Rain, which could be heavy today, will make wet slabs today’s primary avalanche problem. Today’s precipitation type will be the primary driver of instability in the snowpack. If we receive more plain rain, wetting and warming will decrease stability through the day. A greater proportion of frozen mixed precipitation will have a lesser impact on the stability of our currently refrozen and stable snowpack. Today’s Moderate rating is distinctly different than Moderate ratings issued a week ago. Last week, new snow and wind made small avalanches possible in a number of areas. Avalanches were much more likely and much smaller than the large avalanches which are possible but less likely today. It’s a low probability and high consequence kind of day. If the weather doesn’t keep you out of the mountains, be mindful that though natural avalanches are unlikely today, they could be large and run a great distance.
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We are currently in the freeze portion of a prolonged melt-freeze cycle that began on Friday which is largely eliminating avalanche concerns for the day. With continued below freezing conditions today, long sliding falls will be much more of a hazard today. Crampons, an ice axe, and the ability to use them will make travel in avalanche terrain possible. Climbers should be aware of the potential for ice dams as the current freeze may trap flowing water. This trapped water will be looking for a pressure release valve in the form of an ice tool or screw placement. The thaw over the past two days has also reopened holes in the snow on the Little Headwall, making this no longer an advisable route to exit Tuckerman Ravine. Many skiers and riders were seen removing skis and boards and trying to down climb the combination of refrozen snow, ice, and verglassed rock. Hiking down the Tuckerman Ravine Trail to Hermit Lake will be both faster and safer at this point.
Wet avalanches due to melting and liquid precipitation will be possible today. These avalanches are likely to be limited to loose-wet or point release type but a larger, wet slab avalanche is not out of the question in areas that previously had thicker layers of dry wind slab. The Lip, Center Bowl, and Central Gully fit this profile and deserve extra caution. A small wet slab or loose-wet avalanche can also serve as a trigger for a larger avalanche or entrain more snow than expected. Mind your runouts today if you venture into steep terrain and protect yourself and your belayer when climbing into larger snowfields. Avalanche concerns will diminish with the return of freezing temperatures later today and be replaced by slide for life conditions once surfaces have refrozen.
Wet slabs will become the primary avalanche problem today as prolonged above freezing temperatures continue to warm the recently dry upper snowpack. The sun may make an appearance today. If we see sunshine for an extended period, human triggered wet slab avalanches may push toward likely on southerly aspects. Without lengthy sun affect, the soft slabs that blanket much of our terrain should behave similarly on all terrain in the ravines. Loose-wet sluffs and point releases should be on your radar, and like wet slabs, will be possible to human trigger on many aspects. It’s worth noting that wet slab avalanches are notoriously difficult to forecast, with natural avalanches tricky to predict and the tipping point between unreactive and touchy to a human trigger often being a very fine line.
Small to medium sized wind slabs will become more of a threat later in the day as southwest wind increases and loads slopes further. Wind sheltered locations, especially Left and Chute in Tuckerman Ravine, yielded good powder skiing yesterday and also escaped the wrath of the hot April sun. These same aspects may receive more wind loading later today and become increasingly susceptible to triggering. If you are out in the afternoon, be on the lookout for increasing SW wind blowing and cross-loading snow from the alpine into start zones above. Wet loose avalanches may also become a problem, first as sun heats south facing slopes and later as slopes receive some rain or mixed precipitation. These smaller point release type avalanches can have remarkable pushing power as they entrain snow. Monitor your choice of slope for increasingly moist and dense snow and consider the consequences of a slide. Either avalanche type would occur over a bed surface which is icy and hard enough to be a challenge to arrest a fall on.
Wind slabs will be the primary avalanche concern today, with medium sized avalanches in moderate rated areas causing the most concern. If winds remain as light as forecast, our avalanche problem will be limited to small human-triggered, dry loose avalanches and small to medium sized wind slabs. Avalanche activity early this morning points to the fact that the new snow remains unstable and poorly bonded to older, firm wind slabs and ice crust. While likely to be small, avalanches today may occur on a bed surface which is icy and hard enough to be a challenge to arrest a fall on. Recent avalanche activity and variable spring weather is creating a wide range of snow conditions with older, stubborn wind slabs and a hard, icy crust hiding beneath the low density new snow. Our slopes are not at all the stable corn snow conditions that you might expect for mid-April so continue to reduce your exposure in avalanche paths, carry avalanche rescue gear and manage your risk of long sliding falls carefully.
Wind slab formed since late Sunday is our primary avalanche problem. Generally on the thin side and thus not likely to produce large avalanches, this layer remains possible to human trigger. It will likely vary from stubborn to touchy across the terrain. A secondary avalanche problem is the older and now generally stubborn wind slabs which formed on Friday and Saturday nights, the culprit of our widespread cycle of somewhat small human triggered avalanches over the weekend. This layer is beneath the more recent slabs in some areas and at or near the snow surface in others. Low rated areas have significant old refrozen crust at the surface, presenting a long sliding fall hazard while still holding small pockets of wind slab. It’s certainly a “Low” doesn’t mean “No” avalanche danger kind of day. Moderate rated terrain holds more widespread wind slabs, though it will be difficult for travelers to visually discern thin from thicker slabs as well as the older and more stubborn slabs from the newer and touchier slabs.
Wind slab formed Friday night is our primary avalanche problem, with a secondary problem of smaller wind slab formed from the few inches of new snow that fell on strong W wind last night. The older wind slab has been observed up to 14” thick. It varies in thickness and bonding to the old refrozen crust beneath it, but has gained some strength since Saturday’s widespread human-triggered avalanche cycle. Visibility is limited this morning, but we expect that the new wind slab exists in relatively small pockets throughout the terrain. Today is a “small avalanches in many areas” kind of Moderate rating. We don’t expect any terrain to produce particularly large avalanches, with the old wind slab showing variable stability and the new wind slab suspected to be small. Relatively small avalanches can still bury, injure, or kill a person, especially if they happen in high consequence terrain with rocks, cliffs, vegetation, or other hazards in the runout. If avalanches aren’t enough to make you choose lower consequence terrain, be aware that the refrozen crust which exists at the surface in areas is very hard and could easily allow a long sliding fall.
The 6” of snow that arrived Friday night on increasing west wind formed wind slab in all forecast areas. This wind slab proved touchy to human triggers yesterday with skier-triggered avalanches in Lobster Claw, Chute, Hillman’s Highway and the Little Headwall. Slab depths were up to 14” thick and while nobody was buried, enough snow was entrained to carry at least five people downhill in the Hillman’s avalanche. Size and distribution of this avalanche problem is aspect driven. East facing slopes (Considerable rated areas) contain more widespread wind slab capable of producing large avalanches. North and south-facing slopes (Moderate and Low rated slopes) contain smaller areas of wind slab that could produce a small avalanche.
On April 7, 2018, seven avalanches occured in Tuckerman Ravine. All were human triggered. This write-up discusses the weather and snowpack that lead up to these events, an objective summary of the events, and an analysis of factors that lead to at least five people being caught in one of the avalanches. Weather In the […]
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