Avalanche Forecast

Snow and Avalanche Discussion

By Wednesday morning we expect 5 nights without a serious refreeze. This follows on the heels of 6 weeks of below-normal temperatures that produced a layered winter snowpack. The Pacific Northwest is undergoing a major spring transition with one of the sharpest spring transitions this region has experienced. The snowpack has not been adjusting gracefully and the Washington Cascades have been reporting widespread loose wet avalanches in all zones, with more isolated reports of large and dangerous wet slabs.

Snow near the surface has warmed to the melting point on all but high-elevation north-facing aspects across Washington State. Minimal overnight refreezes are the only thing holding that wet surface snow in place to prevent natural or human-triggered loose wet avalanches. Expect conditions change very rapidly during and throughout the day with a stable supportable slope becoming unsupportable and dangerous in a matter of minutes, not hours. The best way to stay safe is to keep your exposure low and limit your exposure to any slopes where you find deep boot penetration and soft, wet snow.

Wet slabs are the avalanche problem keeping professionals returning home early in the day. These avalanches are scary, powerful, and release without warning. The longer we go without an overnight refreeze, the more concerned we are that water will be reaching layers of the snowpack where weaker grains or crusts may be lubricated and may release on slopes greater than 30 degrees, without warning. Keep these avalanches in mind before traveling under or on larger terrain features.

Steer clear of traveling on or below cornices. Many have melted back or already released but sunny skies and warm daytime temperatures will continue to stress cornices once again and they have the potential to trigger more dangerous wet slabs.

Recent avalanches

NPS rangers reported that a small slab avalanche released naturally on a layer of buried surface hoar roughly 4-8 inches below the snow surface Saturday afternoon on a north aspect at 5200'. It's becoming less likely and harder to find, but this layer may be still reactive on steep and shady aspects at higher elevations.  

Sunrise Slide: N, 5200', 38 degrees, ~8" crown. Photo Credit: NPS Rangers, ONP

Forecast schedule

For the 2018-19 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays.

During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. While this avalanche forecast is focused on backcountry avalanche conditions expected in the Hurricane Ridge area, we want to hear about your observations from other parts of the Olympics as well.

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 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.