This information was published 04/19/2019 at 7:02 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wet avalanche types will come into play today. In addition to managing your sluff, consider that the most recent wind slabs will continue to heat up and lose strength. As our snowpack continues to warm and become saturated, the concern for deeper weak layers being impacted or water flowing onto buried ice crusts will increase. In addition to avalanche hazard, remember that falling ice and rocks, undermined snow above drainages, emerging crevasses, and moats around rocks will become a problem starting today. Consider what hazards exist overhead and beneath your feet, and don’t linger in runouts. All forecast areas have LOW avalanche danger though more rain than forecast today, or greenhouse solar gain, could create more hazard. The exception is the Headwall area of Tuckerman Ravine which has MODERATE avalanche danger due to the increasing volume of flowing water there.
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The summit temperature rose above freezing at 6pm last night. A quarter inch of rain fell overnight with another tenth of an inch possible today. High temperature on the summit will rise further and ultimately reach the high 40’s F. The warm and rainy trend will continue through the weekend with heavy rain on the way Saturday and Sunday bringing up to two inches of rain to our snowpack and no freeze overnight.
Primary Avalanche Problem – Wet Slab
Newer, whiter snow that loaded in earlier in the week will be first to react to the warm-up. Be increasingly cautious when softening allows foot penetration into wet snow to reach your boot tops. Wet loose activity often precedes wet slabs. Water flowing in known drainages also increases the risk.
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.
Rain on snow caused this wet slab avalanche release last year in the Lip/Headwall area of Tuckerman Ravine. The potential for this type of avalanche will increase today through the weekend. The crown height on the right side is close to 20′ thick. Our snowpack is currently much deeper than that pictured.
Wet slab avalanches require weather that allows extended periods of temperatures above freezing, intense solar radiation, and rain on snow, especially dry snow. The combined effect of more than one of these is greater than the sum of the parts. While our snowpack is not dry in the upper meter of so, it is likely that preserved weak layers still exist deeper in the snowpack. Wet avalanches are notoriously hard to predict. That fact, combined with a snowpack that is thicker than in recent memory, brings a lot of uncertainty to any predictions of when or where avalanche activity will occur and how large the resulting avalanche will be. Fortunately heavy rain tends to keep most people home but footprints in the snow following other warm-ups suggest that some folks can’t resist the urge to be out in the weather. If that person is you, this weekend may be a good time to at least avoid major avalanche paths.
A view of the runout that reached into the drainage and almost to the rescue cache. Note where the trail enters the floor of the Ravine. Rocks and wet debris would make this avalanche unsurvivable.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, though this is changing by the day. Expect ice patches, opening stream crossings and the occasional bare patch.
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 04/19/2019 at 7:02 AM.
USDA Forest Service
White Mountain National Forest
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