Continental

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–1.5

  Snowfall will continue to accumulate today above 2000'.  Forecasted snowfall amounts are expected to vary depending on elevation and proximity to the coast.  On 1/22, 4 inches of dry new snow was observed on the north side of Thompson Pass at mid elevation.  Today an additional 6-8 inches of snow is expected for Thompson Pass along with rising temperatures that will likely produce denser snowfall.  This rise in temperatures will create storm slabs that are top heavy, making it possible for a person or machine to trigger an avalanche 1 foot or more in depth.  Weather stations have reported generally light southerly winds overnight, however, these may increase today creating deeper, more sensitive pockets of windloaded snow on the lee side of terrain features.

  In the Maritime climate zone, the set up is similar, however, snowfall amounts are expected to be much higher with 12-18 inches of additional snowfall expected today.  Top heavy storm slabs up to 2 feet or more in depth are expected to exist by late today above 3000' near the coast.  Human triggered avalanches will be likely in this area or any other location where new snowfall totals are greater than 1 foot in depth and top heavy.  

  Today it will be important to pay attention to new snowfall amounts in the area you choose to travel, hand shear pits (click link for example) can be a useful tool to quickly gather information on how well the new snowfall has bonded with underlying layers.  This test can also point out weaknesses within the storm snow that are common when the snow is heavier on top (upside down).  Watch for red flags such as shooting cracks and collapsing.  If either of these are present, the snow is clearly telling you that it has enough energy to produce an avalanche on slopes steeper than 30°.

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  The rain line is expected to rise to 2000' today which will create the potential for wet loose avalanches.  This problem is expected to be confined to the few inches of snowfall that fell yesterday at lower elevations. Below this new snow a rain crust exists with a previously rain saturated snowpack below.  Shallow natural and human triggered wet loose avalanches are possible today.  Pay attention to not just the terrain you are on, but what is above and below you.

   The potential size of wet loose slides is directly correlated to the depth of wet snow and the vertical relief and shape of the slope(does the slope fan out? or does it funnel? Is the wet snow shallow? or Deep?).  The consequences of wet loose activity can be compounded by terrain traps or cliffs.  Dragging a pole basket through the surface of the snow downward on a slope can give you an idea of how much entrainment potential the surface snow has (does it drag straight through? or does it collect snow and spread out as you drag the basket?).  The more entrainment that happens the greater the potential for wet loose to occur in terrain steeper than 35°.

  This problem is expected to grow in depth and distribution as rain is expected to continue through the weekend and become heavier.

   

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2.5–3.5

  Our snowpack has undergone a major change since the beginning of January.  What began the month as a fairly thin snowpack with very thick vegetation below brushline has trasformed to a thick midwinter snowpack with the alders being mostly buried above 2000'.  Below 2000' the snowpack remains thin.

  This transition is a very welcome sight but it has put significant stress on our snowpack, especially during times of heavy precipitation.  On the 11-12th we received 100" of snow on Thompson Pass in a 48 hour period.  This caused a significant natural avalanche cycle that failed at weak layers in the mid snowpack.  As the new snow was allowed to settle  a much stronger snowpack was observed the following week.  Again on the 19th we received a significant snowfall event with 30" on Thompson Pass in 24 hours.  This produced a few large avalanches (see avalanche tab below bottom line) but was much less significant in distribution than the previous cycle.  

  Triggering a deep slab avalanche remains unlikely today, however expect for the likelihood to increase this weekend as heavy precipitation is expected.