Avalanche Forecast
Regions: Cascades - South West.
A change in the weather to cooler and cloudier conditions has dramatically lowered the wet snow hazard. Continue to avoid very steep slopes that have wet snow deeper than your ankle where you can trigger a small but powerful loose wet avalanche.
Discussion
Snow and Avalanche Discussion
After a week of widespread above freezing temperatures and a wet snow avalanche cycle that peaked last Sunday-Tuesday, we have turned the corner toward more typical cooler/showery spring-time weather.
Professionals traveling in the Chinook Pass and White Pass backcountry Thursday and Friday respectively, reported colder snow layers still discernible even on south aspects with a deep snowpack below the top 1 ft (30cm). Also, drier colder snow could still be found on north aspects at higher elevations. No new loose wet snow avalanches were observed in either area.
New snowfall amounts Friday night are forecast to be light (1-3") and temperatures above treeline should cool and remain below freezing through Saturday afternoon. This should help limit the loose wet potential to very steep solar (E-S-W) aspects near and below treeline.
If more new snow accumulates than forecast Friday night, anticipate a greater loose wet hazard and plan to avoid all steep sunny slopes in the afternoon.
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.
Examples of the dramatic difference in snow cover on shaded slopes versus sun-exposed slopes. Left: south-southwest aspect. Right: northwest aspect covered in about 4 feet of snow. Blewett Pass, 4,600ft, 3/22. Photo: Josh Hirshberg
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. Â 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.
Avalanche Problems
Loose Wet
Cooler weather and a mix of sun and clouds will limit the loose wet hazard on Saturday to steep sunny slopes at lower and mid elevations. However, you may still trigger a small loose wet avalanche on steep sunny slopes near and below treeline. Be suspicious of slopes that don’t see a firm refreeze Friday night.
Monitor the surface snow and change your aspect and move to lower angled terrain if find wet surface snow deeper than 6”. Be especially cognizant of the consequences of even a small avalanche around terrain traps like gullies or cliff bands.
We haven’t had any reports of wet slab avalanches in several days. The lack of observed slab avalanches plus the cooling has allowed us to remove Wet Slab from the problem list. If you see evidence of new wet slab avalanches, dial back your terrain use immediately to deal with the uncertainty surrounding this dangerous and hard to predict avalanche problem.
Despite the cooler weather, avoid traveling on cornices as they often break back further than you anticipate. Limit your exposure if traveling on slopes below large overhanging cornices.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1