Avalanche Advisory for Thursday, March 23, 2017

This advisory expires at Midnight.

Huntington Ravine has MODERATE and LOW avalanche danger. Central Gully has Moderate avalanche danger. Natural avalanches are unlikely but human-triggered avalanches are possible. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely. Watch for unstable snow on isolated terrain features.

Tuckerman Ravine has MODERATE and LOW avalanche danger. Sluice, Lip, Center Bowl and Chute have Moderate avalanche danger. Natural avalanches are unlikely and human-triggered avalanches are possible. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely.

AVALANCHE PROBLEM: Wind slabs remain the primary avalanche problem. Low rated areas are a mix of scoured old surface with areas of heavily wind sculpted and eroded sastrugi snow. You may encounter unstable snow in isolated areas but options to travel on the stable, icy old surface will be plentiful in low rated areas. Stubborn and firm wind slabs may provide easier travel but will also harbor a low probability risk of triggering a stubborn, hard wind slab. Moderate rated areas contain much more smooth wind slab. This typically means that the area was dominated by deposition throughout the wind and snow event. Anticipate firm, continuous thicker slabs in these areas that will also be stubborn and fairly strong but with weaker layers of new snow beneath that bring a higher probability of fracture and failure. As is often the case, these slabs have good bridging power but the possibility of finding and triggering a thin spot in the slab remains. In general, our moderate rated areas are at the lower end of the moderate rating while low rated areas are nudging towards moderate unless you are on the icy, old surface.

WEATHER: The needle on the gauge has once again swung around towards winter. The current temperature on the summit is -17F with 9F at Pinkham Notch. High pressure to our west will keep winds elevated from the northwest, though at a bit lower velocity than yesterday and last night. Expect 70 mph northwest winds to gradually diminish through the day, ultimately blowing around 50 mph with higher gusts, by dark. Temperatures will increase to around 0F during that same time frame. Expect another cold day on the mountain as the epic battle between winter and spring continues.

SNOWPACK: The 5” of snow recorded on the summit on Tuesday night and early Wednesday morning was blown around the mountain and redeposited by the usual, prevailing nuking winds that grace Mount Washington in winter. Several hours of sustained NW winds in the 80’s mph flanked by 18 hours of winds in the high 60’s and 70’s mph tend to beat up snow particles and either blow them out of our start zones or pack them into firm slabs. A combination of fairly low density snow, high wind speeds and a poor bonding surface in the form of the old, icy surface allowed the snow that fell 24-36 hours ago to be scoured and transported through, and out of, many avalanche start zones. That said, remember the forecasters rule of thumb that a typical ridgetop wind can double or quadruple the amount of snow deposited on a lee slope. Our ridgetop is atypical with a large flat expanse in the Alpine Garden that provides enough snow to easily quadruple and more likely octuple (yep, that’s a word) the amount of snow in wind slabs on lee slopes. Again, expect these slabs to be tough and challenging to trigger in most areas but the hard slab that would result would make for a harsh ride downslope. Stay tuned, winter ain’t over yet!

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 Harvard Cabin.

Posted 8:23 a.m., Thursday, March 23, 2017. A new advisory will be issued tomorrow.

Frank Carus, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856

2017-03-23