Avalanche Advisory for Monday, April 16, 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 MODERATE avalanche danger. All forecast areas in Tuckerman Ravine have Moderate avalanche danger.  Evaluate snow and terrain carefully to identify features of concern. The Little Headwall is once again an open stream and is not receiving an avalanche danger rating

AVALANCHE PROBLEM: Rain, which could be heavy today, will make wet slabs today’s primary avalanche problem. Today’s precipitation type will be the primary driver of instability in the snowpack. If we receive more plain rain, wetting and warming will decrease stability through the day. A greater proportion of frozen mixed precipitation will have a lesser impact on the stability of our currently refrozen and stable snowpack. Today’s Moderate rating is distinctly different than Moderate ratings issued a week ago. Last week, new snow and wind made small avalanches possible in a number of areas. Avalanches were much more likely and much smaller than the large avalanches which are possible but less likely today. It’s a low probability and high consequence kind of day. If the weather doesn’t keep you out of the mountains, be mindful that though natural avalanches are unlikely today, they could be large and run a great distance.

 WEATHER: We’re rolling into day three of mixed precipitation on the mountain, and today should bring the greatest amount with approximately 2” of water. Temperatures should hover near freezing on the summit today. Freezing rain is currently falling on the mountain, though it’s forecast to shift towards plain rain soon. Precipitation should be heaviest this afternoon and evening and continue to flirt with the line between rain and freezing rain. A changeover to snow is forecast to occur early tomorrow morning and result in 2-4” of snow during the day. It will also be a quite windy day today, with current ENE summit wind at 50 mph slated to increase with peak gusts over 100 mph occurring this evening.

 SNOWPACK: New snow last week was followed by a significant melt and refreeze over the weekend, with small amounts of mixed wintry precipitation not resulting in new stability concerns. The upper snowpack is currently stable as a result. Instability should increase as the day progresses due to a significant rain event. While this rain is falling on refrozen surface snow, a number of robust melt/freeze crusts and dry snow exist deeper in the snowpack. This structure presents the possibility for rain water percolating down through the snow to weaken bonds or provide lubrication between layers. It’s unlikely, but a large natural wet slab avalanche could certainly occur today. Large wet slab avalanches are particularly tricky to forecast, and today’s forecast of either rain or freezing rain further complicates this puzzle. Pay close attention to the precipitation type and degree of wetting the snowpack receives as you assess conditions and make terrain decisions today.

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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  7:35 a.m., Monday, April 16, 2018. A new advisory will be issued tomorrow.

Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2858