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East Slopes Central - Lake Chelan to South of I-90

Issued: 7:48 PM PST Saturday, March 18, 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.

The avalanche danger should decrease on Sunday. Fresh wind slab has likely built on NW to SE aspects, mainly near and above treeline. Watch for generally small loose wet avalanches at lower elevations and on solar slopes. 

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Avalanche Problems for Sunday

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 first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes. 

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from earlier in the week had many massive slides that covered Hwy 20 near Washington Pass up to 40' deep in places! Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. Far less precipitation was seen further east of the crest during this period for areas like Blewett Pass and Mission Ridge. 

After a short respite from the active weather pattern on Thursday, another strong low pressure system brought 6-12+ inches of snow to the northeast and central-east slopes of the Cascades Friday night and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 feet in the northeast and central-east Cascades and likely 7000 feet in the southeast by early Saturday morning.  Rapid cooling began mid-day Saturday. Strong W-SW winds were likely transporting new snow above treeline by mid-day Saturday. 

Recent Observations

North

The North Cascades Heli Guides were near Silver Star on Thursday 3/16 reported many previous natural storm slab avalanches up to size 3, likely releasing midweek. They also reported several small triggered storm slab avalanches. This activity was on N-NW slopes in the 8000 ft range.

More observations from Friday in this zone, indicated a significant distinction in the recent storm snow with elevation, with up to 4 more inches of storm snow above about 6500 feet. The several recent warm precipitation events this week appear to have limited the upper elevations of rain to about 6500 feet. At elevations up to 8000 feet, the recent precipitation has fallen as snow. In these higher elevations, some shallow triggered storm slab avalanches have been noted over the past few days.

Central

NWAC observer Jeff Ward was in the Cle Elum drainage on Mt Hawkins and Red Mountain up to about 6000 ft on Thursday 3/16 and noted evidence of an impressive avalanche cycle the past couple days. The current conditions were 3 cm of new snow over 3-7 cm thick, firm crust with recent rain penetration to about 40 cm in the snowpack. The Valentine's Day crust was found at 90 cm but tests did not give results.

NWAC observer Tom Curtis was at Blewett on Diamond Head up to about 5500 ft on on Thursday 3/16 where there were no recent avalanches just consolidation. SW-W-NW slopes were scoured, with a breakable or supportive crust elsewhere over moist 1F or P rounded crystals down to the ground. He found some reactivity in a layer at about 50 cm down, which should not be a current problem, but might have contributed to avalanches a week ago.

South

No recent observations. 

Detailed Forecast for Sunday:

Decreasing snow showers along with rapid cooling Saturday night should quickly transition to mostly sunny skies on Sunday. Freezing levels will be on the cool side Sunday but late March sunshine will help bump up temperatures to near or above freezing at lower and mid-elevations. Winds are forecast to be fairly light on Sunday.

Fresh wind slab has likely built on NW to SE aspects, mainly near and above treeline. Watch for firmer wind transported snow on other aspects, especially in areas of complex terrain.

Generally small loose wet avalanches are likely at lower elevations and on solar slopes but some main entrain deeper layers at higher elevations that received more recent snowfall. Watch for surface wet snow deeper than a few inches, rollerballs or increasing small natural releases.

New snow accumulated with a cooling trend and diminishing winds, but small storm slabs may be possible in specific areas on Sunday especially near the Cascade crest for the central-east and northeast Cascades. 

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

No Corresponding Mountain Weather Forecast Available