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Stevens Pass

Issued: 8:43 AM PST Wednesday, February 7, 2018
by Robert Hahn

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. 

Updated 8:40 AM: Warmer than forecast temperatures Wednesday resulted in an update include loose wet avalanches in all elevation bands on the west slopes of the Cascades and to add the problem near and below treeline on the east slopes. Lower-than-forecast wind speeds have resulted in a reduced wind slab hazard in both the Cascade West-North zone and Cascade East-North zone above treeline. The potential for wind slab development has been reduced and the anticipated hazard has been reduced accordingly from considerable to moderate.

Loose wet avalanches will become more dangerous on Wednesday due to warming temperatures in all elevation bands. Loose wet avalanches may be a problem where previously dry snow moistens due to warming or light rain; or where old wet snow entrains significantly. Watch for roller balls indicating instability and avoid areas where wet snow is more than a few inches deep, where steep or unsupported slopes may slide.

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Avalanche Problems for Wednesday

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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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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Forecast for Wednesday:

Light rain or snow in some areas, rising snow levels, and light to moderate crest-level winds with moderate to strong alpine winds are expected on Wednesday. 

With a return to unseasonably warm temperatures, a host of avalanche problems will be revived from last weekend's wet and wild weather.

Loose wet avalanches were present on Tuesday in the Snoqualmie Pass zone and this problem is expected to develop in all elevation bands as water still moves through the lower elevation snowpack and warming or light rain moisten shallow new snowfall near and above treeline. These avalanches can entrain a significant mass of snow and run further than you expect, so avoid deep wet snow on steep or unsupported slopes and travel through or above terrain that could increase the consequences of being caught in an avalanches such as or cliffs, gullies, or rocky slopes.

Above treeline terrain in the Cascade West-North zone, expect the possibility that new winds form given the moderate to strong winds forecast and significant recent snow available for transport in this zone where crust may not be present above treeline. If warming temperatures don't mitigate the wind transport, these wind slabs will be reactive and dangerous. Significant uncertainty exists surround the forecast in this area.

Elsewhere, warm temperatures should moisten the snow surface and mitigate the potential for snow transport.

Older wind slabs formed Sunday night through Tuesday and will become stubborn to trigger on Wednesday, near and above treeline. Moderate westerly winds with strong gusts blew shallow snow around on top of a crust in many areas throughout this period forming mostly small, but in isolated locations large wind slab by Tuesday. Wind slabs were found primarily on east-facing aspects on Tuesday, but the gusty nature of the wind suggests variability in the wind-loading patterns. Also, where no new snow has fallen since the weekend's rain event, don't expect wind slabs to be a problem, such as in the Crystal Backcountry. 

Cornices are increasingly large now and they have been sagging with the recent wet weather. Don't be tempted to spend time under corniced slopes as warming, rain or direct sunshine will all increase the chances of an unpredictable cornice failure.

With warming temperatures and water still moving through the snowpack near and below treeline, glide Avalanches and cornice falls could be big enough to bury or kill you.  Both of these types of avalanches form in specific types of terrain. The best way to stay safe is to avoid areas where these avalanches form and release. Watch out for slopes with visible glide cracks or where known rock slabs lie under the snowpack. Give yourself an extra wide margin of safety near ridges that could hold cornices, and avoid slopes with cornices overhead. Be aware that water running at the bottom of the snowpack can trigger a glide avalanche long after the rainfall and with significant rainfall over the weekend, we're leaving the possibility in the forecast for another day.

 

Avalanche Summary:

Light relatively insignificant rain and snow showers are moving into the region on the heels of post-frontal showers which continued on a cooling trend behind the front on Sunday night through Monday night with NWAC stations from Snoqualmie Pass northward in the near treeline band receiving 4-6" of snow since the cooling began Sunday evening, with less snow to the south. This new snow now sits on and is well bonded to a refrozen crust above 4000 feet in most locations.

Winds have been moderate with strong gusts in the Cascades with the new snow forming reactive wind slabs on east-facing aspects capable of producing small to large avalanches.

A very moist system that arrived over the weekend, bringing rain totals in the 1.8-3.5" range from Saturday night through Monday morning and creating wet snow up to 6500 feet. Crystal was rainshadowed and only picked up .45" during this time frame. A widespread natural Loose Wet avalanche cycle occurred Friday through Sunday. Observers reported both natural cornice falls and glide avalanches. We’ve received minimal reports of wet slab avalanches. Poor visibility and stormy conditions have limited observations at higher elevations. Below 4000 feet, many areas will likely find moist or wet snow extending up to a meter or more in depth. Loose wet avalanches were easy to trigger in some locations.

The above treeline terrain at Mt. Baker likely received significant snow during this past weekend's storm cycle without changing to rain, so deep snow conditions along with large wind slabs are likely present here and should be starting to heal slowly as the weather inputs decreased on Tuesday. 

Observations

North

Mt. Baker Ski Patrol reported not much avalanche activity since Saturday. A few inches of new snow now sits on top of large, wet rounded grains.

Mt Baker Ski Patrol reported rain to 5000 feet Friday and Saturday with a natural loose wet avalanche cycle occurring in the adjacent backcountry terrain. They observed debris from larger slab avalanches on the Shuksan Arm and Mt Herman as well as cornice fall near Table Mountain.

NWAC Forecaster, Dallas Glass, was greeting backcountry travelers at the Heather Meadows trail head on Saturday and reported snow lines wavering between Heather Meadows (4,200ft) and ridge tops. Dallas observed numerous wet loose avalanches in the Bagley Lakes area.

Stevens Pass

On Monday, Stevens Pass Pro Patrol reported evidence of a widespread wet loose/glide/wet slab natural cycle from the past weekend. The most notable events were large avalanches that ran off steep rock faces during the day yesterday. A patroller triggered a small wind slab in an isolated wind-loading prone feature where the 3 inches of fresh snowfall had been redistributed by winds gusting to nearly 50 mph. At 5000 feet, rain saturated rounded grains constituted the upper 150 cm of the snowpack with the 1/5 rain crust still notable down 150-180 cm with no recent activity on that layer. The 1/29 is discernible down 30-40 cm, but the 1/16 is simply a change of grain size at this point.

NWAC Avalanche Forecasters and observers noted numerous natural wet loose avalanches on all aspects Friday and Saturday. These avalanches ranged from small to large, with some travelling 1000 vertical feet.

Snoqualmie Pass

On Tuesday, NWAC Pro Observer Ian Nicholson was on Granite Mountain. The party easily triggered loose wet avalanches below 4100' on steep, unsupported terrain: They were heavy and traveled far in the moist or wet snow extending at least a meter down. Above 4100', up to 5-6" of new snow was bonding very well to a crust. Winds had recently transported significant snow, scouring ridgelines, cross-loading the south face, and creating the potential for small to large avalanches. The party intentionally triggered a small wind slab in a wind-loaded location. Winds had generally loaded E-facing aspects. Evidence of 3 recent cornice-fall avalanches was also observed near the summit of Granite Mountain. Over the weekend, all of the 3 main slide paths of granite released naturally as large to very large wet slabs or loose wet slides.

Ian Nicholson on a corniced ridgeline, Granite Mountain, 2/6/2018. Note debris from cornice failure (bottom right).

Wind slabs up to 16" or deeper on Granite Mountain 2/6/2018. Ian Nicholson. Rotate your head left to view.

On Sunday NWAC observer, Matt Schonwald, found a saturated snowpack with rain water more than 2 feet below the snow surface. Alpental Ski Patrol reported audible cornices collapses along ridges in the Alpental Valley on Saturday. Both patrol and NWAC observers reported Glide avalanches, visible glide cracks, and many wet loose avalanches.

South

Crystal Mountain Ski Patrol reported mostly firm surface conditions with minor melt freeze on solar aspects on Monday and Tuesday.

Crystal Mountain Ski Patrol reported seeing debris from a widespread natural loose wet avalanche cycle. The avalanches were large and ran far into the runouts. Cornices are sagging, but many didn't release during the recent warmup. The snow surfaces were mostly refrozen and firm on Monday and Tuesday with some softening on solar aspects.

Crystal Mountain Ski Patrol reported skier triggered wet loose avalanches in terrain with an uncompacted, backcountry-like snow on Saturday.

No Corresponding Mountain Weather Forecast Available

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.