The strength of the Spring sun is in full affect and with it comes a variety of avalanche problems.   These include wet loose, cornice/ice fall, wet slab and glide crack avalanches.

Wet loose avalanches will be the most common problem to encounter today.   This is the last day of a clear weather cycle before cooler weather and clouds arrive Saturday, expect for wet loose activity to be on the increase during the day. This occurs when the sun heats up the snow surface enough to become wet and lose strength. This process creates point release slides that entrain more snow as they move down hill.  These can be small-very large depending upon the steepness and relief of the slope they occur on.  Wet loose activity goes through a daily cycle with east aspects being the first to lose strength then moving to south and finally west.  Steep rocky terrain is the most easily affected due to rocks soaking up the suns energy and transferring it into the surrounding snow.

   Other hazards of the spring include cornice fall.  This is a problem with a lower likelihood, although  currently poses the greatest danger to  people traveling in avalanche terrain.  Cornices are losing strength each day as they are baking in the sun.  Avoiding exposure to cornices is highly recommended.  Two cornice fall events in the last week have resulted in very large natural avalanches, with multiple step downs occurring resulting in failure at the ground (see avalanche activity section).

Avoid traveling on or below steep solar aspects during the heat of the day and traveling on or exposing yourself to cornices.

Triggering a persistent slab avalanche remains possible 1-2 feet in depth, although the likelihood is decreasing as more diurnal melt freeze cycles occur.  Near surface facets and buried surface hoar have been identified 6 inches -2 feet deep in mid -upper elevation shaded terrain.  This weak layer has recently been unreactive in snowpits and to ski cuts the last few days, although human triggered avalanches have been reported as recently as 4/17.  Near surface facets and buried surface hoar exist in specific locations and may remain reactive in isolated locations where storm totals from early April are deeper (>18 inches).

Settled storm snow depths since 4/5 vary significantly across our forecast zone, with some areas only having a few inches and others having as much as 2 feet on upper elevation north facing terrain.  Assess the depth and sensitivity of the interface between the settled powder at the surface and hard older layers beneath on a slope by slope basis.

Deeper in the snowpack faceted snow exists in most location at and below hard wind affected snow surfaces in the mid snowpack (3/17 interface).  A very large natural avalanche occurred on Meteorite on 4/15.  This avalanche was triggered by cornice fall that created a series of step-down avalanches.  This a clear indicator that large triggers can tip the scales on weak layers in the mid and lower snowpack (see problem 4).  The changing of seasons bringing warming temps increases the likelihood for more events similar to Meteorite to occur in the coming weeks.

Moderate north winds on 4/17-18  kicked up through Thompson Pass, high elevation ridge lines and other wind exposed terrain.  Shallow wind slabs formed in specific areas that may be reactive to human triggers less than 1 foot in depth.  Pay attention to wind affect at the surface and the sensitivity of the surface snow where you choose to travel.  Wind slabs may have been deposited on near surface facets that have been forming at the surface during the last couple days of clear weather.  This would make the sensitivity of wind deposits higher in those locations.

Winds were light on 4/19 and are forecasted to remain so over the next few days.  This will lower the likelihood of this avalanche problem as time goes on.

This may be the last problem on the list and least likely to encounter, but it is the most dangerous one.   There have been 2 reports in the last 4 days of cornice fall causing very large persistent slabs to full depth avalanches (see avalanche activity section).  Avoiding traveling on or exposing yourself to cornices is highly recommended.

Weak snow exists at the base of our snowpack.  On 4/15 our area saw the first deep slab activity occur since late February.  This occurred on Meteorite (see avalanche activity section) and was a cornice fall that triggered a series of step down avalanches that eventually stepped down to the ground.  This is a clear indication that large triggers can have an affect on depth hoar.  Increasing temperatures could produce more significant activity later in the season while our snowpack is undergoing a transition from a winter snowpack to a spring one.  

This problem is very difficult to assess due its depth and the fact that signs of instability will likely not be present.  You can stack the odds in your favor by avoiding steep terrain that is undergoing rapid changes such as strong solar and avoiding exposure to large triggers such as cornices.