Spring has made a resurgence after the last late season Atmospheric River event with steadily warming temperatures and the occasional fast moving spring storm passing through the region, temporarily ushering in cooler temperatures, gusty winds, and a few inches of snow followed by rebounding temperatures and clearing skies. The pattern sets the stage for Wind Slabs in the mid and upper elevations followed by the inevitable Loose Wet cycle that follows as temperatures climb after frontal passage. Pretty standard spring conditions.
The last significant weather system to impact the region was an unusual late season Atmospheric River event that impacted the region last week (4/6 - 4/8) with 1.2” to 6” of water along the eastern Sierra by the evening of 4/8. As per usual with these storms, it started out warm, with heavy wet snow above about 7500’, then cooling toward the latter half of the storm with lighter density snowfalls as the system exited the region. Storm totals for the 3 days ranged from 19” (Tioga Pass) to 37” (Mammoth Mountain) with the heaviest snow amounts recorded from June Mountain south. The new snow bonded relatively well to the old snow surface. However, the system came in with moderate to strong Southwesterly flow, which is typical of Atmospheric Rivers systems, with winds gusting over 100mph along ridgelines. The moderate to strong Southwesterly winds formed Wind Slabs primarily on NW-NE-SE aspects in the mid to upper elevations Thursday thru Sunday, which remained reactive through Monday (4/10) with reports of natural avalanches observed in Lundy canyon. Mild weather and light winds returned Monday and Tuesday, allowing the recently formed Wind Slabs the time needed to strengthen and stabilize. As the sun returned, temperatures began to quickly rebound post-storm, resulting in the usual springtime Loose Wet avalanches cycle on W-S-E aspects in the mid elevations, expanding to include NW - NE aspects in the lower elevations where a snowpack is present. Wednesday (4/12) winds began to pick-up again in advance of an approaching storm system forming another round of Wind Slabs on NW-NE-SE aspects. This primarily affected slopes above ~9000’ with extended upwind fetches with plenty of fresh snow available for transport. Thursday (4/13), the storm system had moved in land with moderate to strong winds and a quick shot of moisture with up to 9” (3 to 6” most locations) of snow before moving off to the east overnight. The moderate to strong winds associated with the system redistributed the new snow onto leeward slopes forming Wind Slabs predominately on NW-N-NE-E-SE aspects in the mid to upper elevations. Friday thru Saturday (4/14 – 4/15), sunny skies returned with warming temperatures with a few Loose Wet avalanches reported. Sunday started out sunny and spring like but by midday clouds where on the increase with snow showers in the upper elevations and beginning the next round of Wind Slab formation.