Avalanche Forecast
Regions: Olympics.
Large Wind Slabs will propagate surprisingly far on Friday with the danger of trigger or remote trigger greatest on steep northerly slopes with overhanging cornices. In less wind-affected terrain, expect Loose Wet avalanches where the mild temperatures warm the snow. Approach ridgelines with extra caution, avoiding slopes steeper than 35 degrees where you find more than 6” of snow over the crust.
Discussion
Conditions were deteriorating significantly on Thursday with winds picking up blowing around about 4†of dry snow on a crust. Snow levels dropped to below 2000 ft on Thursday.Â
NWAC professionals were able to trigger numerous widely propagating (up to 60-70 ft) wind slabs on test slopes that were up to a foot or more deep. The slabs were particularly large and sensitive below corniced ridgelines as the snowfall and winds increased during the afternoon hours. The setup was ideal for wind slab reactivity with a fresh wind slab sitting, lying over several inches of lower density snow from the tail end of Wednesday’s system, above a firm crust, created very touchy conditions.
Winds scouring lee slopes, building very touchy cornices, and depositing snow to form touchy slabs on Thursday afternoon. Photo Credit: Dennis D’Amico at Hurricane Ridge
A major storm brought 1.63†of water equivalent since it began on Tuesday morning at Hurricane Ridge, but most of that water came as rain with a few inches of snow on top. The rain event created a solid crust with dry snow underneath near treeline and above.
A very shallow snowpack exists below 4000 ft. On wind-scoured slopes you will encounter the crust, make navigating steeper slopes challenging.
Forecast Schedule
For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations with us and the greater community.
Snowpack Discussion
Happy New Year!Â
The pacific northwest rung in the new year with a winter storm cycle that brought changing conditions to our region. This storm exhibited three characteristics: warm, wet, and windy. Let’s take a look at the end of 2019, the beginning of 2020, and where we can go from here.Â
Wrapping up 2019
The last few days of December were generally quiet and cold. The snowpack seemed to enjoy this break in the weather. Lingering unstable snow from the pre-Christmas storms gained strength, persistent weak layers appeared to stabilize, and avalanche hazard decreased in all forecast zones. All in all 2019 ended quiet and uneventful… until the New Year’s Eve weather party showed up … Â
Quiet weather led to lower avalanche danger during the last week of 2019. Hogsback, White Pass, WA. Photo: Andy Harrington
Blowing into 2020
A warm, wet, and windy weather system blew into the northwest for New Year’s Eve and New Year’s Day. This brought rapidly changing conditions and increasing avalanche hazard to all areas.Â
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Warm: Unfortunately this system brought with it warm air. Freezing levels measured near the coast reached 9000’ on the afternoon of the 31st. Many weather stations recorded above freezing temperatures during the onset of precipitation.Â
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Wet: While this system wasn’t as wet as the atmospheric river prior to Christmas, it still produced impressive water numbers in many areas. The bulk of the precipitation seemed to be focused on the Passes and Volcanoes, and water spilled over the crest to places like Washington Pass and Leavenworth. Sadly, when combined with the warm temperatures, this translated to rain well into the near treeline band (or higher) for most areas. The main exception appeared to be in the northeastern cascades, where locations like Washington Pass remained all snow.Â
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HurRidge
MtBaker
WaPass
Stevens
Leanenworth
SnoqPass
MtRainier
MtHood
Precipitation (in)
1.84
3.39
1.42
4.56
1.06
6.19
5.39
5.41
Snow (in)
-
15
-
12
0
3
10
8
Table 1: Precipitation and storm totals from selected weather stations during the New Year’s Eve Storm. “-†24hr storm snow not measured.Â
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Windy: While the warm and wet were impressive, it’s the winds that may set this storm apart. Most weather stations recorded very strong and extreme winds during the storm. Alpental exceeded 100mph just after midnight to ring in the new year. Any dry snow at high elevations was redistributed by the wind and snow surfaces were transformed.Â
Table 2: Wind speeds from New Year’s Eve from selected wind sites. Note the sustained period of winds between 40-60mph.
Eventually, temperatures cooled, the rain turned back to snow, and winds calmed. Many locations picked up additional snow as the storm wound down, but 2020 was already off and rolling with its first major storm.
New Year’s Resolutions
The active weather pattern that started the new year appears to continue. The snowpack and avalanche conditions will continue to change. So, what can your New Year Avalanche Resolutions be?Â
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Read the forecast. This is a great way to monitor conditions even if you aren’t heading into the mountains.Â
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Get out in the snow!Â
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Submit an observation. Tell the avalanche center what you saw while out in the snow by submitting an observation and sending in a photo.Â
Thanks for all of your support in 2019 and here’s to 2020!
-Dallas
Â
Avalanche Problems
Wind Slabs
You are likely to trigger a large and dangerous wind slab on Friday. We think that the reactive Wind Slabs encountered on Thursday will become larger and more dangerous due to rapidly warming temperatures adding extra cohesion to the slab and additional loading Thursday night. You can trigger them easily, particularly at higher elevations where the snow will be more reactive with added wind loading. Cornices are very sensitive right now, so they require extra caution as you traverse ridges. They also provide an excellent indication of where wind-loading has occurred and where you might find Wind Slabs which may present as pillowy snow surfaces. Give these regions a wide berth or stick to forested terrain.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Loose Wet
Expect smaller Loose Wet avalanches in areas where you don’t find Wind slabs. With snow levels rising above 7000 ft and rain returning all day, wet snow surfaces and rollerball activity are distinct signs that natural or human-triggered Loose Wet avalanches may take you for a ride. It might not be big enough to kill you, but it could ruin your day if it pushes you into hazardous terrain.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1