Avalanche Forecast

Issued: Mar 6th, 2018 11:01AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Watch for generally shallow recent wind slabs on mostly easterly aspects near and above treeline. Avoid these avalanches by staying off of snow drifts, deeply pillowed features, and fresh cornices on leeward slopes 35 degrees and steeper. Small loose wet avalanches are possible on Wednesday on steep solar slopes and below treeline.

Summary

Detailed Forecast

Watch for generally shallow recent wind slabs on mostly easterly aspects near and above treeline. Avoid these avalanches by staying off of snow drifts, deeply pillowed features, and fresh cornices on leeward slopes 35 degrees and steeper.

Small loose wet avalanches are possible on Wednesday on steep solar slopes and below treeline. Avoid steep sunny slopes near terrain traps if the surface snow becomes moist and watch for signs of natural pinwheeling and rollerballing as a clear signal to change aspects. 

Snowpack Discussion

Small loose wet avalanches likely occurred Tuesday on steep sunny aspects.

Mt. Hood received 3" of new snow Sunday night through Monday morning and west winds were transporting snow onto lee slopes (mostly easterly aspects) forming shallow wind slabs about 8" deep that were not particularly reactive. The sun also came out on Monday, warming snow on sun-exposed aspects and creating small loose wet avalanches below treeline.

A few inches of snow fell during the day Thursday 3/1, followed on the heels of about 8-10 inches of snow that was deposited Wednesday night 2/28. Strong, mostly SSW winds accompanied Wednesday night's snowfall. A variety of snow conditions exist within the upper snowpack, ranging from sandwiches of soft snow with several crusts on shaded aspects with a multitude of crusts existing on sun-exposed aspects. Wind affected snow and unconsolidated surface snow can be found in shaded sheltered areas.

In many locations more than 3’ of snow now sits on a firm buried crust layer (2/17). This crust has been reported up to 6600’ by professionals in the region. There are currently no significant layers of concern below the 2/17 crust.

Observations

NWAC pro-observer Laura Green was in Mitchell Creek Monday and observed active wind loading occurring near and above treeline. No new or recent avalanches were observed.

On Monday, Mt. Hood Meadows Pro Patrol reported west winds transporting snow, 8" of new and generally non-reactive wind slab on lee aspects limited to E facing slopes. Winds eased mid-day. Small loose wet avalanches occurred on solar, especially below treeline.

On Sunday, an avalanche professional reported no avalanches and no cracking or whumphing observed in the Salmon River Canyon. A test pit at 6200' on a NE aspect indicated right-side up, low-density snow in the upper 12" of snow, with a very thin crust in the upper snowpack and a more significant crust down 1.5'. 

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

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.

Aspects: East, South East, South, South West, West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 7th, 2018 11:01AM