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Mt Hood

Issued: 6:00 PM Friday, April 17, 2015
by Garth Ferber

The last daily NWAC forecasts for this season were issued on Saturday, April 11th. For the remainder of April, NWAC forecasts will be issued on April 17th, 18th, 24th and 25th to provide coverage those weekends. At any time this spring, blog posts and Special Avalanche Advisories will be issued if unusual or unusually severe avalanche conditions are expected.

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.

The snowpack is in transition from winter to spring conditions in the Olympics and Cascades. This should keep the focus on likely loose wet avalanches and possible cornice failures mainly during the midday and afternoon hours.

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Avalanche Problems

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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Cornicesi

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

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Snowpack Analysis:

Cold fronts crossed the Northwest last weekend and early in the week. Snowfall totals for these storms ranged from about 14 inches at Hurricane, to anywhere from about 4-28 inches west of the crest and Mt Hood, to about 3-12 inches east of the crest. Avalanche conditions were touchiest in the north Cascades with NWAC observers and staff reporting storm and wind slab and skier triggered storm slab. Skiers also found great late season powder conditions during this time.

The weather started to become sunny and much warmer late in the week. The sun is getting high and strong and this started causing a very rapid transition to spring loose wet avalanche conditions.

NWAC observers began to report loose wet avalanches in the Mt Baker back country on Tuesday and Wednesday. A private party reported impressive pinwheels there on Thursday.

The DOT crew at Washington Pass triggered small to large loose wet slides during control work that reached the highway Thursday morning. A natural cornice release late Wednesday in Spire Gulch triggered a very large loose wet avalanche.

 A private party reported a natural storm or wind slab with a 4-5 foot crown crossed skier tracks at 7000 feet above Paradise on Thursday. The Paradise ranger also reported extensive pinwheels and small loose wet avalanches around Paradise on Wednesday and Thursday.

The mid and lower snowpack is most areas should consist of stable rounded grains and crust from warm periods this winter. Many areas at low elevations especially in the Olympics and east of the crest do not have enough snow to cause an avalanche danger.

Detailed Forecast for Saturday:

Sunny warmer weather will continue on Saturday. This should keep the focus on likely loose wet avalanches and possible cornice failures mainly during the midday and afternoon hours.

Morning crusts and midday to afternoon wet snow should be common on Saturday. There is some uncertainty to the danger levels on Saturday since the snowpack in transition and  there was already some avalanches and consolidation late in the week but it will be warm over the weekend. It will be beautiful to be in the Olympics and Cascades Saturday but remember to make careful snowpack evaluations and cautious route finding decisions especially above treeline.

Wind slab will not be listed as an avalanche problem but watch for signs of lingering firmer wind transported snow if you visit any high, cool, lee slopes.

There was a widespread weak layer of small reactive faceted crystals found at Washington Pass a week ago about 5-8 inches below the surface. Skiers have not reported any triggered storm or wind slab avalanches due to this layer and it is unknown if the layer might still be reactive. It should be a good idea to check for this layer around Washington Pass in quick snow pits on cool lee slopes where it might still be preserved.

Remember that this forecast applies to elevations up to the Cascade crest and not to the high parts of the volcanoes where conditions at this time of year are often more dangerous.

The work of the 3 NWAC forecasters is transitioning to other essential parts of the program before the NWAC closes for the summer. Going Deep session #6 is next Thursday and the topic is Making the Transition From Winter to Spring, when daily NWAC forecasts are not available. The session is at the Seattle REI from 6-9 pm.