Avalanche Forecast for Thursday, March 14, 2019
This information was published 03/14/2019 at 7:10 AM.
The Bottom Line
Wind slabs sitting on old ice crusts will experience rapid warming today. Most of these slabs are firm but they are also over softer snow. As these slabs warm and weaken, the unlikely possibility of a larger sized avalanche will increase. When surface slabs begin to get wet and sloppy today, human triggered avalanches will increase in likelihood. Loose wet avalanches or sluffs are one sign that this process is getting started. Larger slopes and gullies below 3500’, like those in Crawford Notch, may produce an avalanche large enough to hurt you or even bury you as well. MODERATE avalanche danger exists today with larger avalanche possible.
A warm front arrives today and will push temperatures to near 40F on the summit and near 50 in Crawford Notch. Expect calm winds this morning from the west at 10-12 mph before shifting southwest and increasing to 25-40 mph. Currently, the temperature on the summit is 28F with about ¾” of new snow in the past 24 hours. Some drizzle is possible later this afternoon. Warm temperatures and showers will continue overnight and into tomorrow before temperatures drop and snow showers return. It looks as if dust on crust and long sliding fall conditions will return on Saturday.
Primary Avalanche Problem – Wet Slab
Weak layers of softer snow slowly gained strength in the past 24-48 hours. Expect reduced stability today as warm temperatures and a bit of rain weaken bonds in surface snow and send free water into the snow pack. Older wind slabs scattered around the range are bordering on the definition of persistent slabs which could create a larger avalanche than expected.
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
Areas of softer snow will be affected more quickly by sun and warm temperatures. Wet loose sluffs may occur on or below steep areas, especially on or below cliffs that absorb sunshine. This type of avalanche is a warning sign for the more dangerous wet slab avalanche type. Wet loose avalanches or sluffs can add load to and trigger a slab.
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.
Our snowpack structure bears a resemblance to the 2013-14 snowpack that produced a very large and destructive avalanche on the summit cone on March 29th of that season. This year’s melt/freeze crusts have facilitated large avalanches this season as well. Icy crusts and cold temperatures are one recipe for weak crystal types to form deep in the snowpack. Weather conditions this season have created enough crusts and cold temps to put deeper weak layers on the list of things to consider when travelling around the forecast area when temperatures rise and melting begins. Our wind driven snowpack and avalanche problems result in a very spatially diverse snowpack which fortunately keeps that type of avalanche unusual here. Unusual avalanches are one reason that make it a good idea to carry a beacon, probe and shovel and minimize your exposure in avalanche terrain.
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 03/14/2019 at 7:10 AM.
USDA Forest Service
White Mountain National Forest