Avalanche Forecast
Regions: Snoqualmie Pass.
Expect enough sun and warm temperatures to maintain the threat of wet avalanches and cornice falls. They could be big enough to bury or kill you. Time your travel to be off of slopes before the snow becomes wet and weak.
Discussion
Snow and Avalanche Discussion
After a week of very warm temperatures and minimal overnight freeze, sun-exposed slopes have transitioned to mostly melt forms but have yet to see a solid freeze. Shaded, northerly aspects are holding cold, dry snow. In addition to the avalanche problems below, warming has created a few other concerns. Watch for loose wet avalanches on most slopes steeper than 35 degrees. Don't stand on or under cornices, as they typically fall with warm temperatures. Expect clouds, light precip, and cooler temperatures in the next couple days. By the end of the weekend, we may see a trend to less elevated danger.
At 4:30p on the 20th a glide avalanche released on a known rock slab at Alpental Ski Area. The avalanche slid 50 feet and stopped before reaching the track of the path. Glide avalanches were also reported from neighboring Stevens Pass and East Central forecast zones. Observers reported size D1.5 wet slab avalanches occurring on the 17th on a south aspect of Granite Mtn and an east aspect of Bryant peak. Widespread loose wet activity has occurred since the 13th. We've received observations of dry snow on steep northerly aspects and upper elevations. There was even wind blowing snow at mid-week.Â
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 soared 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
Wet Slabs
Wet slabs are difficult to predict and could be large and 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 few wet slab observations from Snoqualmie Pass are the most compelling pieces of evidence that these slides could occur in similar locations.
Loose wet activity is occurring less frequently now than five days ago. Some slides may gouge into old snow layers and be big enough to injure or kill you. Look for signs of instability such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches as clues that you could trigger a loose wet slide.
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 about 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
Glide Cracks
Observers have reported natural glide avalanches at Snoqualmie Pass as well as in the neighboring East Central and Stevens Pass forecast zones. Two recent glide avalanches released late in the afternoon. Glide avalanches are very difficult to predict and can be large and destructive. Watch for steep slopes with large cracks in the snow. Stay out from under known rock slabs that could produce these avalanches.
A release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
Predicting the release of Glide avalanches is very challenging. Because Glide avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide avalanches.
This Glide avalanche broke to the ground on a smooth, grassy slope. From all the mud on the bed surface, water pooling at the base of the snowpack likely caused the failure.
Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 2