ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday February 22nd, 2019
Posted by Heather Thamm on 02/22/19 at 7:00 am.
Avalanche risk
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

The avalanche danger remains CONSIDERABLE in the Alpine due to strong Northwest winds that have loaded leeward terrain. Human triggered wind slabs 1-2′ are likely above treeline on slopes steeper than 35 degrees. Natural wind slabs are possible. Additionally, give cornices a wide berth and avoid travel under glide cracks. Blowing snow, shooting cracks and any recent avalanches are signs of unstable snow.  

At treeline and below, MODERATE avalanche danger exists for triggering an isolated wind slab in areas exposed to recent winds.

GIRDWOOD / PORTAGE / PLACER:  Wind slabs could be deeper, up to 2′ thick, due to more snow that has fallen over the last 6 days and significant wind loading. Mid and lower elevations are also suspect due to 8-12″ of snow sitting on a slick bed surface.

SUMMIT LAKE / JOHNSON PASS: This area has a very poor snowpack structure with multiple weak layers. Large natural avalanches were observed in this zone yesterday. Triggering a larger, more dangerous slab, breaking deeper in the snowpack is a concern today. Conservative terrain choices and a cautious mindset is advised.

SEWARD / LOST LAKE: Recent snow has fallen this week and strong Northwest winds were observed in this zone as well. Wind slab avalanches are possible and more uncertainty about a deeper more dangerous avalanche hazard exists due to very little info about the snowpack.

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Below Treeline
/ Below 1,000'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Special Announcements

Heading to Hatcher Pass?  Large human triggered avalanches have occurred this week and dangerous avalanche conditions are expected to continue.  Be sure to read the  mid-week snow and avalanche summary,  and check out  recent observations  and follow  Hatcher Pass Avalanche Center Facebook  page for more updates.  

Avalanche Problem 1
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
  • TYPE
    Wind Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Yesterday an impressive Northwest outflow wind event impacted our region. Starting around noon numerous large natural avalanches released in Summit Lake on East and Southeast aspects along the road corridor on Fresno Peak, Colorado Peak, and Summit Peak. This includes a close call by a group (myself included) on Fresno Ridge who turned around due building winds and rapid loading. Just after our descent a large natural avalanche ran adjacent to the route we had descended. This avalanche had a powerful powder blast that blew over some of our tracks. See the report HERE for details.

In some parts of our region winds were much stronger and more sustained than forecasted. Mile 43 along the Alaska Railroad reported average speeds in the 40s and gusts in the 60-80s mph. In Summit Lake wind speeds were likely gusting into the 50s mph and higher at times, but was not accurately captured by MP 45 weather station. In Turnagain Pass the winds were milder, Seattle Ridge weather station and Sunburst averaged 15mph with gusts in the 20-30s mph. Blowing snow was observed throughout this area, but not nearly as much as in Summit Lake. Several fresh natural wind slabs were observed on the SE aspect of Seattle Ridge near the motorized lot. By comparison these avalanches were much smaller in size and only ran a few hundred yards.

Today Northwest winds will start to diminish this morning and become light by the afternoon. Natural avalanche activity is becoming less likely as winds decrease, but human triggered avalanches will be a concern on aspects that received significant loading. East and Southeast aspects are the most suspect, but cross loading may exist on all aspects. Be suspect of smooth supportable surfaces or pillowed snow on steep or unsupported terrain. These slab may have formed on weak faceted snow buried last weekend (2/16) and may initiate easily with the weight of a person or snowmachine. Shooting cracks will be an obvious clue wind slabs are touchy. Wind slabs could be small and isolated or large enough to bury a person if one releases in an older mid-pack layer. More on this below.

*Although there is not much loose snow available at this point, solar radiation may warm up Southerly aspects when winds calm down. Roller balls and small wet loose point releases are possible on steep Southerly aspects near rocks. 

This photo above illustrates the difference between the large avalanches that released naturally in Summit Lake yesterday compared to the photo below of the avalanche activity on Seattle Ridge in Turnagain Pass. These two pictures are a good example of how different the snowpack and weather can be from one part of our zone to another.  Also note the ski tracks on looker left. Our group feels lucky we chose to descend when we did.  

These 3 wind slabs were the only recent avalanches observed yesterday in Turnagain Pass where winds were much milder than Summit Lake.

  

Avalanche Problem 2
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

In the periphery zones of Summit Lake and Johnson Pass an overall poor snowpack structure exists. Yesterday’s natural avalanche cycle in Summit Lake was clear evidence that these older layers are still reactive to additional loading. In Turnagain Pass, where the snowpack is deeper and less loading occurred, the balance was not tipped as of mid-day. However… a layer of widespread buried surface hoar is lurking 1.5’-3’ below the surface across the region. Stability tests over the last 10 days have not been very reactive and the last human triggered avalanche on this layer was over two weeks ago in Turnagain. With that said, some uncertainty exists due to yesterday’s loading event. It is good to keep in mind triggering an avalanche today could be larger than expected on slopes that have recently been loaded. Northwest winds this morning may still be actively loading leeward aspects. Triggering a deeper more dangerous avalanche is more likely in and around Summit Lake where the snowpack has proven itself guilty. Conservative decision making and cautious route selection is recommended. Be on the lookout for obvious clues like whumpfing, shooting cracks and recent avalanches.

 

Rapid loading due to strong winds overloaded an old weak snowpack and caused avalanche activity to run farther and faster than expected in Summit Lake yesterday. This powder blast from the second avalanche crossed over the terrain we descended. For the full report click HERE. 

Additional Concern
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches

Glide cracks are unpredictable, not associated with human triggers, and can release without warning at any time. New glide cracks are opening up around our region and the most recent glide crack to avalanche was a few days ago on the south side of Goat mountain in Girdwood Valley on Tuesday. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks.

Mountain Weather

Yesterday: Skies were overcast becoming mostly sunny by the late afternoon. Moderate to strong Northwest winds were observed across the region. Most of the local ridgetop weather stations recorded average speeds around 15mph with gusts in the 30s mph, but in some areas winds were more sustained and much stronger. Temperatures average in the 20s F. At sea level temps reached a high of 30 F and upper elevations temps dipped to low-teens F overnight.

Today: Clear skies and sun will dominate as high pressure settles in over most of Southcentral, Alaska today. Gusty Northwest winds will quiet down by early this afternoon to 5-15mph. Temperatures will be in the teens F at ridgetops and low-mid 20s F at lower elevations. Valley fog is likely. Expect inverted temperatures wherever fog develops.

Tomorrow: Sunny clear skies will persist through the weekend and into next week. Expect inverted temperatures with ridgetops averaging in the 20s F. Valley bottoms will be in the low teens F to single digits F. Daily temperatures swings with solar warming and warmer air aloft may allow ridge tops to reach warmer than normal temperatures for this time of year. Valley fog is possible and will keep temps in the teens F in the lower elevations. Winds will be light to calm.

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   0   0   62
Summit Lake (1400′) 25   0   0   31  
Alyeska Mid (1700′) 23   0   .01   57  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13   NW   11    39
Seattle Ridge (2400′) 21   NW   11   33