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Stevens Pass

Issued: 6:00 PM PST Tuesday, March 19, 2019
by Matt Primomo

The snowpack is undergoing a major thaw. Dangerous wet avalanches may continue to occur. This is a good time to step back, and allow the mountains to make the transition. 

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Snow and Avalanche Discussion:

After a prolonged period of snow and below average temperatures, we are experiencing a heat wave with record high temperatures. The snowpack is making a transition from cold and dry, to a spring-like pack. Meltwater is beginning to make its way through the upper snowpack. Along with this, there is lots of variability with the depth of water infiltration on the stratified and inclined snowpack. This leads to uncertainty with avalanche conditions on various slopes. Dry snow can still be found on direct north facing slopes, but anything with a northeast or northwest tilt, and more southerly aspects, wet snow is commonplace. Cornices are beginning to sag and break off. The most active period for wet activity has been observed to be in the later half of the day.

On the 19th, a new wet slab big enough to injure, bury, or kill a person (D2) was observed on Lichtenberg Mountain. There have been several reports of wet slab avalanches on southerly slopes since the 16th. A large slide into Tunnel Creek on a south aspect at 5,200ft occurred on the 17th in the afternoon. Also, a handful were spotted at Snoqualmie Pass, one at Crystal Mtn, Excelsior Ridge near Mt Baker, and one on lower Mt. St. Helens. From the 13th-18th, observers reported numerous natural wet loose avalanches on the southeast face of Lichtenberg Mtn, southwest aspects near the Skyline, and Windy Mtn. A couple of these avalanches were big enough to bury or kill a person. Significant wet avalanche cycles have also occurred in neighboring zones. A new regional synopsis is out, click the Regional Synopsis tab for more.

Cornices are looming and may break off in this heat.

Avalanche Problems for Wednesday

Loose Weti

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Example: Loose wet avalanches (3/15/2019)

Loose avalanches are likely to begin by mid morning on northeast and east aspects, and continuing to south, southwest, and northwest as the day continues. Some slides may be quite large, gouging into old snow layers, or running on the ground. 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. 

It's a good idea to choose conservative terrain, or just stay out of the mountains until we get a solid freeze. Many slopes have produced wet avalanches, but until every steep slope has run, theres potential for more with this weather. Take note of how much the sun is affecting slopes by aspect, elevation, and slope angle to help you assess which slopes may be most problematic on a given day. If you do choose to go into the backcountry during these times of transition, get out of the mountains early, stay off steep slopes as soon as any superficial crust begins to break down. Watch what looms above, as cornices and wet slides may start up above you.

 

Wet Slabi

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Recent wet slab in Tunnel Creek (3/18/2019)

Steeper slopes that face the sun have become quite weak and dangerous. The snowpack has a few layers that could begin to "wake up" as meltwater percolates down to them. The two recent wet slabs in the area appear to have failed at one of the recent storm interfaces, though one of the more concerning layers are the facets above or below a thin crust that was buried in early February. As each day warms and only superficial refreezes occur, unpredictable and large avalanches may become more likely. The recent activity is enough evidence to make me want to stay off these slopes for now.

You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow. As day-time temperatures warm and any surface crust breaks down, stay off of and out from under sun-exposed avalanche start zones, rocky alpine walls, unsupported slopes, and terrain over 30 degrees where meltwater may be changing deeper weak layers.

 

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