Avalanche Forecast for Wednesday, April 29, 2020
This information was published 04/29/2020 at 7:08 AM.
The Bottom Line
Significant snowfall at higher elevations on Monday formed wind slabs which may become unstable from direct sun and warm temperatures today. Wet slab avalanches are possible with a human trigger today, therefore the avalanche danger rating is MODERATE. Small wet loose avalanches may occur in specific areas and may have enough force to carry you into undesirable terrain. Steep terrain where wind slabs exist from the previous storm should warrant caution. Wet avalanche problem types share the same indicators; roller balls, rapid warming and increasing depth of boot penetration. Areas to be mindful of today are aspects that receive strong solar effects, terrain features that absorb solar energy, such as near rocks and cliffs.
Travel has been difficult with untracked heavy wet snow at lower elevations, if you find yourself in mountain terrain today floatation is highly recommended.
Yesterday, NE wind decreased from 29 MPH into the teens by dawn. Small amounts of snowfall accumulated at the summit from upslope snow showers. Visibility was limited as fog and low clouds moved slowly through the Presidential range. Summit temperatures peaked at 27 F midday on the summit and 42 F at Harvard cabin.
Today, clouds will part and make way for clear skies. Summit temperatures are forecast to reach the upper 20s F. Wind will make its way around the compass rose, starting from the N shifting E and ending daylight hours from the SE. Gradually the wind speed will increase from 10-25 to 25-40 mph, with gusts up to 50 mph.
Tomorrow, wind speed will increase to 40-60 mph. Summit temperatures will gradually increase from low 30s to upper 30s F near dusk. NWS forecast for .05” of rain to fall during the day.
Primary Avalanche Problem – Wet Slab
Storm totals vary at weather plot locations, the summit received 4” while Harvard cabin recorded 13.3”. Wind slabs that were formed during the recent snow storm will receive a strong degree of solar radiation today. Warming in the snowpack could loosen bonds between old surfaces and old wind slabs. Predicting when and where a wet slab avalanche will occur is difficult, best practice is to avoid slopes that are being heavily affected by solar radiation and recognize that wet loose point releases are a sign of rapid warming.
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
A wet loose avalanche today could become more than sluff management. Heavy dense snow exists in the mountains today and could cause you to be pushed into an undesirable outcome at the bottom of a slope. If you must enter avalanche terrain the best way to mitigate wet loose avalanches is to choose terrain with clean run outs, make sure no one is above you on a slope and be mindful of signs of rapid warming.
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.
Direct sun and calm winds today will create warming within the snowpack which will, in turn, produce wet avalanche problems. Wet slab hazards are difficult to forecast and come with an inherent degree of uncertainty. Above freezing temperatures were present at some locations yesterday and temperatures will rise even higher today. The effects of heat transfer within the snowpack are difficult to forecast and require careful route selection and snowpack evaluation when in avalanche terrain. Clear skies will help provide good visibility today which will help with route selection and decision making.
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 04/29/2020 at 7:08 AM.
USDA Forest Service
White Mountain National Forest