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East Slopes South - South of I-90 to Columbia River

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 large avalanches in many 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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Storm Slabsi

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.

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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 warm, wet storm arrived on Valentines Day and formed the uppermost strong rain crust in our snowpack in the central-east and southeast Cascades while a thinner but prominent freezing rain crust exists throughout the Washington Pass and Harts Pass areas.

Strong southwest flow carried a strong front across the Northwest on Friday evening March 3rd. Along the Cascade east slopes this caused strong southwest alpine winds, heavy, moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft. A region wide avalanche cycle was seen late Friday 3/3 and Saturday 3/4.

NWAC and NRCS Snotel stations reported 6-12 inches of snow from the weather system Tuesday and Tuesday night. Another 4-8 inches of snow accumulated mainly in the southeast and central-east Cascades Wednesday night.

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. As of 6 pm Thursday, several inches of heavy wet snow had accumulated along the east slopes of the Cascades. 

Recent Observations

North

Jeff Ward was out on Monday 3/6 in the Cutthroat drainage and found a generally right side up snowpack with no significant results from ski tests. The thinner crust from 2/17 was spotty and generally not an issue. The Valentine's Day crust was down 125 cm in this location and not reactive.

The NCH were out on Tuesday 3/7 in the Silver Star drainage and reported that on a NE slope at 7000 feet ski tests only gave small loose dry avalanches. On a north slope at 6300 feet the Valentine's Day crust was not found.

On Wednesday, the NCMG on Delancey Ridge observed crowns from recent natural storm slabs about 20-30 cm deep on S-SW aspects near and above 5500 feet. In a test pit on a SE aspect at about 5400 feet, the 2/17 melt freeze crust gave hard but sudden planar results in compression tests failing on facets above the crust. 

Central

Mission Pro-patrol reported surprisingly minor results during avalanche control Thursday morning despite the new snow and moderate west winds. The new snow was generally bonding well with only a few areas of soft wind slab releasing below ridge-lines. The releases were shallow and did not step down to any deeper layers.  

South

No recent observations. 

Detailed Forecast for Friday:

Dense snow received along the east slopes of the Cascades Thursday afternoon and evening should be followed by a period of rain Thursday night and Friday morning that reaches into the alpine only in the southeast and central-east Cascades. W-SW winds will increase Thursday night and stay strong through Friday. Snow levels will not lower much along the east slopes of the Cascades on Friday. 

With a complex weather pattern afflicting the Cascades Thursday and Thursday night and stressing our deep storm snow received over the last week in the northeast zone and near the Cascade crest, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and avoid consequential terrain. Realize that 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. 

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.

Storm slab should still be sensitive on Friday due to denser storm slab building over weaker snow received earlier in the storm and potentially stepping down to deeper storm layers from earlier in the week.   

The persistent slab problem has returned to the Northeast zone due to recent avalanches and some reactive tests on the Valentine's Day or 2/17 crust. Avalanches stepping down to these depths would be large and very dangerous.

Loose wet avalanches won't be listed in the northeast zone due to a plethora of avalanche problems but are likely in all zones on steep slopes below treeline during rain showers. Loose wet avalanches along the east slopes of the Cascades that begin small may become large by entraining deeper layers.

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