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Mt Hood

Avalanche Warning i

Issued: Thu, March 9, 2017 at 8:35 PM PST
Expires: Fri, March 10, 2017 at 6:00 PM PST

Issued: 8:35 PM PST Thursday, March 9, 2017
by Dennis D'Amico

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

Numerous serious avalanche involvements occurred in the Olympics and Washington Cascades over the March 4th-5th weekend. NWAC has posted brief summaries of the incidents here: http://www.nwac.us/accidents/accident-reports/. Full reports will be posted when finished. 

Due to the potential for very large avalanches in specific areas following Thursday's complex storm, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and stay out of consequential terrain. 

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Avalanche Problems for Friday

Wind Slabi

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.

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Wet Slabsi

Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.

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Loose Weti

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.

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Snowpack Analysis:

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. This crust may be a bed surface for very large avalanches during the current storm cycle. 

Strong southwest flow carried a strong front across the Northwest on Friday evening March 3rd. At Mt Hood this caused strong southwest alpine winds, heavy, moist, dense new snow above about 4000 ft and wet snow or rain below about 4000 ft.

NWAC stations at Mt Hood received about 4 ft of snow earlier this week. A strong warm frontal system brought several inches of water, first as snow and then as rain into the alpine of Mt. Hood on Thursday. SW winds were routinely gusting above 100 mph at the Cascade Express station at Mt. Hood Meadows Wednesday night and Thursday.  

Recent Observations

The Meadows patrol on Tuesday morning reported strong winds and heavy snowfall but with limited avalanches. Upside down wind slab layers were building with lots of snowpack cracking and whoomping due to wind slab on ridges. Patrollers released 2 sensitive cornices remotely.

Warming, strong W-SW winds, and rainfall following heavy snowfall Tuesday caused a very large natural avalanche cycle according to the Mt. Hood Meadows pro-patrol by early Wednesday morning. Control work on Wednesday reported widespread large avalanches triggered with explosives releasing within the new storm snow. Cornices along ridges have become very large. 

On Thursday, rainfall was beginning to saturate the upper snowpack with un-supportive wet snow allowing several feet of boot penetration by mid-day. 

Detailed Forecast for Friday:

Snow levels should peak around 8000 feet Thursday evening and be followed by a slow cool down with quickly diminishing shower activity on Friday. W-SW winds will stay very strong through Friday morning.

With a complex weather pattern afflicting the Cascades and Mt. Hood Thursday and Thursday night and stressing our deep storm snow received over the last week, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and avoid consequential terrain. Realize that very large avalanches are possible if initially small avalanches step down to deep and lingering storm instabilities or if a large natural trigger like a cornice collapse occurs. 

Deep wind slab should be suspected on all aspects near and above treeline but most likely found on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.

Loose wet avalanches that begin small may become large by entraining deeper layers. Large wet slab avalanches will be possible in isolated areas due to the significant rainfall received Thursday and Thursday night.  

Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time. Cornices have been reported as large and in charge in many areas. They will have likely been weakened during the recent storm cycle becoming more likely to fail. 

No Corresponding Mountain Weather Forecast Available