Six inches of new snow overnight arriving on increasing NW wind makes wind slab our avalanche concern today. No visibility this morning adds a degree of uncertainty to today’s forecast. We expect the snow to struggle to adhere to the ice crust which will act as today’s bed surface. This highlights a key characteristic about today’s avalanche problem; wind slab will likely be touchy to human triggers. In Moderate rated areas, the combination of wind and the slick bed surface may allow scouring to take place leaving areas of bed surface exposed. Considerable rated slopes, those in the lee of our fetch, should hold much larger areas of wind slab with limited to no old surface exposed. The potential for large avalanches exist in these areas with the possibility of natural avalanches due to continued loading through the day. The floor of Tuckerman Ravine today will be avalanche terrain as we have seen avalanches run far this year on well-developed slide paths. The hard bed surface will demand the use of crampons and an ice axe for those choosing to travel in avalanche terrain.
Due to the rugged ice crust on the mountain, you’ll find two distinct threats to safe travelling in steep terrain. The recent sleet storm finished with freezing rain which created a thick ice crust over top of a widespread glaze of sleet, wet snow and ice. New snow fell at the end of the storm on Tuesday afternoon and was able to stick in some areas, but not very well. Due to scouring of the snow, wind exposed, scoured areas above tree-line will require crampons for safe travel while many lee areas have collected enough snow to build some wind slabs. Low visibility, and other duties kept us from gathering many observations yesterday and this morning, but it seems that the ice crust will be the dominant surface in steep terrain. In areas where the new snow adhered to the ice, hand shears low in Hillman’s Highway showed that the new wind slabs failed easily in the new snow. Only a small amount (2-3”) of the recent snow was light enough to be carried by the wind and blown into our terrain but the icy bed surface will up the ante if you get swept off your feet. Human triggered avalanches are unlikely today unless you seek out these areas of wind slab. Additionally, light winds may allow the 1-2” of new snow that falls to build into small but more sensitive, new wind slabs. If we receive the upper end of the forecast amount, human triggered avalanches will become more likely and increasingly widespread.
The amount of snow available for transport by increasing westerly winds is driving the avalanche problem today. Deeper areas of new snow may be attractive for skiing and riding but bear in mind that the new snow may not be well bonded yet to the old refrozen surface. As wind increase today, be on the lookout for wind slabs growing over a foot thick and may fail on softer new snow beneath. Lower angled terrain may have the best skiing since the icy bed surface will be lurking just below the new snow. The potential for wind slab avalanches in many areas is a reminder that it is not springtime in the mountains yet.
Wet slab avalanches will be possible to human trigger this morning, with colder temperatures gradually refreezing the snowpack and reducing instability late today into tonight. Multiple layers near the surface may be capable of producing varying sized avalanches. First, sluffing action of ice pellets yesterday proved to create unstable and reactive, though isolated, slabs low in avalanche paths. These and similarly new slabs formed from the mix of sleet (ice pellets), snow, freezing rain, and rain may present pockets of instability in much of the terrain, though wetting from rain overnight has likely reduced sensitivity to a human trigger. Second, deeper slabs which have become increasingly wet could produce large avalanches, also possible to human trigger and unlikely to avalanche naturally, though we’re not ruling out this possibility entirely. Finally, snow falling today and tonight on increasing W wind may build to form new wind slabs by the time this advisory expires at midnight tonight.
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.
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.