Turnagain Pass

Good morning backcountry travelers. This is Wendy Wagner with the Chugach National Forest Avalanche Information Center on Sunday, May 1st at 7am. We are no longer issuing daily avalanche advisories for the 2010-2011 season. However, this does not mean that the avalanche season is finished.

ANNOUNCEMENTS

All winter motorized recreation areas are now closed for the winter season.

We would like send out a HUGE THANK YOU to all who have submitted observations this year. They are invaluable to us, as well as help steer this operation in the right direction.

We’d also like to thank the Friends of the CNFAIC. You are an amazing group of folks with a passion to help keep people safe in the backcountry. THANK YOU for all your support; we could not do this without you.

Additionally, we would like to thank:

-Alyeska Ski Patrol

-and many CNFAIC Staffs

for sharing important avalanche information to pass on to the backcountry community.

BOTTOM LINE

We have finished issuing avalanche advisories for the season. Below is a description of general springtime avalanche concerns.

AVALANCHE DISCUSSION

Wet slab avalanches:

These are very dangerous, unpredictable and un-survivable avalanches. They are common in the spring as the snowpack warms from the increase in solar energy and generally warmer temperatures. This cycle of avalanche activity typically occurs for a period of a couple weeks and is often referred to as the ‘shed cycle’ or the ‘spring melt-down’. This year, large wet slabs began to release around April 24th.

Wet slab avalanches are a complex equation. Once the snowpack warms to near freezing it becomes isCNFAIC Staffmal and water begins to percolate through the pack. This free water can lubricate the ground or rocks (glide avalanches) or impermeable layers within the snowpack (wet slabs). This spring the Thanksgiving Rain Crust is a very suspect impermeable layer. This means water percolating through the pack can accumulate on the crust and lubricate the weaker snow grains above the crust to the point the grains lose so much strength the snow above will slide. This process can occur 24/7 since it is deep in the pack and insulated from much of the daily surface melt-freeze cycle.

The take home with wet slabs: When conditions are ripe they are often not forecastable but easily triggered (natural or human) by a wet loose snow avalanche running on top of the slab. Keep an eye out for any recent activity as well as watch the weather stations for overnight above freezing temperatures or rain on snow. When uncertainty is high, conservatism is key.

Wet loose snow avalanches:

Wet snow point release slides are common in spring and can be small and manageable or large and destructive. A small wet sluff can quickly become large on slopes with any distance to run. Wet snow is heavy and once moving can entrain large quantities of snow and run far.

These slides are often in response to daytime warming and can be easily managed by the time of day. Under clear skies, the snow surface usually freezes at temperatures under 40F; the warmer the temperature the weaker the refreeze. When frozen, the snow is strong but can rapidly weaken as the surface softens with the sun. The general rule of thumb is, once the snow penetration becomes ‘boot top’ it is weak enough to slide. East slopes weaken first, then south then west. Tours are best planned with this in mind. Early starts and afternoon tail-gates are good strategies.

Storm snow avalanches:

Storm snow avalanche activity occurs during or shortly after storms. This involves typical wintertime new snow instability concerns (wind slabs and storm snow soft slabs). The usual suspects of recent avalanches, cracking and collapsing are what to watch for. A second peak in new snow activity will often occur once the sun comes out by initiating wet loose and wet slabs (usually confined to the new storm snow).

Rain on snow can trigger wet loose snow avalanches as well as wet slab avalanches. This is most prone at elevations where the snowpack as not seen rain yet.

WEATHER ROUNDUP

A quick look at our season snow numbers:

Turnagain Pass SNOTEL (1800′ on Center Ridge)

SNOW DEPTH – Roughly 60% of last year and 80% of the long term average

Remember, for all things weather see the CNFAIC weather page!

Thank you for checking the avalanche advisories this season. Have a great spring and summer!