The spring melt-freeze cycle is in full swing. Snow surfaces are thawing throughout the day into wet corn snow and generally re-freezing overnight into hard crusts. Several factors come into play when it comes to the extent, depth, and speed of thawing at the surface. Sunny skies and warm temperatures are the source of the warming on solar slopes, whereas thin clouds can hold in the heat and increase thawing on northerly aspects, especially overnight where temperatures struggle to get below freezing. That’s the story that played out earlier in the week with daytime highs in the 50s and overnight lows mostly above 32 deg F. Hazy clouds triggered the greenhouse effect and increased the thawing. This generally saturated the snow with melt water and the loss of cohesion at the surface made loose wet avalanches possible. Observations of small natural and skier-triggered rollerballs and sluffs were an indication that larger wet slides were possible during the heat of the day.
Recently however, slightly cooler temps and cold winds in the upper elevations have helped reduce the scope of the melting. As of Wednesday morning, only a few areas below about 9500’ are reporting low temps above freezing. Cloudy skies overnight may have helped keep lower elevations a little warmer. But with mostly cloudy skies forecasted through Thursday, solar radiation will be less intense. At upper elevations cooler temps and increasing winds will help keep surfaces from melting too extensively. Loose wet activity is not impossible in the coming days, but it is expected to be smaller and less widespread.
Terrain and time of day both amplify the risk of encountering wet instabilities. As the sun and temperatures climb throughout the day solar radiation will melt the snowpack more and more. Cirques and gullies receive and retain more heat. Rocks and vegetation can increase local thawing and loose wet avalanches often originate from just below these features. Rollerballs and boot-top penetration into wet snow are good indications that loose wet avalanches are becoming possible. You can avoid the problem by moving to shady and less steep slopes before these signs appear. Even small loose wet avalanches can be dangerous if they carry you into a terrain trap, so continue to use appropriate caution in your backcountry travels.
Slide-for-life conditions early in the day are of equal concern. Using crampons, an ice ax, and a helmet may reduce your exposure to a slip and fall with high consequences when the snow surface is hard and fast.
Finally, an unusually late-season and strong atmospheric river is expected to begin impacting the region Friday. Expect conditions to change rapidly going into the weekend.