Avalanche Forecast for Sunday, December 2, 2018
This information was published 12/02/2018 at 7:09 AM.
The Bottom Line
Rain on snow will take place today, driving our concerns of wet slab and wet loose avalanches. With few areas in the lee of a southerly wind (save the Great Gulf Headwall), wet slab development this morning should remain minimal. As the day progresses, concerns are directed toward how the incoming rain will affect this morning’s snow. As rain totals will be on the small side and spread out over a long duration, the snowpack should accept the rain well, leading us to believe avalanche activity is unlikely today. All forecast areas will have LOW danger today.
Clear skies, no wind, and warm temperatures were the theme on Friday and Saturday. Today, Sunday, precipitation began just after midnight. With a 6am temperature on the summit of Mount Washington (6288′) of 21F and Hermit Lake (3800′) of 25F, light snow showers are widespread. As temperatures rise, this will change over to sleet, freezing rain, and likely rain at all elevations by the early afternoon. By the time we see the switch to rain take place at all elevations, forecasts are calling for 2-4″ of new snow and sleet with a SWE of 0.25″. This should arrive on S wind in the 30-50mph range. As the changeover of precipitation takes place, winds will quickly switch to the West. Rain totals for higher elevations will be around 0.35″ with more at treeline and below as we transition to rain sooner.
Primary Avalanche Problem – Wet Slab
Liquid precipitation totals for the day will be between 0.5″ and 0.75″. At high elevations, this will be about half snow/mixed precip and half rain while lower elevations will see more rain. Precipitation that falls today will create an upside down new layer on top of our snowpack with snow at the bottom, topped with sleet and finally rain. Due to the relatively low amount of precipitation and slow rate at which it should fall, it is unlikely to produce natural or human-triggered avalanches. Areas of melt/freeze crust do exist, but it seems the amount of rain forecast is small enough to prohibit the lubrication of the overlying slab.
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 – Wet Loose
Wet loose avalanches are unlikely today, but will be more prone than wet slab at lower elevations due to greater amount of rain. Pay attention to when the transition to rain takes place and how quickly it saturates the new snow.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Prior to this morning, wind slab was widespread through avalanche terrain. Signs of natural avalanche activity were visible Friday morning, likely having occurred mid-week during the loading event. Many slopes were tested by skiers and climbers on Friday and Saturday with no further avalanche activity. South facing slopes saw significant solar gain and have a currently have a refrozen melt/freeze crust. North, east, and west aspects stayed cold enough to avoid this crust, though the warm temperatures increased stability to the point where we now see avalanche activity in this wind slab as unlikely.
Rain on snow will create tough hiking conditions as the day progresses. Be prepared with flotation for all modes of travel to avoid post-holing.
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 12/02/2018 at 7:09 AM.
USDA Forest Service
White Mountain National Forest