Expect the snow surface to change rapidly as the another day of sunshine and warm temperatures quickly thaws a poor overnight refreeze. Avoid traveling on any steep slope that isn’t frozen, and steer around areas where avalanches can run and stop.
Summary
Discussion
The heat just keeps on coming. We’re in for another day of above freezing temperatures and sunny skies. Anytime we go this long without a solid freeze, it should catch our attention. There is quite a bit of water moving in the snowpack, and that brings with it lots of uncertainty.
You will find a thin frozen snow surface in the morning, but that will change quickly. Once the surface crust thaws, you may find yourself standing in a foot for more wet snow. Make sure to plan for these rapidly changing conditions and expect slopes you travel in the morning to be very different by mid-day. Be leery of traveling near or under cornices. These waves of snow are experiencing the same stress from the heat and may fail without warning.
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.
Snowpack Discussion
March 19, 2019
Turning Up The HeatMy 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.
Problems
Loose Wet
NWAC staff traveling in the West North zone the last few days reported an ongoing loose wet avalanche cycle. Many of these avalanches have been small, but several notable avalanches have grown large, entrained significant amounts of snow, and run long distances. With all the debris, piles one could get the impression that this cycle has run its course. We don’t think that’s the case. Another day of heat will allow more slopes, on different aspects, and at higher elevations to become wet and avalanche. When you find wet surface snow, avoid traveling on steep open slope and consider traveling away from places where natural avalanches could run or stop.
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
Expected Size
Wet Slabs
You’re not likely to see wet slab avalanches everywhere, but several days of above freezing temperatures and a cold, layered snowpack keeps them on our radar. Wet slabs are difficult to assess and tough to predict. However, when they release they often carry with them substantial water weight, which makes them dangerous. 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.
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
Expected Size
Valid until: Mar 20th, 2019 11:00AM
