ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday February 28th, 2019
Posted by Heather Thamm on 02/28/19 at 7:00 am.
Avalanche risk
The Bottom Line
Low Avalanche Danger
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

A generally LOW avalanche danger exists across all elevations bands for the Turnagain area. LOW danger doesn’t mean NO danger. The sun and above freezing temperatures will be heating up Southerly aspects and a person may initiate a wet-loose avalanche in steep South facing terrain in the afternoon. Give cornices a wide berth and avoid travel under glide cracks. Good travel habits, such as exposing one person at a time, watching your partners and re-grouping in safe zones are key ways to minimize risk.

Any sign of natural wet-loose activity is evidence the avalanche danger is increasing on Southern aspects. If this happens, be prepared to adjust your route in the afternoon.  

SUMMIT LAKE / SILVERTIP / JOHNSON PASS: *** We want to emphasize the difference here. More caution is advised South of Turnagain Pass.*** Keep in mind a thin snowpack with buried old weak layers exist. There is more potential for triggering a large slab avalanche in this zone. Choose terrain wisely and pay attention to how the sun may be affecting solar aspects in the afternoon. Please read the Additional Concerns below.

1. Low
Alpine
/ Above 2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
1. Low
Treeline
/ 1,000'-2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1
Normal Caution
Normal Caution means triggering an avalanche is unlikely but not impossible.
  • TYPE
    Normal Caution

The Turnagain, Girdwood and Placer zones have entered the “Normal Caution” regime of backcountry hazards. What this means is triggering an avalanche is unlikely but not impossible. Watch for south facing slopes to become wet and punchy by the afternoon and keep in mind the following:

  • Wet loose avalanches – Unusually warm air associated with a high pressure system is upon us. Temperatures along ridge tops are at or above freezing. Sun and daily warming will allow lower elevation temperatures to rise into 30s F and possibly 40s F at ridge tops. In this case, triggering wet sluff will be possible if the slope is steep enough. Most southerly slopes have already gone through several melt-freeze cycles, but should surface snow become wet natural wet-loose activity is not out of the question, especially under rocky terrain.
  • Outlier slab avalanches – It’s unlikely but not out of the question that a person could trigger an old wind slab in steep terrain, all aspects – shaded or sunny.
  • Cornice falls – Cornices that are baking in the sunshine can become weaker and more unstable. Give these features lots of space if you’re traveling along a ridge
  • Glide avalanches – It’s always best to limit exposure under glide cracks. Please let us know if you see a glide crack release into an avalanche. The last known glide avalanche was on Goat Mt. in the Girdwood Valley 9 days ago.

Today will be the 7th day of sunny and springtime weather. The warmest day so far was Monday 2/25 and today looks similar. Several wet loose avalanches were seen on South facing steep rocky slopes, like this photo of a mountain near Portage Valley. Keep close tabs on how the snow surface is heating up during the day.

 

Surface crust on steep Southern aspects is 1cm to 1″ thick. If this crust becomes wet and saturated avoid steep South aspects. 

Additional Concern
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

South of Turnagain – Summit Lake/Silvertip/Johnson Pass zones: A shallow snowpack with a generally poor snowpack structure exists in these areas. A variety of weak layers sit in the mid and base of the snowpack and were re-activated last Thursday by the outflow wind event. Many natural avalanches were seen on windloaded slopes in the Summit Lake area. Although whumpfing has been observed in the Summit area, no signs of instability may be encountered before a slab is triggered.

Again a quick recap of the weak layers sitting in the pack:

Valentine’s Layer: Small facets, 10″-16″ deep, last avalanches on 2/21 due to wind event region-wide. Not showing signs of reactivity in Turnagain/Girdwood/Placer currently. Layer was responsible for the 2/19 snowboarder remotely triggered slab on Seattle Ridge.
MLK Jr Layer: Buried surface hoar, 2-3′ deep, last avalanches seen in this layer 2/21 in the Summit Lake region. All natural slides.
Basal facets (large faceted snow near the ground): This layer has only been found in the Summit Lake region and produced at least one very large natural avalanche on Thursday, 2/21.
If you’re headed this way, remember the snowpack becomes more complex – evaluate terrain exposure and the snowpack as you travel.

Mountain Weather

Yesterday: Skies were clear and sunny. Winds were light from the Northwest. No precipitation fell. Temperatures were inverted, teens F rising to low 20Fs in valley bottoms with daily warming. Temps along ridge tops were in the upper 20s F rising to low 30s with daily warming. Overnight ridge top temperatures have been on the rise. Sunburst weather station temps have been above freezing 32F since midnight. Lower elevation temps have dropped into the single digits F and low teens F at sea level.

Today: Another clear and sunny day is in the forecast with high pressure centered over the Gulf of Alaska. The only difference today is temperatures are expected to be warmer, especially in the upper elevations. Temperatures are already at 34F at Sunburst and may rise to the upper-30s F. Sun and daily warming will cause inverted temperatures at sea level to climb into the 30s F. Winds will be light and variable.  

Tomorrow: Looks similar with another day of clear and sunny weather. Unusually warm temperatures will persist through tomorrow. Highs at ridge tops will be in the mid 30s to low 40s F. Overnight temps will drop into the teens at valley bottoms with warmer temps in the upper elevations. Light and variable winds are expected.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   0   0   59  
Summit Lake (1400′) 15   0   0   28  
Alyeska Mid (1700′) 21   0   0   54  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 32   W   3   11  
Seattle Ridge (2400′) 24   N   2   9