Avalanche Forecast for Saturday, December 15, 2018

This forecast was published 12/15/2018 at 7:07 AM.
A new forecast will be issued tomorrow.


This is an archived avalanche forecast and expired on 12/15/2018 at midnight.

The Bottom Line

Our avalanche terrain, which harbors existing wind slab, will go through a period of warming today. For this reason and the period of instability it will bring to our snowpack, avalanche danger will be MODERATE for our forecast areas. This scenario of warming wind slabs has many nuances that can be hard to predict. While wind slabs are larger in size and distribution on east facing aspects, the greater amount of solar gain may have more of an impact on shallower wind slabs that reside on south aspects. Heightened avalanche conditions exist on specific terrain features today that can be avoided with good route planning and careful navigation.

Printable 2018-12-15

Forecast Area

Mountain Weather

Following a mostly clear Thursday, winter weather brought increasing wind speeds and snowfall  to the summits on Friday. The summit received a total of 1.3” of snow with no new snow at Hermit Lake or Grey Knob. During hours of recorded snowfall, wind started at WSW just under 50 mph and increased to 60 mph shifting due west. Today, upslope snow showers may bring up to another inch of snow to higher elevations with skies clearing by afternoon. Temperatures increased from the high-teens F to 23F on the summit where they currently reside with elevations below 3300’ above freezing at 6am. Temperatures will increase today with a forecast high on Mount Washington of 29F. Wind speed will decrease gradually through the day.

Primary Avalanche Problem – Wind Slab

Wind Slab




Warming wind slab is the avalanche problem today. This particular scenario has a great amount of uncertainty due to temperatures being close to the freezing mark, clearing that will allow solar gain but a short window of actual daylight due to being close to the Winter Solstice, and varying thickness of existing wind slab. While today’s warming will eventually lend stability to the overall snowpack, the first time wind slabs see warmth does decrease stability of the slab for a period of time. South and east facing aspects will be of greater concerns, but for differing reasons. South aspects will see the greatest amount of warming due to length of time in the sun while east facing slopes have the largest size and distribution of wind slab that can be affected.

What is a Windslab Avalanche?

  Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Snowpack and Avalanche Discussion

The surface of our snowpack is a mixture of melt-freeze crust and wind slab. Wind slabs range from recently formed to those formed over a week ago. Our snowpack has been very dynamic this season and today will add another change in conditions that will keep avalanche danger elevated. Keeping an eye on the weather today will be key to safely navigating avalanche terrain. Wind slabs that warm are notoriously hard to predict the behavior of, particularly with the various thickness and distribution we have currently in our terrain.

Additional Concerns

Ski trails in the Pinkham Notch area are snow covered with the exception of the rare waterbar that is only half frozen.

Snow Plot Information

Density (%)HSTTotalAir TT MaxT MinSkyPrecipComments
Trace Trace NC213 CM-4.0 C3.0 C-4.0 COvercastSnowView
0 CM 0 MM0CM213 CM-1.5 C0.0 C-11.5 COvercastNo precipitation
0 CM 0 MM0CM214 CM-11.0 C-4.5 C-14.0 CClearNo precipitation
0 CM 0.0 MMNC215 CM-13.0 C-9.5 C-15.0 CFewNo precipitation
Trace 0.1 MMNC216 CM-15.5 C-13.0 C-15.5 CBrokenSnowView

Avalanche Log and Summit Weather

Daily Observations

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
03/20/1917 F1 F 0.0 in 0.0 in32 MPH58 MPH

270 (W)

03/19/194 F-4 FTrace Trace 43.4 MPH63 MPH

300 (WNW)

03/18/193 F-6 F 0.03 in 0 in52.1 MPH82 MPH

280 (W)

03/17/192 F-6 F 0.07 in .5 in61.5 MPH110 MPH

270 (W)

03/16/1925 F2 F 0.24 in 2.3 in67.5 MPH97 MPH

280 (W)

03/15/1941 F25 F 0.04 inTrace 54 MPH105 MPH

250 (WSW)

03/14/1940 F17 F Trace in Trace in27.7 MPH75 MPH

200 (SSW)

03/13/1924 F11 F .07 in .8 in29 MPH52 MPH

290 (WNW)

03/12/1911 F-1 F 0.14 in 1.9 in65 MPH104 MPH

280 (W)

03/11/1917 F6 F 0.35 in 4.4 in77 MPH114 MPH

280 (W)

03/10/1925 F4 F .35 in 3.7 in47.2 MPH94 MPH

150 (SSE)

03/09/1919 F0 F 0 in 0 in36.1 MPH70 MPH

290 (WNW)

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 12/15/2018 at 7:07 AM.

Helon Hoffer
USDA Forest Service
White Mountain National Forest