Persistent slab avalanches have been a standout feature of the avalanche cycles brought on by recent storms. Maintain conservative terrain selection with this problem in mind and avoid assuming avalanche danger will end as surface snow stabilizes.
Large, dangerous wind slabs have formed as a result of recent snowfall and strong winds. Newly formed wind slabs combined with deeply buried persistent weak layers makes the consequence of triggering any avalanche high.
Two surface hoar layers are found 50-100 cm and 90-140 cm below the surface. These layers continue to produce large natural and human triggered avalanches, particularly at shaded treeline elevations where the surface hoar is well preserved.
Saturday night: Clear. Light north winds.
Sunday: Sunny, becoming cloudy by evening. Light to moderate west winds, increasing over the day and overnight. Alpine high temperatures around -10.
Monday: Cloudy with flurries bringing 5-10 cm of new snow by end of day, increasing overnight. Moderate to strong south or southwest winds, increasing overnight. Alpine high temperatures around -7.
Tuesday: Cloudy with continuing flurries bringing about 10 cm of new snow and 2-day snow totals to 20-30 cm, easing overnight. Moderate southwest winds easing over the day. Alpine high temperatures around -6.
A parade of storms over the past week produced continuous natural avalanche cycles. Recent avalanches have failed on a mix of storm interfaces and persistent weak layers. Natural avalanche activity has now begun to ease off, however human triggering remains a serious concern.
Improved visibility on Friday allowed for more observations of recent avalanches, which included size 1.5 to 2.5 persistent slabs failing on our March 1st surface hoar layer and storm slabs as well as a size 3 (very large) glide slab. No new avalanches were noted during the day, indicating that conditions had transitioned from a natural cycle toward likely human triggering.
Several more size 1 to 2 (small and large) persistent slab avalanches were triggered with skier traffic in the Stewart area on Thursday, noted for releasing in remarkably low angle terrain. Weather limited observations in the Terrace area, although one new size 2.5 storm slab was observed failing on a reloaded slope and increasing the debris in its runout zone.
15-25 cm of new snow over Thursday brought snow totals from the first week of March to about 100 cm or more. Each round of snowfall has been accompanied by strong south winds, leaving us with wind slabs in leeward terrain and two layers of surface hoar from February that are now deeply buried.
The first of these layers was buried on March 1st and is currently 50-100 cm below the surface, the other was buried on February 19th and is closer to 90-140 cm below the surface, with the greatest depths found at higher elevations. On solar aspects (south through west) this surface hoar may be resting on a buried crust which may continue to act as a sliding surface for overlying slabs. Reduced accumulations of recent snow overlie a rain crust below about 1000 m. The most suspect slopes can be found at treeline elevations and at the higher end of below treeline where preserved surface hoar most likely exists.
A layer of weak and sugary faceted grains that formed in January may be found about 130 to 180 cm deep, and an early-season melt-freeze crust lingers at the base of the snowpack. These layers have produced a few very large natural and explosive triggered avalanches over the past two weeks.
