Avalanche Forecast for Tuesday, April 9, 2019
This information was published 04/09/2019 at 6:58 AM.
The Bottom Line
Wind slabs formed overnight have been capped by a thin freezing rain crust in many areas but remain possible to human trigger. New wind slabs may also develop late today from new snow and increasing wind. Variability and variety define our recent and forecast weather, with snow, sleet, graupel, freezing rain, and rain all observed in our terrain yesterday. If you have good visibility today, you may be able to avoid travelling on the avalanche problem by choosing distinctly grey, old crust. Weird weather makes weird avalanches, so make careful and continual observations and realize that familiar terrain may act in unfamiliar ways. All forecast areas have MODERATE avalanche danger.
Snow, sleet, and freezing rain yesterday totaled 3.3” of accumulation on the Summit of Mount Washington. Lower elevations received similar precipitation types with some rain also mixed in. The Hermit Lake snow plot recorded 2.4” of snow capped with a thin freezing rain crust, while Gray Knob recorded only a trace of snow but also a crust. Low temperatures in the upper teens F yesterday occurred around midday throughout our terrain and have since wavered around 32F. Southerly wind under 40 mph on the summit yesterday shifted W and increased to 60 mph overnight. Today, wind should decrease and briefly blow from the south before shifting W 20-30 mph and increasing slightly this evening. Additional snow and mixed precipitation is again forecast, with peak precipitation late this afternoon resulting in 1-3 inches total. Another 1-3 inches of snow may fall tonight. Temperatures near or above freezing throughout our terrain today will drop slightly tonight but be similar tomorrow. Expect precipitation to taper off tomorrow and wind to shift NW and increase.
Primary Avalanche Problem – Wind Slab
Slabs formed of yesterday’s snow and sleet from both wind transport and sluffing action. These mostly small slabs have likely been capped by a thin freezing rain crust. Variable precipitation types yesterday should keep the possibility of new slabs that aren’t capped with a crust on your mind, as well as larger pockets. You’re most likely to find these slabs on the eastern half of the compass rose, in the lee of last night’s W wind. We expect this new layer will be stubborn to a human trigger where capped by a crust and more sensitive if not capped by a crust. Additional snow late today could also form small and sensitive wind slabs into the evening hours.
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.
Spring conditions have tried to take hold in the past week, but winter is hanging on. New snow, sleet, graupel, freezing rain, and some rain yesterday fell on top of a mixed snow surface of warmed wind slabs and crust. Westerly wind overnight combined with additional mixed precipitation which likely varied across the range, including significant freezing rain. Both new crust and wind slabs, generally variable conditions from an odd precipitation mix should be on your radar today. A significant melt/freeze event just over a week ago resulted in a surface crust and stable conditions, and it has since been followed by several small snow storms. Wind slabs formed primarily on the eastern half of the compass rose last week. These were small in size and somewhat isolated, interspersed with exposed old crust. Slight warming Saturday and Sunday, especially on southerly aspects, helped these wind slabs bond with each each other and to the underlying crust. We have minimal stability concerns outside of new slabs formed since early yesterday.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with some open water at low elevations.
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. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit 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 04/09/2019 at 6:58 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest