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Archived

Avalanche Forecast

Feb 25th, 2017–Feb 26th, 2017
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Watch for generally shallow new wind slab on lee easterly aspects. Fresh wind slab should mainly be found near and above treeline but may also form in the upper portion of the below treeline band. Fast moving loose dry avalanches are possible on steeper slopes in non-wind affected terrain.

Detailed Forecast

A low pressure system moving south along the Washington Coast on Sunday should produce light to moderate amounts of low density at Mt. Hood during the daylight hours Sunday. 

While older wind slab continues to exist on isolated lee slopes mainly above treeline, we will shift our focus to new wind slab formed Saturday night through Sunday. Existing weak surface snow in addition to new low density snowfall will be transported to lee slopes mainly near and above treeline. Watch for generally shallow new wind slab on lee easterly aspects. Fresh wind slab should mainly be found near and above treeline but may also form in the upper portion of the below treeline band. 

Watch for loose dry avalanches on steeper slopes in non-wind affected terrain. Be especially wary of fast running loose dry avalanches near terrain traps. 

Storm slabs will not be listed as a primary avalanche problem on Sunday but watch for shallow storm slabs if new storm layers form a cohesive slab above weaker snow in specific terrain on Sunday. 

Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.

Snowpack Discussion

Weather and Snowpack

The most recent atmospheric river arrived on Valentines Day 2/14 forming the uppermost significant rain crust in our snowpack. A short period of fair weather on Friday 2/17 caused another surface crust, especially on solar aspects.

A storm Sunday night 2/19 and Monday 2/20 deposited 10-12 inches of snow at the NWAC Mt Hood stations with a warming trend. Mid-mountain winds were strong W-SW above treeline for much of Monday 2/20, forming hard slabs on lee terrain above treeline.  

Light to moderate snowfall Tuesday changed to showers in the afternoon with moderate W winds. More light snow showers occurred Wednesday, bringing the 4 day snow total to about 2 feet at NWAC Mt Hood stations by Wednesday evening, 2/22.

Saturday started off mostly sunny but high clouds increased in the afternoon.   

Recent Observations

By Thursday 2/23 more stabilizing had taken place relative to earlier in the week and the Mt Hood Meadows patrol reported no results from ski tests within the ski area, but there was still some reactivity of wind slab layers 10-35 cm down on a ENE slope at 6600 ft.

NWAC observer Laura Green on Friday 2/24 reported about 20 inches of low cohesion snow on wind sheltered north aspects, providing excellent conditions with no avalanches. On solar aspects, a less desirable breakable sun crust was covered by the last few days of snowfall received during showers.

Avalanche 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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

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