ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday February 1st, 2019
Posted by Heather Thamm on 02/01/19 at 7:00 am.
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
The avalanche danger is MODERATE at Treeline and in the Alpine. Human triggered slabs 2-4′ thick are possible above 2,000′ on slopes steeper than 35 degrees. Remote triggering an avalanche is becoming less likely, but not out of the question. Localized winds may have created wind slabs along some upper elevation features and loading patterns may be unusual. Triggering a windslab or large cornice could initiate a deeper more dangerous avalanche on a slope below. Identify high consequence terrain and use safe travel habits. A lot of uncertainty exists in our current snowpack.  
 
SUMMIT LAKE / JOHNSON PASS: Poor snowpack structure and multiple weak layers in the snowpack exist in this zone. Recent natural avalanches have occurred in this area and wind loading may have added additional stress overnight. Triggering a windslab or a more dangerous avalanche 1-3′ deep is possible.
2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
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.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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Avalanche Problem 1
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

Over the last week two storms impacted our region with strong winds and heavy snow causing natural avalanche activity. Also last weekend, between storms, numerous human triggered avalanches occurred in Seattle Creek drainage. All remotely triggered without anyone involved. The weather over the last 48 hours has significantly improved, but the snowpack is not to be trusted. A widespread layer of large sized buried surface hoar  sits 2-4’ below the surface and remains a weak layer of concern.

We are now in a situation where triggering this layer is becoming less likely, but the consequences remain high if someone finds a trigger spot. In places that haven’t avalanched already this problem will be difficult to evaluate. Slab thickness varies across terrain features and identifying thinner areas will be impossible without x-ray vision. Obvious clues like whumpfing or shooting cracks may not be present until its too late. Previous tracks on a slope do not mean its safe. The 3rd, 4th or 8th person could find a trigger spot. Large connected slopes steeper than 35 degrees are the most suspect. This problem is a concern on all aspects above 2000’ including smaller terrain features. Below this elevation a strong crust has improved stability.

This type of avalanche problem is tricky to navigate and requires safe travel habits and a good deal of skepticism. Take a moment before entering into steeper terrain to visualize what a large avalanche might look like or if debris could funnel into a terrain trap. Today will be important to minimize exposure by traveling one at a time in avalanche terrain, regrouping in safe zones, and watching for other groups in the area. 

South of Turnagain – Johnson Pass/Summit Lake zone: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present, including buried surface hoar. A storm that ended on Tuesday triggered numerous large avalanches including cornice triggered avalanches on Spirit Walker. Northwest winds over the last 24 hours have added more stress to the snowpack in some areas of Summit. Keep in mind that an avalanche triggered in this area could release on buried surface hoar or step down into deeper weak layers.

Girdwood Valley: More precipitation has fallen in this area and a facet/crust interface 2-4′ below the surface is also suspect in this zone. In general less information is known about the snowpack in Girdwood, but we do know that some slopes have seen large natural avalanches this week. A cautious mindset and similar travel advice to Turnagain Pass is recomended. 

MLK buried surface hoar in a pit at 2700′ on Tincan from Tuesday 1/29/18

 

Buried surface hoar on the bedsurface of a stability test from Tuesday 1/29/18. This layer is large and easy to indentify if you take the time to dig a pit. 

Avalanche Problem 2
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

WIND SLABS: Localized Northwest ridgetop winds overnight may have transported snow in some parts of our region and could continue today. This wind direction is opposite of our normal storm track pattern and can create unusual wind loading patterns. Drifting snow was observed in Summit Lake yesterday. Places like Summit Lake, Seattle Ridge and Johnson Pass are vulnerable to this direction. Remember any active wind loading could form tender wind slabs or overload a deeper more dangerous problem. Hard supportable snow that sounds drum-like should be suspect. 

CORNICES: Large cornices are present along many ridgelines across the region. These ridgtop hazards can be difficult to see and can break further onto a ridge than expected. Triggering a cornice could initiate a large avalanche on the slope below. Avoid traveling on or underneath these unpredictable hazards. 

Mountain Weather
Yesterday: Thick mid elevation and valley fog occurred region wide with a mixing of cooling upper elevation temps and warmer air at sea level. Temperatures were in the mid to low-20F’s and dropped into the teens overnight in the mid and upper-elevations. Ridgetop weather stations recorded Light NW winds, but stronger winds were seen in Summit Lake. Whittier experienced SW winds 20-45mph most of the day. No precipitation was recorded.
 
Today: Valley fog is likely with clear skies above. Expect some inverted temperatures. Valley bottoms may be in the single digits. Upper elevations temps should remain in the teens F. Light Northwest ridgetop winds 5-20mph should transition to an Easterly direction overnight.      
 
Tomorrow: Overcast skies are expected with single digits to temperatures in the teens. A chance of snow flurries is possible without any expected accumulation. Winds should remain calm to light from the East.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.   We are currently working to replace it.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20   0   0   58  
Summit Lake (1400′) 16   0   0    22
Alyeska Mid (1700′) 21   0    0 47  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   W   7   16  
Seattle Ridge (2400′) 18   *N/A   *N/A     *N/A