Avalanche Forecast

Issued: Jan 30th, 2020 10:46AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Mild weather and wet snow conditions will drive the avalanche danger Friday. Loose wet avalanches will be most dangerous in the near treeline band where they can grow large. Wind slabs above treeline will become more sensitive as rain pushes above treeline Thursday night and Friday morning.

Summary

Discussion

A dose of sunshine and rapidly warming temperatures Thursday increased the avalanche danger on specific terrain features. Small cornices broke near treeline (see photo below), kicking off small avalanches in the gullies below. It’s in the same sort of terrain and elevations that we are most concerned about the loose wet hazard on Friday.

Photo Wes Baumann 1-30-20

Snowpack Discussion

January 30th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

Looking Back at January

As we turn the corner from January to February, this is a good time to look back at the last month’s snow, weather, and avalanche conditions. January was notable for continuous storms and elevated, yet quickly resolving avalanche danger. In most parts of the region, the snowpack grew significantly. Snow depths went from far below average at the New Year to near normal by the end of the month. While numbers are still being compiled, January 2020 could rank high for total precipitation amounts in some locations. As we move forward, there are currently few layers of concern in the existing snowpack. 

Snotel weather stations report well above average precipitation amounts for the month of January. Source: www.nrcs.usda.gov

 

Weather

January was marked by an onslaught of storms with very few breaks in precipitation lasting 24, or even 12 hours. Most notable was the sheer amount of precipitation that these storms delivered. Most NRCS Snotel weather stations are reporting at least 125% of average precipitation amounts for January, with many over 200% of average. Not all of this fell as snow. For the month of January, the Mt. Baker, Snoqualmie Pass, Paradise, and Mt. Hood Meadows weather stations measured an astounding 42”, 33”, 32” and 28” of water (both rain and snow) with 1 day still left in the month. If you do some quick math, those 4 stations average near or above an inch of water a day!  

Snow levels and temperatures were less consistent than the overall stormy weather. On the heels of a warm December, the first week of the New Year followed suit. From about January 8th through the 19th a cold period had its grasp on the region, bringing snow to the lowlands and copious amounts of light, champagne powder to the mountains. The last third of the month saw continued storms. Temperatures moderated and warm air brought occasional rain to middle and even upper elevations. 

 

A small skier triggered avalanche (D1) within new snow on a NE aspect at 5600ft on Mt. Herman. 01/28/20 Photo: Zack McGill

 

Snow and Avalanche Danger

After a meager start to the winter, the snowpack grew exponentially in January. In fact, the storms were so intense in the middle of the month that they challenged the accuracy of remote weather stations. Snow blocked precipitation gages and knocked out cellular towers and phone lines used to transmit data. Currently, snow depths from reporting sites are near average.

While the storms often brought rapid spikes in avalanche danger, the hazard was quick to taper off. Throughout the month, there were 12 days when NWAC issued High Danger in one or more zones and 11 days when there was at least some Low Danger in forecasts. It’s notable that there were no days with all Low Danger in any forecast zone. In addition to the snowpack growth, a positive side to the storms was that weak layers had little time to form. In general, any facets or surface hoar that formed were very short-lived weak layers. Most avalanche problems involved new snow, wind or wet snow. There are still some areas of shallow, weak snowpack in typical places like the Wenatchee Mountains and the most eastern extent of the Cascade Mountains. Aside from that, the snowpack layering in most zones is marked by a few lingering storm interfaces in the upper snowpack and intermittent crusts from rain events. If you dig in the snowpack around the region, the most prominent crusts you’ll find formed from rain events roughly around January 1-3rd, 7th, MLK weekend, 23-25th, and the 31st.

Looking Forward

The winter snowpack is now well established for recreating and traveling through the mountains. Much more so than it was a month ago. Looking ahead, there’s a good chance this stormy pattern will continue at least into early February. We’ll keep monitoring how the snowpack continues to change near the surface with each weather event. You can check the daily Avalanche and Mountain Weather forecasts for the most up to date information. 

Problems

Wind Slabs

An icon showing Wind Slabs

Winds distributed snow onto a variety of aspects prior to the incoming storm. Firmer, denser snow sits on top of the most recent rain crust. Depending on how much rain penetrates the upper snowpack before tapering off Friday afternoon, we could have a wet slab avalanche problem. 

However the problem is defined, the treatment is the same - stay off of wind loaded slopes steeper than 35 degrees Friday. Wind slab avalanches may transition to wet loose avalanches on larger slopes, growing more powerful and dangerous. Do not travel on slopes with overhanging cornices. If traveling along ridgelines, give cornices a wide berth. Cornices will weaken further Friday and release naturally or break back further than you expect. 

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.

Likelihood

Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Although loose wet is listed at all elevations, more snow is available for entrainment in the 5000-6000’ range and it’s at these elevations that loose wet avalanches could become large and dangerous. Above treeline, strong winds will likely limit the loose wet potential. 

If the sun makes an appearance Friday afternoon, pay attention to steep sunny slopes for increasing signs of instability. If you see roller balls, pinwheeling or natural loose wet avalanches, it’s time to avoid nearby slopes. Remember that loose wet avalanches are often surprisingly powerful and even small loose wet avalanches can be dangerous when combined with terrain traps like gullies, cliff bands, or open creeks.

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

Valid until: Jan 31st, 2020 10:46AM