Avalanche Advisory for Wednesday, December 17, 2014

This advisory expires tonight at 12:00 midnight.

All forecast areas of Tuckerman and Huntington Ravine currently have Low avalanche danger. Natural and human triggered avalanches are unlikely. Watch for unstable snow in isolated terrain features which may be capable of producing small avalanches.

AVALANCHE PROBLEM: Today’s avalanche problem #1 will be small Wet Slabs. This will depend greatly on the timing of the temperature drop and if  we receive rain before the forecasted mixed precipitation and snow set it later today.  Problem #2 are the lingering pockets of Wind Slab that have been hanging on in completely shaded gullies that don’t see direct solar effect.  I wouldn’t quite call them persistent, but they are getting a little long in the tooth compared to the secondary wind slab problem with new snow expected today.  The timing and accumulation of new snow is critical and the most important issue to watch today. A rating of Low danger is the most appropriate assignment to both Ravines, but it includes the possibility for unstable snow in isolated terrain features or in extreme terrain. Today will also be dynamic with changes occurring to the snowpack……read on for more insight.

WEATHER: Yesterday witnessed a pretty interesting inversion around the beltline of Mount Washington. As winds shifted to the SW around 3pm the temperature peaked on the summit to just shy of 36F.  The Ravines pushed even higher due to ample sun, but below Hermit Lake in the undercasted clouds it remained below freezing.  As already mentioned today will be dynamic.  A total expected water equivalent (QPF) of 0.4-0.5″ is forecasted by very late tonight.  A wintry mix should transition to snow after lunch time with the mercury drop and continue into Thursday delivering 3-6″ (7.5-15cm) by dawn. Anticipate the potential for some rain early this morning.

SNOWPACK: Yesterday the solar gain continued to drive heating down into the snow pack.  We found the differences of a slight aspect change to be a remarkable player in how deep you could find wet snow.  As you would expect, the S facing slopes were far more gloppy, wet, and harder to deal with when traveling than N facing areas.  Even with fairly warm temperatures close to 40F, areas completely shaded remained cold and could actually harbor some propagation potential.  This threat is minimal, but worthy to remember. As the temperature drops the current snowpack is moving towards strength as free water refreezes from the surface down.  The issue of wet grain depth based on aspect will be a player in the speed of a complete lock up.

The main thing to pay attention to today is exactly what form precipitation takes and when transitions occur.  Forecasts are somewhat conflicting with one showing a frozen mix moving to rain before a late snow change and another showing mixed precipitation moving to snow. Be prepared for a variety of changing forms today.  If we get rain for a period I would be most concerned about the isolated cold pockets of snow that still exist, such as in North gully or perhaps Pinnacle.  The vast majority of areas that have been cooking and settling should accept a bit of liquid with little effect.  The second thing to watch is the timing and accumulation of new snow.  If weather forecasts are realized with snow accumulations becoming more consistent this afternoon expect some new loading late in the day by an increasing NW wind. However, some light new snow is currently falling with the summit temperature of 25F and Hermit Lake at 32F.  If snow continues from this point forward and does not change forms, which is contradictory to the forecast, I expect some thin new slabs to develop hedging us to a Moderate rating this afternoon. I mention this because it is a possibility that we may see a changing avalanche danger with weather differing from the expected forecast.  Tomorrow anticipate danger ratings and the avalanche forecast to increase.

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 or the Harvard Cabin.
  • Posted 7:50 a.m. December 17, 2014. A new advisory will be issued tomorrow.

Christopher Joosen, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856