Avalanche Forecast
Regions: Stevens Pass.
Prolonged very warm temperatures and sun are maintaining dangerous avalanche conditions during the warmest hours of the day. Wet avalanches and cornice falls can be large enough to bury or kill you. Time your travel to be off of slopes before the snow becomes wet and weak and avoid being on or below cornices.
Discussion
Snow and Avalanche Discussion:
Thursday will be the sixth day in a row of very warm temperatures, minimal overnight freeze, and melting of snow surfaces. The snowpacks spring transition is far from over, with many slopes holding cold, dry snow under the surface. There was a peak of wet avalanche activity on Sunday. Expect the avalanche danger to remain elevated until temperatures cool.
An observer reported a natural wet slab big enough to injure, bury, or kill a person (D2) on a sun-exposed slope on Lichtenberg Mtn on the 19th. Another large wet slab occurred in Tunnel Creek on a south aspect at 5,200ft on the afternoon of the 17th. Observers have reported similar avalanches at Snoqualmie Pass, Crystal Mtn, near Mt Baker, and Mt. St. Helens. Since the 13th, observers reported widespread natural wet loose avalanches. Locally these have occurred on the southeast face of Lichtenberg Mtn, West aspect of Snowgrass Mtn up to 7,200ft, and southwest aspects near the Skyline and Windy Mtn. Some of these avalanches were big enough to bury or kill a person.
Cornices are looming and can typically break during warm weather.
Snowpack Discussion
March 19, 2019Turning Up The Heat
My how the weather has changed. After nearly six weeks of below average temperatures, spring roared in like a lion. Temperatures March 17-19 sorrowed into the mid ’50s at many of our mountain weather stations. This has been a big change for our cold winter snowpack, and you can see the effects of several days of warm temperatures in the mountains.
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Temperatures from selected weather stations for the past week. Notice the long stretch of above freezing temps over the last few days. (Dates March 12-19, 2019)
The Loose Wet Avalanche Cycle
A prolonged small loose wet avalanche cycle occurred in some areas March 14-16, Â as daytime temperatures crept above freezing. Recent snow from a storm on March 11-12 fueled these generally small avalanches, while thin clouds minimized the impact of the warming trend. Due to their small size and specific locations, the avalanche danger stayed moderate. This initial cycle played out in different areas at different times.
On Sunday, we noticed a marked shift. Poor overnight refreezes, continued warming temperatures, and clear skies finally tipped the balance. Loose wet avalanches on Sunday afternoon began to grow larger and run farther in some locations. Avalanche conditions became dangerous. Subsequent similar days allowed this cycle to impact higher elevation terrain and move onto more shaded aspects. As of Tuesday, we’re still very much in the middle of this cycle.
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Loose wet slides hit the groomed road near Blewett Pass. Photo: Matt Primomo
Why Wet Slabs? And Why Now?
Here in the NW we're used to seeing wet slabs associated with rain on snow events, but we don’t always see them as part of a spring shedding cycle. So, what’s different this year?
As the loose wet avalanche cycle ratcheted up a notch Sunday afternoon, this also began to indicate that more water was moving in the snowpack. Over February and early March, several winter storms formed a cold and layered mid-winter snowpack. How would these old layers respond to the influx of water? This is one of the more difficult questions in avalanche forecasting. The first indications came over March 16 and 17 with a few reports of isolated wet slab avalanches. Would these be the precursors to a more widespread cycle? Well, we're still waiting to see. We know there have been several days now of completely above freezing temperatures and the snowpack is still cold and layered. With a lot of uncertainty about the possibility of wet slabs, we’re approaching any avalanche terrain with a high degree of suspicion and dialing back when, where, and how we travel.
A wet slab from Mt St Helens, Sunday, March 17, 2019. Photo: NWAC public observation page.
Variability in Time and Space
So what does this all mean? Well, two things come to mind. 1: You may experience a wide variety of conditions depending on where you travel. Changes in aspect, elevation, and feature can lead to changes in sun exposure, overnight freezing, and timing of the thaw. Other than steep due north aspects, the sun and temperature appear to be finding every snow surface. Conditions will change rapidly during the day. Don’t expect slopes you travel on in the morning to be the same by mid-day. That leads us to point 2. Be informed, monitor conditions, and prepare to respond to changing conditions. Use the Weather and Avalanche Forecast to make sure you are up to date on what we think of the current and forecasted conditions. As you travel, make observations. How is the snow responding to the heat, sun, etc? Don’t forget to think about the slopes above your head. Expect conditions to change quickly, and plan for travel options that allow you to avoid potentially dangerous overhead slopes.
A Shout Out to Low Elevation Snowpacks
Cold temperatures in February built deep low elevation snowpacks, especially east of the Cascades. This snowpack has been very weak. As it becomes warm and wet, you may see odd, full depth avalanches occur. Don’t let your low elevation fool you. Just because it’s not a big mountain avalanche path doesn’t mean it can’t slide.
Full depth slabs next to full depth loose wet avalanches. Swakane Canyon near Wenatchee. Photo: Matt Primomo
When Will This End?
Transitions like this take time. Don’t be in a rush. Until the snowpack undergoes a solid refreeze, continue to be leery of avalanche terrain. We’ll keep monitoring the snow and the weather to keep you informed.
Avalanche Problems
Loose Wet
Loose Wet activity is less frequent now than on Sunday. However, observers have reported new natural avalanches, some big enough to bury a person, almost every day this week. Loose avalanches will begin by mid-morning on northeast and east aspects and continue on south, southwest, and northwest aspects as the day continues. Some slides may gouge into old snow layers. Look for signs of instability such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches. If you notice these clues, avoid going on or under steep slopes where loose wet slides may run naturally.
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
Wet Slabs
While wet slabs are less likely than other wet avalanches, they’re difficult to predict and could be larger and more dangerous. Steep, unsupported, and sun-exposed slopes are the most likely areas to encounter these avalanches. There’s a good chance that wet slabs could run naturally, or be triggered by cornices or other avalanches. The two recently reported wet slabs in the Stevens Pass area are good evidence that more could occur.
As day-time temperatures warm, stay off of and out from under sun-exposed avalanche start zones, rocky alpine faces, unsupported slopes, and terrain over 35 degrees where meltwater may be reaching deeper weak layers. If you find areas of unsupportive, wet snow, avoid avalanche terrain. You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow. There are a few recent interfaces in the top 2 feet of the snowpack that could be a problem. The layer of most concern consists of old facets buried in early February that lies 2-3 feet below the surface.
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1