Tuesday, April 10, 2012

This advisory expires in 24 hours

BOTTOM LINE:

Natural avalanches are unlikely and human triggered avalanches are possible on all aspects. Natural avalanches are possible and human triggered avalanches are probable on southerly aspects in the afternoon with the increasing solar input.

TREND:

Decreasing stability on southerly aspects in the afternoon. North and west aspects trending towards stable after new snow settles out, except in thin zones and wind loaded ridges which remain excellent trigger points for future avalanches.

DISCUSSION:

Hatcher Pass received 11″ of new snow on 4/6 and 4/7.   Last months tracked up slopes were once again covered with new spring snow.  Many sluffs, wet-loose avalanches, and a few soft slabs were observed between 4/7 and 4/10.  Whoomping and cracking were observed after the storm as the snowpack attempted  to adjust to its new load.  Our current issues are twofold. Spring weather conditions are rapidly changing the winter snowpack, shedding surface snow from southerly slopes during the heat of the day. Secondly, buried near surface facets and buried surface hoar lay hidden dormant under variable crusts on north and west aspects waiting for enough stress (ie..new snow) to fail and create more widespread avalanches. A few of these slab avalanches were whittnessed on Sunday.

Three human triggered soft slab avalanches (SS-ASu-R2D1.5–O (x2) and SS-ASu-R2D2-O) were whitnessed on Sunday on north and west aspects. All of these avalanches failed 10″-12″ deep underneath the old crust on buried near surface facets and/or buried surface hoar. Note that buried surface hoar is mysterious, sometimes hard to find, and accounts for widespread avalanche fatalities every year (mostly in Colorado). These persistent weak layers are famous for making their debut this time of year as new snow and an increase in temperature change the existing stability regime.

Watch out for southerly aspects in the afternoon as the stout crusts begin to deteriorate and the new snow adds enough stress to tip the balance of the snowpack. Several wet-loose avalanches (WL-N-R3D2-I) were observed the past several days with the increasing temps. One wet-loose avalanche ripped to the ground on a south face at low elevation. These wet-loose avalanches entrain more snow as they slide and have the ability to take you down and  break a leg. Make sure you know which aspect you are on and the time of day! Be careful!

Paying attention to aspect will be critical for safety as the season continues.  An entire season of low energy and a stable snowpack is no longer true. There are safe places to ski, but don’t let the new snow cloud your vision and fall into a heuristics trap leading to an accident. A heightened awareness and ability to assess snowpack and changing weather will help keep the outcome of your decisions in the safe zone.

-Allie Barker

Friday, April 6, 2012

This advisory expires in 24 hours

BOTTOM LINE:

Natural avalanches are very unlikely and human triggered avalanches are unlikely at high elevations on all aspects. Natural avalanches are unlikely and human triggered avalanches are possible on south aspects at low and mid elevation.

TREND:

Stable on most aspects. Deteriorating stability on south and east aspects in the afternoon when affected by the sun and increasing temperatures.

DISCUSSION:

Temperatures have significantly warmed over the past 10 days . Spring is here to stay. Daytime temperatures are maxing out in the mid to upper thirties, and dropping into the twenties at night. The wind picked up on 4/4 and 4/5 moving snow and creating a thin brittle wind slab on west aspects. Most south and east faces are feeling the effect’s from the month of sun in March.  Hatcher has received new wet snow on a couple occasions over the past week, both storms depositing a few inches. This snow is wet enough to make the perfect snowball, hence its ability to bond with other like minded grains and sluff on its crust counterpart during the heat of the day.

South aspects have changed most dramatically over the past week. The increase in temperature combined with significant sun penetration has formed a 4-5″ thick sun crust. 3-4″ on new snow now lay on that sun crust.  The thicker this crust becomes, the more challenging it is for moisture to escape from the snowpack. The result is facet growth underneath the crust creating a greater temperature gradient below and above the crust. The crust can act like visquene by suppressing the moisture and not allowing it to metamorphose into stronger bonds while the upper layers of the snowpack (above the crust) heat up, creating more weight and slab like characteristics. This disagreement below and above the crust creates friction which can create a recipe for an avalanche. To better understand what is going on,  pay attention to see how these crusts grow and change over the next few weeks to help predict avalanche activity.

Weather for the week calls for more clouds, increasing temps, and little precipitation. Paying attention to the upcoming weather patterns will dictate how the snowpack changes. Constant warm temps during the day AND night will deteriorate the snowpack and weaken grains making it more likely to see larger wet-loose avalanches on most aspects. On the contrary, warm days AND cold, clear nights will create the kind of Alaskan corn we like to eat, and the only kind we can successfully grow here.

Continue to watch out for cornices collapsing as well as the growth of elephant holes near boulders (large enough to fall in).  Test your snowball making ability throughout the day to see how quickly the snow is heating up.  If water is dripping out of the snowball you are in the wrong place at the wrong time. Making decisions to stay off of and away from slopes that are heating up during the day can significantly decrease your chances of starting or entraining yourself in an avalanche.

-Allie Barker