Avalanche Forecast for Friday, December 21, 2018

This forecast was published 12/21/2018 at 7:24 AM.
A new forecast will be issued tomorrow.


This is an archived avalanche forecast and expired on 12/21/2018 at midnight.

The Bottom Line

Rain falling on snow can result in wet avalanches though these avalanche types are notoriously hard to predict. This type of avalanche can be quite destructive, though slower moving than dry slabs, due to their greater density and pushing power. Rain on snow also weakens the snowpack for foot travel and makes for miserable post-holing conditions. And then there is hypothermia. If those reasons aren’t enough to choose something other than going into the Presidential Range today, then consider the possibility of a large natural avalanche caused by an erupting waterfall blowing out the snowpack in the Lip and generating a wall of slush, water and boulders. Photos of a similar historical event can be seen here with a view of the debris track here and the crown here taken last year. Though there is the possibility of natural avalanches in other isolated areas of steep terrain, it will be unlikely in most places. The Headwall area of Tuckerman Ravine has CONSIDERABLE avalanche danger due to a confluence of terrain factors, a thicker snowpack, and a stream channel under the snow. The floor of Tuckerman Ravine is threatened by a natural avalanche from this area. Human triggered avalanches are possible elsewhere in avalanche terrain throughout the range which has MODERATE avalanche danger today.

2018-12-21 Printable PDF

Forecast Area

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Mountain Weather

Skiers and climbers enjoyed the gift of light wind, warm temperatures and sunshine yesterday before today’s lump of coal. Rain began overnight and will continue through the day with rain falling heavily at times. The most recent forecast totals indicate around an inch and half will fall, but we could get more due to banding and orographic lift. Temperature on the summit will slowly fall from a high in the lower 40’s starting around midnight tonight and reach freezing levels by tomorrow morning when snow showers begin. The potential for long sliding falls on a hard icy snowpack will become a problem tomorrow.

Primary Avalanche Problem – Wet Slab

Wet Slab




Wet Slab avalanches will become a concern as recent wind deposited snow becomes wet and loses strength. Softer snow will be more likely to fail though larger areas of harder old wind slab may be more consequential. This problem will increase through the day as water and heat penetrate the snowpack.

What is a Wet Slab Avalanche?

  Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Secondary Avalanche Problem – Wet Loose

Wet Loose




Wet loose avalanches could be triggered in steep terrain today. These sluffs are most likely to occur in areas of softer snow. They could entrain enough snow to cause problems in bigger terrain or on slabby ice climbs.

What is a Wet Loose Avalanche?

  Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Snowpack and Avalanche Discussion

Warming yesterday allowed recently built, firm wind slabs to further settle. The majority of the thicker wind slabs were firm and stubborn while thinner, softer wind slabs were more scattered. No avalanche activity has been observed or reported in the past two-three days. In steep terrain, you can find buried crusts with small facets nearby. Stability tests and a fair amount of traffic supported the theory that a crack would not propagate in these layers.

Additional Concerns

The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. The Little Headwall streambed has remained open in many places so far this season and will become a full on waterfall today. Don’t plan on skiing out from the Bowl anytime soon.

Snow Plot Information

Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
0 CMTrace 0CM0 CM8.0 C11.0 C0.5 CBrokenNo precipitation
0 CM 0.0 MM0CM0 CM7.5 C7.5 C1.0 CScatteredNo precipitation
0 CM 22.3 MM0CM0 CM1.5 C4.0 C0.0 COvercastNo precipitation
0 CM 0.0 MM0CM0 CM0.5 C11.0 C0.5 CClearNo precipitation
0 CM 0.0 MM0CM10 CM8.0 C15.5 C8.0 CClearNo precipitation

Avalanche Log and Summit Weather

Thank you Mount Washington Observatory for providing daily weather data from the summit of Mount Washington.

DateMax TempMin TempTotal (SWE)24H Snow & IceWind AvgWind Fastest MileFastest Mile DirAvalanche Activity
05/30/1946 F36 F 0 in 0 in27.9 MPH55 MPH

290 (WNW)

05/29/1947 F33 F 0 in 0 in20 MPH48 MPH

290 (WNW)

05/28/1934 F28 F .71 in 3.7 in20 MPH48 MPH

290 (WNW)

05/27/1940 F27 FTrace 0 in38.9 MPH68 MPH

300 (WNW)

05/26/1948 F39 F .77 in 0 in48.7 MPH75 MPH

290 (WNW)

05/25/1947 F31 F .42 in 0 in17.7 MPH63 MPH

240 (WSW)

05/24/1942 F32 F .66 in 0 in44.8 MPH105 MPH

05/23/1944 F30 F .16 in 0 in26.8 MPH71 MPH

270 (W)

05/22/1934 F21 F 0 in 0 in36.2 MPH115 MPH

330 (NNW)

05/21/1934 F23 F .57 in 1.9 in73 MPH135 MPH

330 (NNW)

05/20/1951 F33 F 0.57 in 0.0 in48 MPH82 MPH

250 (WSW)

05/19/1951 F34 F .6 in 0 in34.2 MPH66 MPH

250 (WSW)

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 12/21/2018 at 7:24 AM.

Frank Carus
USDA Forest Service
White Mountain National Forest