Avalanche Forecast for Sunday, April 21, 2019
This information was published 04/21/2019 at 7:00 AM.
The Bottom Line
Wet slab avalanches today are becoming unlikely though could be large and destructive should they occur. These events are hard to predict and make lingering under avalanche terrain an unwise choice. In addition to wet avalanches, objective hazards that appear as the spring snowpack falls apart are now coming into play. Obvious holes in the snow, less obvious undermined snow that may collapse, cracks in the snow around rocks and cliffs, and icefall should all play into your decision-making process today. All forecast areas of the Presidential Range have LOW avalanche danger with the exception of the Headwall in Tuckerman Ravine, which has a MODERATE rating. This exception is made for the historic tendency of the Lip blow out from the flooding stream that runs under the snowpack in this location. Keep in mind that Low avalanche danger does not mean no avalanche danger, especially when the concern is hard-to-predict wet avalanches.
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Just over 1” of rain fell on the snowpack yesterday. Above freezing temperatures continued with the Summit (6288’) reaching 48F midday and Hermit Lake (3800’) peaking at 52F. Temperatures cooled slightly overnight with most locations currently sitting in the 40sF. Wind from the SW in the 35-50 mph range should diminish to 10-25 mph later today. Temperatures today should climb slightly, ultimately mimicking yesterday’s highs. Rain showers should cut off today, but fog and moisture in the air will likely still make the air feel wet. Wet weather should continue for the start of the week with cloudy skies and rain likely at times tomorrow.
Primary Avalanche Problem – Wet Slab
Wet, soupey, and gooey are unscientific terms that accurately describe the snowpack and are great indicators of the possibility of springtime wet avalanches. Wet slabs that are unlikely today tend not to follow any clear pattern, resulting in avalanches that are isolated, but also large and destructive. The driver of these avalanches is likely shifting from lubricating deeply buried crusts to flooding streams being directed into the snowpack. While the result is the same, this may help pin down likelier locations if you have intimate knowledge of the terrain and know where stream channels flow under the snowpack.
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
It is possible that skiers and riders will initiate sluff avalanches today, particularly in the steepest terrain. These will be small in size and not destructive, but they always have the potential to capture skis and carry a person to an undesirable location. With cracks opening in many locations, wet loose avalanches can have severe consequences in terrain that just days ago was seemingly benign.
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.
While the rain will largely pause for the day, our snowpack will continue to see above freezing temperatures. The long range forecast is hinting at a freeze for higher elevations finally returning Wednesday night. Falling apart or shrinking rapidly are accurate ways to describe our snowpack this weekend. Watch out for the following hazards that should play into today’s decision making:
- Opening creeks and streams
- Holes near trees, rocks, and cliffs
- Glide cracks on wide open snow slopes
- Falling ice
The speed of change the snowpack has seen over past few days is moderating. We believe the time of rapid change has passed and the current malleability of the snowpack can be seen as cracks in the snow. These cracks indicate the crack initiating in the avalanche process, but the snowpack is becoming uniform and can handle these stresses. While the result may be no slab avalanche, it does leave a permanent scar on the snow surface that will develop into a hole that must be negotiated for the remainder of the season.
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. 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 04/21/2019 at 7:00 AM.
USDA Forest Service
White Mountain National Forest