An intense winter storm is delivering heavy snowfall and strong winds. Widespread avalanche activity is expected. Avoid avalanche terrain.
Two surface hoar layers are found 50-100 cm and 90-140 cm below the surface. These layers produced large natural and human triggered avalanches as recently as Friday. They are most problematic at shaded treeline elevations where the surface hoar is well preserved.
Heavy snowfall and strong to extreme southwest winds are expected to create dangerous avalanche conditions on Tuesday.
MONDAY NIGHT - Snow, 20-30 cm / southwest wind, 40-80 km/h / alpine low temperature near -8
TUESDAY- Snow, 10-20 cm / southwest wind, 40-70 km/h / alpine high temperature near -2
WEDNESDAY - Mainly cloudy with scattered flurries / west wind, 20-40 km/h / alpine high temperature near -4
THURSDAY- A mix of sun and cloud / west wind, 20-30 km/h / alpine high temperature near -5
With heavy snowfall and strong to extreme winds overnight on Monday and continuing during the day on Tuesday, a widespread natural avalanche cycle is expected to occur on Monday night and throughout the day on Tuesday.
With several weak surface hoar layers in the snowpack, the possibility exists for very large natural avalanches to occur, potentially reaching the ends of their run out zones.
20-30 cm of new snow with strong to extreme southwest winds on Monday night will mean that fresh storm slabs will be widespread, and likely very reactive on Tuesday. The snow and wind is expected to continue throughout the day on Tuesday, with an additional 10-20 cm expected by the end of the day. This new snow is sitting on a weak layer of surface hoar in many areas. See this recent MIN post from the Shames area that shows this layer when it was on the surface.
There are two more layers of surface hoar that are now buried 70-120 cm, and 110-160 cm deep, and are likely the most prominent around treeline. On south through west facing slopes, this surface hoar may be sitting on a crust, which may increase the potential for triggering avalanches on these layers. Below about 1000m, the recent new snow is sitting on a melt-freeze crust.
A layer of weak and sugary faceted grains that formed in January may be found about 150 to 200 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.
