ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday February 21st, 2019
Posted by Heather Thamm on 02/21/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

The avalanche danger is CONSIDERABLE in the Alpine due to strong Northwest winds blowing new snow and loading leeward terrain. Human triggered wind slabs 1-2′ are likely above treeline on steep terrain and unsupported slopes. 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 steep terrain.

GIRDWOOD / PORTAGE / PLACER: Slabs could be deeper due to more snow that has fallen over the last 5 days.

SUMMIT LAKE / JOHNSON PASS: This area has a very poor snowpack structure with multiple weak layers. Triggering a larger, more dangerous slab, breaking deeper in the snowpack is possible today as well as natural wind slabs.

SEWARD / LOST LAKE: Recent snow has fallen this week and strong Northwest winds today will elevate the avalanche hazard. Wind slab avalanches are possible and more uncertainty exists in this area due to little info about the snowpack. Look for signs of instability.

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 avalaches have occured this week and dangerous avalanche conditions are expected to continue.  Be sure to read their  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

Today strong outflow winds will be transporting 3-4” of new snow that fell overnight. Expect Northwest winds to increase this morning to 20-30’s mph along ridges. Taller, more exposed ridgetops and channeled terrain may see gusts in the 40’s or 50’s. Blowing snow will be your first clue wind slabs are forming. Wind slabs could be a few inches thick or up to 1-2’ if they break into a older snow buried this past weekend. Two days ago we observed a similar situation where a period of strong NW winds initiated a handful of natural wind slabs and several people triggered wind slabs in steep terrain. 

Keep in mind a Northwest wind direction creates unusual wind loading patterns opposite our normal Easterly storm track direction. Sunburst weather station doesn’t often reflect the full extend of NW winds and sometimes a South or SW wind direction is observed. Smooth, pillowed surfaces on steep leeward terrain will be most suspect. Wind slabs could be small and isolated or large enough to bury a person if one releases in older snow. Although its becoming less likely in Turnagain Pass, the MLK buried surface hoar is widespread and sits 1.5-3’ below the surface. More on this below.

Cornices are large and strong winds today will be adding additional stress. Give them extra space as they can break farther back onto a ridge than expected.

 

NW winds from two days ago (2/19) observered on Taylor Pass. A similar if not stronger wind event is expected today. Blowing snow is an obvious sign of instability. Photo by Ray Koleser 

Wind slab in Placer Valley Tuesday was likely triggered by snowmachiners along the ridge during a period of active windloading. Overnight we’ve recieved a few more inches of new snow and additional loading is expected today. Photo by Graham Predeger.

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

Several persistent weak layers exist within the snowpack across our region, including near surface facets buried on Saturday (2/16.) This is the most likely layer to be initiated today under 1-2’ of snow. In the  periphery zones of Summit Lake and Johnson Pass an overall poor snowpack structure with a variety of weak layers are lurking including the MLK (Martin Luther King day) buried surface hoar and facets mid-pack. The MLK layer has been documented throughout the entire advisory region and was responsible for a number of large human triggered avalanches in late-January and early-February. Although these deeper persistent weak layers have not been reactive lately, additional loading from recent snow and strong winds today may start to tip the balance. It is good to keep in mind that triggering an avalanche today could to step down into old weak layers and initiate a larger more dangerous slide. This is more likely in and around Summit Lake where a thin and weak snowpack is more vulnerable.

 

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. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks.

Mountain Weather

Yesterday: Skies were overcast becoming obscured as snow started falling mid-day. 2-4″ of new snow fell (0.2 -0.4″ SWE) overnight. Easterly winds increased later afternoon to 10-15mph with gusts in the mid-20s mph. Temperatures were in the teens (F) at ridge tops and upper-20s (F) near sea level. All precipitation remained as snow.

Today: Snow will taper off this morning and skies will start to clear by this afternoon. Northwest outflow winds will increase this morning in channeled terrain and along ridgetops. Expect strong winds 25-35mph with gusts in the 40-50’s to last through the day before decreasing this evening to moderate. Temperatures will be in the teens (F) at ridge tops and low-20s (F) at lower elevations.

Tomorrow: Clear skies and sun are on tap for Southcentral this weekend and into next week as high pressure sets up over mainland, Alaska. Outflow winds will dissipate by tomorrow morning. Temperatures could dip into the single digits (F), but are expected to be more in the teens to low 20F’s. Expect inverted temperatures, cooler temperatures associated with valley bottoms and warmer temps in the Alpine. Valley fog is likely.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24   4   .4   62  
Summit Lake (1400′) 23   3   .2   32  
Alyeska Mid (1700′) 23   3   .15   61  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   ENE   11   28  
Seattle Ridge (2400′) 21   SE   9   19