A rapidly rising freezing level, rain, and strong warm wind are expected to initiate large natural avalanches that could run into areas we normally think of as "safe" in the valley bottom. Travel in or below avalanche terrain is not recommended.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain on Tuesday
Weather Forecast
Tuesday delivers a significant warm up to the Columbia Mountains, precipitation is expected to fall as rain as high as 2000 m, possibly even 2500 m. Wednesday is ever so slightly cooler and then we should return to a more seasonal temperature regime on Thursday. TUESDAY: Scattered cloud cover, freezing level beginning around 1500 m, may rise as high as 2500 m, moderate to strong southwest wind, 2 to 8 mm of precipitation possible.WEDNESDAY: Scattered cloud cover, freezing level beginning around 1500 m rising to about 2000 m, light west/southwest wind, trace of precipitation possible.THURSDAY: Broken cloud cover, freezing level beginning around 1000 m rising to about 1500 m, light variable wind, 3 to 6 mm of precipitation possible.
Avalanche Summary
Loose wet avalanche activity to size 2.5 was widespread Sunday involving all aspects as high as 2500 m. A natural wet slab avalanche was observed on a south facing slope at 2600 m as well. This activity is expected to increase Tuesday.Avalanche activity on Saturday was limited to small storm slabs and sluffing in steep terrain.Warming temperatures on Friday initiated loose wet avalanche activity to size 1.5 on a variety of aspects. Small storm and wind slabs were sensitive to skier triggering on steep unsupported features at upper elevations too. On Thursday warming temperatures initiated large loose wet avalanches to size 3 on south facing aspects between 2100 and 2700 m in the neighboring Glacier National Park. A natural size 2.5 wet slab was observed on a southeast facing feature at 2200 m as well.
Snowpack Summary
5 to 15 cm of new snow fell Saturday with moderate southerly wind. This snow rests on a crust that is present on all aspects below 2000 m, and extends up to about 2500 m on south facing aspects. The storm snow be be sitting on surface hoar on polar aspects (north and east) at upper elevations. The main concern is the mid-March Persistent Weak Layer (PWL) which is now 60 to 110 cm below the surface. This layer is composed of a crust on all aspects at low elevations. At upper elevations the layer presents as a crust on solar aspects (those that face south and west) and buried surface hoar on polar aspects. The reactivity of this interface has been steadily decreasing and while the likelihood of triggering a deeper persistent slab avalanche is lowering, the consequence of doing so remains high. Solar aspects are expected to be the most suspect as the freezing level begins to rise over the next few days.Deeper persistent weak layers from January and December are still being reported by professional observers, but are generally considered dormant.
Problems
Loose Wet
Loose Wet avalanches are the 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. 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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.