Loose Wet – Spring is a challenging environment for avalanche forecasting because of the often opposing weather inputs. Strong northerly winds early yesterday helped keep temperatures cool in the Alpine regions while from ~ treeline and below, light winds and mostly clear skies allowed temperatures to climb into mid 30’s to low 50’s (9000’ to 10700’) at a number of reporting stations. This was enough to see the surface snow begin to warm significantly, even up into the Alpine on favored aspects. However, the key element that was lacking in some locations was loose surface snow. Areas where there was loose surface snow (i.e., Mammoth to June), the threat of Loose Wet was more evident. A few Loose Wet releases were noted but not as numerous as expected. Elsewhere, areas with denser surface snow (wind slabs, avalanche scoured slopes, etc.) were slower to thaw because the tightly compacted snow is somewhat resistant to becoming loose and incohesive as it thaws. Today (3/28) overnight lows ranged from upper teens to low 30’s and highs are forecasted to run between upper 30’s to low 50’s, which will drive more significant thawing of the surface snow. For all elevations, anticipate Loose Wet on W-S-E aspects. Today, large natural avalanches of D2 or greater are unlikely (small natural avalanches are possible), triggered releases are possible, trending toward likely in the low to mid elevations, on solar aspects (W-S-E), especially where there is significant loose snow.
Wind Slabs - The recent series of storms were accompanied by strong SW winds forming widespread lingering Wind Slabs on NW-N-NE-E-SE aspects. These Wind Slabs have generally had enough time to strengthen and bond to the underlying snow and as a result, less reactive but a potential lingering concern in steep complex terrain. Strong Northerly winds Tuesday above 10,000’, likely produced small isolated shallow Wind Slabs in favored locations (leeward slopes, under cornices, near rock outcrops, areas with large windward fetches with loose transportable snow). The lingering Wind Slabs have likely had sufficient time to strengthen/bond and are expected to be stubborn to trigger, though not impossible. Stubborn Wind Slabs may be encounter on W-S-E aspects, near and above treeline(~10000’ and above). I don’t expect this to be a widespread problem. More of a concern in favored localized areas.
Persistent Slab - The recent spat of storms brought 4 to 6’ of snow and has helped turn the corner on a rather lean season. The series of storms began to roll into the region in early March and continued through 3/22 with the last storm producing the biggest avalanche cycle of the season with impressive debris fields in many of the runouts throughout the range. The heavy dense snow accumulations have buried the persistent weak layers deep in the snowpack reducing the potential for triggered release. However, we have limited information concerning this problem. We can use your help with obtaining additional information on the distribution and sensitivity of this layer, especially outside of the Mammoth and June region.