Avalanche Forecast

Issued: Feb 20th, 2018 11:27AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cold temperatures will slow recent wind slabs from stabilizing. Avoid steep wind loaded terrain especially above treeline on all aspects. Loose dry avalanches are possible on very steep slopes and could become problematic around terrain traps.

Summary

Detailed Forecast

Cold temperatures with continued snow showers Tuesday not will not significantly increase the current danger. The cold temperatures will allow recent slabs to persist longer than usual. 

Due to previous shifting winds, avoid steep wind loaded terrain especially near and above treeline on all aspects. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a new or old Wind Slab avalanche. 

Loose-Dry avalanches are possible on very steep slopes. With nearly 3 feet of new low density snow, any Loose-Dry avalanche may entrain significant low density snow and could become problematic, especially near or above terrain traps, like gullies, cliffs or trees.

Snowpack Discussion

An additional 4-8" of cold low density snow accumulated in the Mt Hood area by Tuesday evening. The newest snowfall is not significantly increasing the danger as it has little to no slab properties. However, the new powder snow may be masking underlying wind slabs formed last weekend.

Winds abated Sunday afternoon through Tuesday but cold temperatures persisted, slowing the stabilization process of recent slabs.

Nearly 3 feet of snow has been received since Saturday along with much colder temperatures. 

The strong storm system that preceded the colder weather brought heavy precipitation in the form of snow, ice and rain to Mt. Hood Saturday along with moderate winds increasing to strong and gusty winds in the afternoon (WSW gusts 40-70 mph were common at the lower NWAC wind sites!). The rain-line reached near 6000 feet Saturday mid-day before cooling ensued.  

Prior to this system, 12-18 inches of storm snow fell late last week and sits on a strong firm crust layer formed earlier in the month. There are no significant layers of concern in the mid and lower snowpack.

Observations

On Monday, Mt Hood Meadows patrol traveled into the above treeline area for the first time since the strong storm cycle. Surprisingly, finding a lack of wind slabs in terrain usually laden with wind slabs. The very cold temperatures may have limited slab formation in this area. However, given the strength of the winds, reactive wind slabs are not being ruled out above treeline. 

On Sunday, Mt. Hood Meadow reported generally soft storm or wind slab avalanches breaking within the new storm snow. Despite significant winds occurring mid-day near treeline, the usual firmer wind slabs present on Mt. Hood were not observed. No observations were available above treeline. 

On Saturday morning, a new rain or freezing rain crust was noted up to 6600 ft. Rime ice had covered most surfaces. Strong winds prevented observations later in the day. 

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 Dry

An icon showing Loose Dry

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 21st, 2018 11:27AM