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Archived

Avalanche Forecast

Mar 1st, 2017–Mar 2nd, 2017
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Mt Hood.

Careful snowpack evaluation and cautious route finding will be essential for travel in back country areas at Mt Hood on Thursday.

Detailed Forecast

The next front will begin to move over the Northwest Thursday afternoon and night. But Mt Hood should be spared significant new snow until Thursday night and Friday. But increasing SW-W alpine winds should generally be seen in all areas on Thursday.

Existing wind slab should linger and possibly further build on NW-SE aspects at Mt Hood on Thursday.  Remember to watch for wind transported snow on other aspects as well.

Existing storm slab may also linger at Mt Hood on Thursday.  Storm slab is generally found on more sheltered slopes that experience rapid snow accumulations for several or more hours.

The cornice accident at Washington Pass on Tuesday is a reminder to avoid ridges or mountain tops where there may be a cornice and avoid slopes below cornices.

Remember to change your plans if the weather deteriorates sooner than expected in any area.

Loose wet avalanches also won't be listed as an avalanche problem but it is March so watch for roller balls and loose wet snow deeper than a few inches in you are on solar slopes in sun breaks.

Further increasing alpine winds and snow with a further increasing avalanche danger should be seen in all areas Thursday night and Friday.

Snowpack Discussion

Weather and Snowpack

The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost very strong rain crust in our snowpack. 

About 2 feet of snow accumulated at the NWAC Mt Hood stations from Sunday 2/19 through Wednesday 2/22. 

Weak weather systems Sunday and Monday deposited about another foot of low density snow by Monday evening with light to moderate W winds.

A stronger than forecast storm late Monday through Tuesday deposited an additional 2-2.5 ft of snow by Tuesday evening! The latest heavy snow has arrived with very strong alpine W alpine wind which only gradually decreased on Wednesday.

See the information near the top of the Avalanche Forecast tab regarding the cornice accident at Washington Pass on Tuesday.

Recent Observations

Several contacts were made with the Mt Hood Meadows patrol through the day Tuesday and they reported widespread natural and triggered wind and storm slabs especially near and above treeline. Avalanches were occurring even on relatively low angled terrain. Over 2 feet of new snow had accumulated by early Tuesday with more snow received through the day. Low visibility limited additional observations but by Tuesday afternoon large hard wind slabs were developing above treeline and several sympathetic avalanches were triggered. The recent relatively stable snowpack had quickly turned into very dangerous avalanche conditions.

By Wednesday conditions had improved and the Mt Hood Meadows patrol reported sunny but windy conditions with windward slopes getting scoured and local sizeable wind slab building on lee slopes. The lower mountain was showing signs of spring with rollerballs and small triggered loose wet avalanches 

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: Likely

Expected Size: 1 - 1

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: All elevations.

Likelihood: Possible

Expected Size: 1 - 1