Avalanche Advisory for Friday, April 13, 2018

Huntington Ravine has MODERATE and LOW avalanche danger. Central, Pinnacle, Odell, South, and Escape Hatch have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. All other forecast areas have Low avalanche danger. Watch for unstable snow on isolated terrain features.

Tuckerman Ravine has CONSIDERABLE, MODERATE, and LOW avalanche danger. The Lip has Considerable avalanche danger. All other forecast areas have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. The Little Headwall is the exception with Low avalanche danger. Watch for unstable snow on isolated terrain features.

AVALANCHE PROBLEM: Wet slabs will become the primary avalanche problem today as prolonged above freezing temperatures continue to warm the recently dry upper snowpack. The sun may make an appearance today. If we see sunshine for an extended period, human triggered wet slab avalanches may push toward likely on southerly aspects. Without lengthy sun affect, the soft slabs that blanket much of our terrain should behave similarly on all terrain in the ravines. Loose-wet sluffs and point releases should be on your radar, and like wet slabs, will be possible to human trigger on many aspects. It’s worth noting that wet slab avalanches are notoriously difficult to forecast, with natural avalanches tricky to predict and the tipping point between unreactive and touchy to a human trigger often being a very fine line.

WEATHER: After a cloudy start to the day yesterday, skies cleared for the middle part of the day while wind speeds dropped to below 10mph. By mid-afternoon, summits fog developed as light snowfall began. This precipitation was capped by two hours of freezing rain around midnight. All told, the storm delivered just over 2″ of snow with a water equivalent of 0.24″. Unsettled weather with a degree of uncertainty will be the story of today. A warm front is moving into the region, creating warming temperatures as well as a drop from current wind speeds. Elevations under 4000′ are already above freezing at 7am and the higher summits should follow suit by mid-morning with highs reaching well into the 30sF all day. Current W wind at 60mph should decrease to the 30-45mph range early. Cloud cover for the day is a big question mark with the incoming warm front possibly allowing periods of clearing as it lifts over the White Mountains. By late afternoon, moisture will bring a return of fog and precipitation of some type, likely a mix of rain and freezing rain. Temperatures will dip below freezing on the summit tonight, allowing snow and sleet to enter the mix. All told, by tomorrow morning, 0.5″ of water should arrive in some form of mixed precipitation.

SNOWPACK: The past week has transformed our snow surface from predominantly refrozen crust to widespread new and soft snow. The crust, while present at the surface in very limited areas, is now buried under several layers of varying hardness though generally soft (4F-F) slab. Bonding between these slabs and to the buried crust was improving though continued to exhibit spatial variability across the terrain. Stability test results have illustrated this point, with varied loading amounts required to initiate a failure and mixed ability to propagate a crack. These layers of recent slabs, which we believe to be up to two feet thick, have been more consistently producing clean, even (Q1) shears. All told, this snowpack structure of layered soft slabs on a robust crust as a bed surface creates potential for a widespread wet slab avalanche cycle. We don’t expect likelihood to exceed “possible” today, but the gloppy snow may provide less than ideal skiing and riding conditions with heavily trafficked areas like the Sherburne being the exception.

Please Remember:
• Safe travel in avalanche terrain requires training and experience. This advisory is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
• Anticipate a changing avalanche danger when actual weather differs from the higher summits forecast. For more information contact the Forest Service Snow Rangers, the AMC at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters.
• Posted   8:00a.m., Friday, April 13, 2018. A new advisory will be issued tomorrow.

Ryan Matz / Helon Hoffer, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856