Avalanche Forecast

Issued: Apr 14th, 2018 11:18AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs, Storm Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Reactive and large, hard Wind Slabs remain widespread near and above treeline on Sunday. They will propagate farther than you expect, so give them a wide berth. Additionally, avoid exposure to large avalanche paths where a slide from the the upper slopes of Mt. Hood would be deadly. You can also trigger a Storm Slab near and above treeline or a Loose Wet avalanche at lower elevations as rainfall returns. Avoid open slopes greater than 35 degrees, particularly near and above treeline.

Summary

Detailed Forecast

Very large avalanches may still release from the upper elevations of Mt. Hood. Avoid large avalanche paths were avalanches descend from the upper elevations of Mt. Hood. Check a map to ensure flat slopes are not connected to larger terrain before crossing open areas.

Dangerous avalanche conditions exist near and above treeline on Saturday. A relative break in the intense weather on Saturday has not allow sufficient time for a variety of avalanche problems to heal from stormy conditions earlier this week. Light to moderate snow is returning Saturday night and Sunday.

Large and rreactive, hard Wind Slabs formed Wednesday afternoon through Friday during periods of heavy precipitation, strong winds, and warming temperatures. These slabs will continue to be reactive and may propagate widely near and above treeline on Thursday. Give wind-loaded aspects steeper than 35 degrees a wide berth. You are most likely to trigger a Wind Slab on NW through SE aspects below recently developed cornices, on convex roll overs, or on cross-loaded features. As new snow arrives, expect these slabs to be disguised by non-wind-affected snow.

You can trigger a Storm Slab near and above treeline on Sunday. These formed during periods of heavy precipitation Wednesday thorough Friday. A warming trend on Friday created unstable snow as denser snow sits on top of lower density layers within the 20-24" of recent snow that sits on a crust. You are most likely to trigger storm slabs on unsupported slopes, near convex rollovers, and on slopes steeper than 35 degrees.

At lower elevations, rain will return on Sunday, renewing wet surface snow conditions. If you see new rollerballs, fan shaped avalanche debris, or experience periods of intense rain, stay off of slopes greater than 35 degrees. Loose wet avalanches can entrain additional snow allowing them to grow large. Use caution if you travel in areas where loose wet avalanches may carry you into or off of terrain with higher consequences.

Watch for other spring hazards such as recent cornices along ridges, open glide cracks and opening creeks.

Snowpack Discussion

Friday night and Saturday saw a break in the recent active weather, following 20-24 inches of new snow fell in the Mt Hood area stations Wednesday afternoon through Friday afternoon. Moderate to strong south to west winds during the Wednesday through Friday time-frame easily transported snow, forming wind slabs near and above treeline on a variety of aspects. Many of these slabs were confirmed to be large, hard slabs that will propagate when triggered. On the upper mountain at Mt. Hood, Wind Slabs are likely to be very large.

The heavy snowfall rates on Friday also created storm slabs with heavier, denser snow on top of lower density layers. However, a crust has formed Friday night up to 6,000 ft due to the high moisture content in Friday's snowfall. The crust likely reduced storm slab activity on Saturday while moderate temperatures on Saturday have helped the storm slab to heal quickly at lower elevations.

This most recent round of precipitation fell on a new melt-freeze crust (4/11) formed during warm weather earlier this week. Observations demonstrate the new crust is strong and supportable. The snowpack now consists of a slab (wind or storm) on top of lower density snow, sitting on an older crust. That is a prime recipe for dangerous avalanche conditions in the upper snowpack.

Reports indicate that the snowpack consist of various layers of melt –freeze crusts, refrozen snow, and strong rounds. This has resulted in a generally strong snowpack without any significant layers of concern.

Observations

Saturday was another day of active results for the MHM ski patrol with 1 ft crown hard wind slabs widespread on leeward slopes which which propagated widely on explosive trigger. They observed two artificially triggered Large Wind Slab Avalanches on ENE and NE aspcts, running 600-900 ft vertically and 700-1200 ft in length. Crowns were 1-2 ft and 4-8 ft, respectively. 

The Azone avalanche is picture here. Photo credit: Paul Klein

On Friday, Pro Observer Laura Green traveled in the Newton/Clark drainage between 4500 and 5900 ft. She avoided higher elevation or larger avalanche terrain due to strong winds, heavy snowfall and poor visibility. With warming temperatures during the day, snow surfaces were moist to wet. On a NNE aspect at 5600' she observed up to 2' of upside down snow on top of lower resistance wet snow layers. Although she didn't find a particularly cohesive slab in the location tested, she suspected they exist higher up.

On Friday, Mt. Hood pro patrol reported reactive 6-18" explosive triggered slabs with limited visibility up high. Snow was falling at the base, but it was very heavy and wet.

On Thursday, Mt Hood Meadows ski patrol reported wind slabs ranging from 8 to 12 inches (20-30cm). Wind and storm slabs were reactive during avalanche control work Thursday morning. The most recent melt-freeze crust was reported as strong and supportable.

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Loose Wet

An icon showing 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.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 15th, 2018 11:18AM