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West Slopes South - South of I-90 to Columbia River

Issued: 6:15 PM PST Thursday, March 21, 2019
by Dennis D'Amico

One more day of warm weather is forecast Friday before temperatures cool over the weekend. Dangerous wet snow avalanches, natural or human-triggered may still occur. Avoid traveling on any steep slope that isn’t frozen, and steer away from places avalanches can run or stop.

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Snow and Avalanche Discussion

Observed wet snow avalanches have decreased from earlier in the week, but our uncertainty remains high as the prolonged warmth continues. 

Continue to monitor wet surface snow conditions and ensure you have options to avoid traveling on or under steep wet slopes. The same heat impacting the snowpack is also stressing cornices. Be leery of traveling near or under these waves of snow that may fail naturally.

Professionals traveling in the Chinook Pass area Thursday reported small to large wet loose avalanches that likely ran earlier in the week. Wet slab avalanches were not observed in the surrounding terrain. Sunny aspects had moist snow down to 1 ft (30 cm) but colder and drier snow from earlier in the winter was still intact deeper in the snowpack. Recent observations also report dry snow on northerly aspects at higher elevations.

Avalanche Problems for Friday

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Natural Loose Wet Avalanche on Alta Vista Near Paradise. (3/18/2019)

You can still trigger a loose wet avalanche on any steep slope with wet snow, especially if we have a period of extended sunshine Friday before the clouds thicken and temperatures cool. 

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.

 

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Wet Slab Avalanche on Mt St Helens (3/17/2019)

While we haven't received any fresh reports of wet slab avalanches in the West South, it's also been a week since temperatures have dipped below freezing. This unusual and prolonged period of warmth continues to keep our hackles up regarding dangerous and hard to predict wet slab avalanches. 

Deal with the uncertainty and sporadic nature of this avalanche problem by avoiding terrain where avalanches can start, and limit your exposure to where avalanches can run or stop until we firmly exit this warm stretch.

 

March 19, 2019

Turning 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.

 

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,  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.

 

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.

No Corresponding Mountain Weather Forecast Available