Avalanche Forecast for Thursday, April 18, 2019
This information was published 04/18/2019 at 7:01 AM.
The Bottom Line
New wet snow falling this afternoon on high winds may contribute to wet snow avalanche problems as older wind slabs turn wet. Wet loose avalanches, or sluffs, are also possible today, even on the older snow surface. Sluffing will be more intense in areas of Monday’s drier snow, which appears white when compared to the darker, melt/freeze surface. Beware of the sliding fall potential that exists today particularly on shady areas this morning. Any avalanche activity today should be easily avoidable and limited to the more recent snow becoming wet in the sun or being further loaded this afternoon by up to an inch of new snow drifting on increasing winds. LOW avalanche danger exists today in all forecast areas.
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A warming trend starts today which will eventually bring above freezing temperatures to the summit elevations. The freeze line is around 4300’ on the Auto Road with Gray Knob at 25F and 36F at Hermit Lake at 5:30am. Temperatures will continue to climb to 39F on the summit by mid-afternoon. Wind from the southwest, currently at 32mph, will increase through the day with 45-60 mph mid-day and 60-70 mph range by nightfall. Mostly clear skies this morning will gradually fill in with cloud cover as precipitation arrives this afternoon. A trace to an inch of wet snow may fall this afternoon. Another low pressure system brings more rain showers tomorrow with temperatures near 47F on the summit.
Primary Avalanche Problem – Wet Slab
Wind slabs that formed from snowfall on Monday have been unreactive but could become wet today if precipitation turns wet. An inch of snowfall or rain on increasing winds today on our existing icy bed surface could set the stage for some small avalanches. Keep a close eye on the weather today.
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 snow avalanches may grow large enough to push you around the terrain today. Consider strategies to manage these and bear in mind that more recent snow may be harder to handle and entrain more snow than the older melt/freeze surfaces.
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.
There has been no reported avalanche activity since April 11th. Yesterday, the inch of new snow which fell on Monday provided quality turns in the Tuckerman Headwall but the shadier Ammonoosuc Ravine failed to soften. Today, the strong April sun should have time to work it’s magic again before clouds move in and new snow or rain falls this afternoon. All aspects should soften today as the freeze line rises to summit elevations. The deeper snowpack is generally isothermal which reduces our concerns for larger avalanches. No freeze overnight will bring a return of significant hazards due to melting tomorrow. Lots of rain on snow increases the risk of undermined snow over swollen rivers, along with ice and rock fall. From a recreationist’s perspective, nothing good comes from rain on snow.
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.
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 04/18/2019 at 7:01 AM.
USDA Forest Service
White Mountain National Forest