Avalanche Forecast for Friday, January 4, 2019
This information was published 01/04/2019 at 7:15 AM.
The Bottom Line
Ten inches of new snow fell in the past 36 hours. West wind, possible sunshine, and rapidly warming temperatures this afternoon will combine to continue the threat of natural avalanches. Wind slabs forming this morning are likely to be touchy to a human trigger and may fail naturally with no help from a person. All upper elevation avalanche paths, particularly larger slopes and gullies in the eastern half of the compass rose, are rated CONSIDERABLE today. Lower elevation slopes and gullies not affected by wind or with less new snow are rated MODERATE. Intense sunshine on a slope could wet and weaken the snow in these wind slabs. Today is a good day to stay in terrain below 30 degrees and out of larger, wind loaded avalanche paths.
Wednesday night brought 7-8” of new, very low density snow (6%) to the higher elevations of the Presidential range. Light snow continued off and on yesterday, with steadier snowfall again last night, bringing another 2” of slightly denser (8%) snow to Hermit Lake. Wind speeds remained low by local standards through yesterday afternoon, blowing in the 50 mph range on the summit while shifting from the WSW to WNW. Wind speed this morning has ramped up to a steadier 50-60 mph, with 70 mph recorded at 6am. Temperatures remained cold at 7F overnight though the warming trend is starting as of 6am. Wind may gust a bit higher this morning before diminishing this afternoon as skies clear and temperatures warm to the upper 20s F on the summit. Periods of sun at lower elevations and facing the sun may push the mercury above freezing.
Primary Avalanche Problem – Wind Slab
Wind slab avalanche danger remains today as winds increase to a speed well suited to moving snow into reactive slabs. These slabs may be touchy in steep terrain due to the soft, low density snow beneath the wind drifted snow. These wind slabs could be triggered from below and may fail wall to wall in a gully. Signs of wind loading may be hard to identify this morning as summit fog lingers.
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.
Secondary Avalanche Problem – Wet Slab
Intense sunshine on a steep slope could wet and weaken the snow in existing wind slabs. Narrow gullies can intensify this effect. Air temperature will warm this afternoon and set the stage for solar gain to affect the snow if skies clear.
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.
Yesterday, thick summit fog obscured visibility and reduced contrast so observations were limited. Dry loose avalanche activity in Left Gully piled up debris around a meter deep and appeared to have run yesterday morning. Though it didn’t appear to trigger a slab, it was a little more than harmless. A trip to Gulf of Slides revealed no obvious signs of avalanches though the somewhat thinner fog but did reveal that avalanche paths remain fully developed though choked with trees in the runouts.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 01/04/2019 at 7:15 AM.
USDA Forest Service
White Mountain National Forest