Avalanche Forecast
Issued: Apr 20th, 2017 2:25PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Cornices.
NWAC,
Northwest Avalanche Center
Don't let the sunshine blind you to spring avalanche hazards on Friday! The loose wet potential will ramp up quickly as temperatures warm along with increasing sunshine. A more winter-like snowpack should be found above treeline where lingering wind slabs may still be sensitive. Cornices are still large so give them a wide safety margin.
Summary
Detailed Forecast
After a cool start, strong late April sunshine and rising freezing levels will quickly increase the loose wet hazard on all solar aspects. If the alpine winds out of the E-SE and high clouds increase in the afternoon as forecast, the greatest loose wet hazard above treeline will likely peak late morning through early afternoon. Fresh snow near and above tree-line will quickly activate with a natural shed cycle likely on steeper solar slopes. Loose wet avalanches at lower elevation may be less likely to initiate naturally, but if skier triggered, could entrain older snow layers and become large in specific areas. Keep this in mind around terrain traps.
Winds have transported recent snowfall over the past week onto lee slopes, with the most snowfall and transport occurring above treeline. Be suspicious of steeper wind loaded slopes below ridges. Expect the wind slab hazard to increase quickly with elevation. Wind slabs may be the largest and most sensitive in the Mt Baker area above treeline.
Cornices are still large so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.
Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches can release at any time, not just during the heat of the day, and are by definition difficult to predict and manage.
Snowpack Discussion
The active weather pattern continues into late April for the Pacific Northwest despite what the calendar says. Over the last few days, Mt. Baker has picked up about 3 inches of water equivalent (WE) while other areas along the west slopes have picked up 1 - 1.5 inches of WE. Much or all of this has fallen as snow above 5000 feet in the north and 5500-6000 feet in the central and south Washington Cascades. Natural loose wet avalanches have likely occurred throughout the week as the snow-line has oscillated or on solar aspects at lower elevations during sunbreaks.
Recent observations
The Chinook Pass DOT on Tuesday reported loose wet avalanches entraining older snow below 5000' on steeper slopes. Wednesday was cooler and cloudier with less avalanche activity.
NWAC pro-observer Lee Lazzara was in the Mt. Baker backcountry on Wednesday. Lee found recent wind slab on lee slopes near and just above 5000' up to 30 cm thick but unreactive in ski or snowpack tests. Lee observed a large recent glide avalanche on Mt. Herman (see Lee's Instagram post) as well as evidence of a recent loose wet cycle from earlier in the week.
Problems
Loose Wet
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine.
Likelihood
Expected Size
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 lips 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.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 21st, 2017 2:25PM
