Avalanche Forecast
Regions: Mt Hood.
While the danger surrounding wet snow avalanches is trending down with a change in the weather, it isn't quite over yet. Monitor the surface snow for changing conditions and head to lower angled terrain without overhead exposure if wet surface snow becomes ankle deep. Isolated but large wet slab avalanches are difficult to predict; they can release naturally or be triggered by loose wet avalanches or cornice falls.
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
Loose Wet
If we get a period of extended sunshine Friday before the clouds thicken and temperatures cool, you may still trigger a loose wet avalanche on any steep slope with wet snow. Even as temperatures cool more quickly on the upper mountain, expect wet snow conditions to linger lower on the hill for most of the day.
Monitor the surface snow and recognize that conditions may change very quickly. You can use a pole or your boot to check surface penetration and if it exceeds 6”, seek low-angle slopes or different aspects. The deeper the wet surface snow, the more snow a wet avalanche entrains and the larger and more dangerous they become. Be especially cognizant of the consequences of even a small avalanche around terrain traps like gullies or cliff bands.
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: Possible
Expected Size: 1 - 1
Wet Slabs
Natural wet slabs finally revealed themselves in the Mt. Hood area this week. Two wet slab avalanches released in White River Canyon not far from the Timberline Lodge just above treeline Tuesday afternoon.
For Friday, cloudy skies and slowly lowering freezing levels will diminish but not eliminate the wet slab potential as temperatures continue their week-long vacation above the freezing mark.
You may not see direct signs of instability prior to a wet slab release. Deal with the uncertainty surrounding this dangerous and hard to predict avalanche problem by avoiding terrain where avalanches can start, and limit your exposure to where avalanches can run or stop.
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: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1