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Snoqualmie Pass

Issued: 6:00 PM PST Tuesday, March 19, 2019
by Dallas Glass

Conditions should change rapidly Wednesday as another day of above freezing temperatures and sunshine stresses the snowpack. Avoid traveling on any steep slope that isn’t frozen, and steer away from places avalanches can run or stop particularly late in the day when natural avalanches may be more common.

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Snow and Avalanche Discussion

We’re in for another day of hot temperatures and sunshine Wednesday. It won’t take long for the thin overnight crust to thaw. Once it does you could find yourself standing in a foot or more of wet snow. Anticipate these quickly changing 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.

On Wednesday we’ll be entering the 5th straight day of above freezing temperatures. This pattern is a clear red-flag, but unfortunately, wet snow avalanches are often more complicated. We still have quite a bit of uncertainty around this transition in the mountain snowpack.

Observations report dry snow on steep northerly aspects and upper elevations. We’ve even hear reported of wind transporting the snow and possibly forming small slabs. This dry snow exists in a very narrow aspect band but could bring with it a unique set of avalanche concerns.

Avalanche Problems for Wednesday

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Natural Loose Wet Avalanches in the Alpental Valley. (3/18/2019)

You won’t have to look far to find evidence of the ongoing loose wet avalanche cycle around Snoqualmie Pass. While many avalanche paths ran earlier in this warm period, there is still plenty of snow that can avalanche. Another day of heat will allow new loose wet avalanches to occur on other slopes, more shaded aspects, and at higher elevations. We don’t think this cycle has run its course. Avoid traveling on or under any slope greater than 35 degrees where you find wet snow.

 

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A Wet Slab Avalanche Triggered by a Loose Wet. (3/18/2019)

Wet slab avalanches are incredibly difficult to assess and predict. The few wet slab observations from Snoqualmie Pass are the most compelling pieces of evidence that these slides could occur in some locations. When they release, they often carry with them a substantial amount of water weight which can make them particularly dangerous. Give this troublesome problem a wide berth. Be suspicious of any steep open slope greater than 35 degrees, and consider not traveling in avalanche terrain, particularly late in the day when natural avalanches could be more likely.

 

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 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.

 

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 avalanche terrain. We’ll keep monitoring the snow and the weather to keep you informed.

No Corresponding Mountain Weather Forecast Available