Avalanche Forecast for Thursday, February 20, 2020
This information was published 02/20/2020 at 7:03 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Natural avalanches occurred late Tuesday or early Wednesday morning. Strong wind since that time has continued to build wind slabs in east facing terrain. These wind slabs are likely to be stubborn in most areas but large in isolated areas. Evaluate these areas of smooth, wind drifted snow carefully for signs of instability such as cracking or soft layers underneath. MODERATE avalanche danger exists where this wind loading has occurred with LOW danger in areas scoured to an ice crust.
Yesterday, the summit observatory recorded 1.1” snow (.13” SWE) with heavier snow squalls to the south and east of the summit yesterday morning following the 5-10” that fell Tuesday. Wind was steady in the 70-80 mph range from west and then northwest.
Today, high pressure will dominate the eastern US with clear skies and cold temperatures on tap for a while. Temps will only reach -10F or so today with wind dying down to 30-45mph through the day. Continued cold tonight, down to -15 to -20.
Tomorrow, the dry spell continues and will persist through the weekend, with the next chance for precipitation arriving Tuesday. Temperature will rebound to the single digits above zero with west and northwest summit winds continuing in the 40-50 mph range.
Primary Avalanche Problem – Wind Slab
Limited observations yesterday confirmed that heavy wind loading occurred from west and south winds on Tuesday and early Wednesday. Include north facing aspects, where that southerly wind created wind slabs, when making your travel decisions. While the wind speed record yesterday makes it likely that these newer slabs will be stubborn and resist triggering, this has not yet been confirmed.
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.
Clearing conditions yesterday afternoon allowed a look into the Gulf of Slides. Though afternoon shadows, continued fog, and blowing snow reduced visibility, the crown near the top of Main Gully was clear at times and only partially reloaded. The crown thickness and debris pile led me to estimate a R3D2.5 size though the knoll at the base slowed and captured most of the flow; maybe an R4 if you exclude the historical runout. In any case, the debris swept through the trees on the lookers left of the gully and is a good reminder of the danger of approaching these gullies from below in periods of active loading. The trees in the start zone and in the path do not serve as anchors or protection in any meaningful way.
We hope to gather some more field observations today to fill in the gaps on the current state of the snowpack following this round of snow and avalanches. Snow accumulations vary widely through the forecast area and we are looking to get a handle on the redistribution due to the south wind and any avalanches that may have resulted. Please submit photos or a quick observation if you brave the cold today.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch and offered up fine powder skiing yesterday.
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 02/20/2020 at 7:03 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest
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