Avalanche Forecast
Issued: Mar 25th, 2018 12:14PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
The Avalanche Danger will rise to Considerable by Monday afternoon. Be prepared to evaluate conditions and dial back your terrain selection as the danger increases. Watch for snow totals over 6" and blowing snow as indicators that the danger is on the rise. You can stay safe by steering around steep slopes where you see more than 6" of snow that slides easily or recent snow drifts, cornices, and firm snow that produces cracking.
Summary
Detailed Forecast
The Avalanche Danger will increase to Considerable by Monday afternoon due to a robust round of new snow and wind. You can trigger Wind Slab avalanches at upper elevations on steep slopes near ridge tops or in cross-loaded terrain features. Watch for clues like variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you could trigger a Wind Slab. You can avoid triggering these avalanches by steering around steep roll-overs, unsupported features, and obvious start zones where you suspect Wind Slabs.
You may be able to trigger small Storm Slabs on steep slopes near and below treeline where storm totals exceed 6 inches. Watch for areas where the storm snow easily slides. Use quick tests and small test slopes to check for a poor bond in the new snow.
It has been more than two weeks since the last reported avalanche involving older weak layers. Deep Persistent Slabs are now very unlikely. The weak snowpack structure could be become a problem later in the spring.
Snowpack Discussion
The main concern for avalanches over the next couple days will lie with incoming snow and wind. Expect enough new snow by Monday afternoon to see dangerous conditions. Snow (and rain at low elevations) will continue into the week. This past weekend, almost 2 feet of snow fell in the Cascade West South region. Observers reported natural and triggered Wind Slab avalanches in the Crystal backcountry on Saturday. Storm instabilities were less reactive by Sunday.
Last weekends storm fell on crusts at low elevations and sunny slopes. Many slopes hold a series of thin crusts in the upper snowpack. You may find some buried softer layers on shaded slopes, but these have generally not been problematic. Below the top 2 feet (60cm) the snowpack is generally strong and lacking interfaces of concern.
An old, thin layer of sugary facets (2/13) can still be found in some locations just above a very firm and wide spread crust (2/8). The overall depth (1.5-2 meters) and significant rounding (melt-freeze cycles in some locations) has allowed this structure to drop off the official list of avalanche problems. NWAC professionals will continue to track this weak layer as it could be a problematic later in the spring.
Observations
South
On Saturday NWAC Forecaster Dallas Glass reported multiple natural and triggered Wind Slab avalanches on North, East, and Southwest aspects above 5,500 ft. The largest was almost 2 feet deep and involved all of the snow that fell since Wednesday. Dallas also reported sudden test results on the 2/13 facets over 6 feet below the snow surface.
On Wednesday NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry. Jeremy found a firm and supportable surface crust on S-facing slopes. On shaded slopes, he reported soft snow. The 2/8 crust was down about 4 feet and still reactive in snowpack tests.
Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 26th, 2018 12:14PM
