This information was published 01/12/2020 at 6:55 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Rapid melting is never a good thing in the mountains. Be wary of hazards above you if you venture into avalanche terrain or beneath any cliff with ice hanging from it. Rain and warm air will add more strain to already weakening wind slabs and make them susceptible to a human trigger. Later today, rapid cooling will lead to a refreeze in the snowpack and may create a long sliding fall hazard. Also, wind slab avalanches may be possible later today if new snow amounts reach the upper end of the forecast 2-4”. Avalanche danger remains MODERATE today. Evaluate snow and weather conditions carefully.
As of 5am this morning, the summit of Mount Washington is 43F, and 50F at 4000’ elevation. Just over half an inch of rain (14mm) was recorded at Harvard Cabin at 3500’. .3” of rain fell yesterday on the summit, with another .3” expected today before temperatures fall to the lower teens with a chance for 2-4” of snow and sleet before this system is done. Rainfall totals could exceed the forecast amount if intense thunderstorms pass over the range. Winds will remain elevated through the day from the NW 75 to 95 mph with gusts to 120. Seasonably cold tomorrow with temperatures in the teens F and W wind 45-60.
Primary Avalanche Problem – Wet Slab
Though natural wet snow avalanches seem unlikely today, there is always great uncertainty with this type of avalanche problem. Warm air and more rain will weaken bonds and add stress to existing wind slabs.
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 – Wind Slab
New wind slabs may develop later today as temperatures cool and precipitation turns to snow. Strong winds from the northwest combined with upslope snow showers have a history of creating this avalanche problem type. The timing of this problem remains in question so keep a close eye on the changeover to snow.
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.
A skier on the Sherburne Trail yesterday suffered a season-ending leg injury in the soft, sodden snow. Previously packed surfaces on trails and slopes demand flotation and if skis are your tool of choice today, remember that ungroomed mashed potatoes can make it very hard to make a turn. This applies to steeper terrain as well. Snow temperature at 20cm down is 0F but it is unlikely that our snowpack is entirely isothermal yet despite 24 hour settlement figures near 17cm! If you are keeping tabs on our snowpack for future ski missions, you’re likely thinking, “Uh-oh, another melt-freeze crust.” Though this rain is resetting our snowpack by erasing any concerns for the previous ice layer and any facets which may have developed nearby, it is creating the same problem all over again in a new location. Be wary of upslope snow fall over the next few days which may bond only tenuously to the new ice layer. As evidenced by the avalanches earlier this week, small amounts of new snow and high winds can create large avalanches in some of our terrain.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch but are heavily damaged by rain and warm temps.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 01/12/2020 at 6:55 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest
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