General Bulletin for Saturday, December 21, 2019

This information was published 12/21/2019 at 7:23 AM.


This is an archived avalanche forecast.

The Bottom Line

Stubborn wind slabs exist in many locations, particularly mid-slope, in strongly sheltered areas, and in the aprons of gullies (sluff piles) in east facing terrain. These firm wind slabs weren’t reactive to a few human triggers that were out and about on Friday even though weak layers of softer snow or graupel exist beneath the slab above. Remain cautious and suspect any slabs in steep, unsupported terrain. Small avalanches would lead to big problems in our snowpack which is riddled with boulders, cliffs and terrain traps, all of which have made small slides fatal here in the past. Use the terrain to your advantage, communicate and move one at a time in suspect areas.

General Bulletins are issued when unstable snow may exist within our forecast areas but before conditions warrant  or available resources allow 5-scale avalanche forecasts. Thank-you to everyone who has submitted observations! We will start 5-scale forecasts on Wednesday, January 15 or earlier, if possible. Please remember that avalanches can and do occur before 5-scale avalanche forecasts are issued.

Sluffs in Tuckerman Ravine

The Tuckerman Ravine and Huntington Ravine trails are challenging climbs with significant avalanche hazard during winter. The summer Lion Head Trail remains the safer choice for accessing the summit of Mount Washington from the east. The Winter Lion Head Route still needs more snow. An ice axe and crampons are needed near treeline and above. The Sherburne ski trail has snow coverage to the parking lot though rocks and the occasional open water bar lurk just below the blown in snow.

2019-12-21 General Bulletin printable

Mountain Weather

Temperatures will rebound over the weekend with mostly clear visibility until Sunday. Some new mixed precipitation may return then. Overall a nice weekend to be out and about though be wary of increasing windspeeds on Sunday and be sure to check the Higher Summits forecast when making plans. The warming trend will encourage settlement and further stability. At this time, the forecast amount of mixed precipitation Sunday does not appear to be enough to adversely affect stability.

You can keep tabs on daily details of snow on the ground and 24 weather at our snowplots at the bottom of the forecast page of this website along with the weather resources, including a daily F6 report, produced by the Mount Washington Observatory on the summit.

Forecast Discussion

Fan is poorly filled in with many exposed rocks in the fan (aka “the runout”)

The 22cm of low density snow that fell through the 19th landed on a refrozen rain crust that developed following the heavy rain event on the 14th. That rain event fell on fairly porous soft snow which allowed the rain to drain through the snowpack with no avalanche activity and surprisingly little damage and settlement overall. Areas of firm snow became mucher firmer and icier following the rain but above treeline, our fetch remained smoothed over. The smooth and filled in fetch meant that most of the 22cm of new snow remained available for wind transport. Considering the sustained west and northwest winds, that is probably an understatement as most available snow from the west side of the range is now appears to be on the east side! SWE from that snowfall was low so our terrain remains poorly filled in though in reality, “seasonably” filled in is a better way to state it. December and January are often drier months though the snowpack seems meager compared  to the previous two seasons. 

As mentioned earlier, the wind slabs that exist through the terrain appear stubborn, despite weaker layers of early storm snow that exist between the slab and the rain crust. Observations indicate that the wind slabs lack energy for propagation with cracking limited to underfoot as you would expect with firm wind slabs. Terrain configuration means everything with this kind of avalanche problem. Bridging power is impressive where it exists and that is in most of our terrain. Any convexity or bulge, particularly over ice, could put strain on the slab and lead to failure despite the strength of the wind hammered snow.

Please Remember:

Safe travel in avalanche terrain requires training and experience. This forecast 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.

Avalanche danger may change when actual weather differs from the higher summits forecast.

For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.

Posted 12/21/2019 at 7:23 AM.

Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest