpack is still deep and layered. Many slopes at this elevation band near and west of the Cascade Crest are holding 6-10 feet of snow. This is also where you’ll find the most dramatic variation in the snowpack based on aspect.

High-elevations

Above 5,000ft you’ll encounter a snowpack similar to what you may have found around the 1st of March. Upper elevations have stayed mostly dry. The most sun-exposed slopes have surface crusts but have not seen much water or change to melt forms below the surface.

Problems

Wet Slabs

An icon showing Wet Slabs

Likelihood

possible

Expected Size

1 - 2

The extended thaw for elevations below 5,000ft has allowed water to slowly infiltrate deeper layers of the snowpack. This happened faster on east, south, and west aspects last week with strong sun. On shaded aspects, the snowpack had largely been quiet, until the recent rains. Here, the snowpack still has very weak facets underneath a crust from early February. The recent occurrence of a large wet slabs that likely ran on the early February facets near the ground is concerning for slopes of similar aspects and elevations, as it tells us water is making its way down to these layers and weakening already weak grains. The temperatures are cooling off into the daytime tomorrow, but not enough to combat the potential deep instabilities associated with this problem.

The best method for dealing with these is avoidance of potential problem slopes until the snowpack freezes solid, or melts away. Places I would avoid are steep northwest to northeast aspects from 3,000ft to 5,000ft in elevation, along with gullies and trails that may be in the potential runout zone. Wet slabs are difficult to predict, but when they do happen, can be large and destructive. 

Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.

 

Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty

 

A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.

Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.

Elevations:

Below Treeline.

Valid until: Mar 26th, 2019 11:00AM