Monday thru Tuesday: Monday, the primary avalanche problems are isolated Wind Slabs becoming more wide spread Tuesday and possible Wet Loose avalanches below ~8,500’.
Wind Slab – Another storm system with moderate Southwesterly winds is forecasted to pass through the region today (Monday) into tomorrow (Tuesday) with up to 2” of new snow today (Monday), Tuesday - 5” to 10” in mid elevations, 6” to 12” in the upper elevations, which will form Wind Slabs in the mid to upper elevations on NW-NE-SE aspects. Wind Slabs will be relatively small and isolated Monday becoming more widespread and potentially larger by Tuesday. Be wary of hollow, drum-like sounding snow or shooting cracks.
Monday - Natural avalanches are unlikely, triggered releases possible on slopes of 35 degrees and steeper in the mid to upper elevations.
Tuesday – Natural avalanches are possible, triggered avalanches are likely.
Loose Wet – Mild overnight lows combined with light rain/snow below ~8,000’ and potentially 5” to 10” of wet snow Tuesday in the mid to lower elevations will maintain the Loose Wet threat below ~8,500’.
Monday/Tuesday: Loose Wet - Natural avalanches unlikely, triggered releases possible.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Trend ?Increasing Danger
Another spring storm system is passing through the region today thru Tuesday with moderate Southwesterly winds and up to 14” new snow forecasted by Tuesday evening in the upper elevations. Monday, as snow begins to accumulate and drift, small isolated Wind Slabs maybe encountered on in the mid to upper elevations, becoming more widespread, numerous, and potentially larger by Tuesday. Wind Slabs will likely be encountered on NW-NE-SE aspects in the mid to upper elevations above ~ 8500’. Riders will likely encounter these below ridgeline, near and around terrain features that promote drifting, crossloaded gullies and depressions. Be wary of hollow, drum-like sounding snow or shooting cracks.
Monday - natural avalanches unlikely, triggered releases possible.
Tuesday - natural avalanches possible, triggered releases likely.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Trend ?Decreasing Danger
Overnight lows were very mild, barely below freezing in many locations. Highs today (Monday) are forecast to be in the mid 40s to 50s in the mid elevations (8000’ to 10,000’elevation) with cloudy skies and snow or light rain, depending on elevations, which will maintain the threat of Loose Wet avalanches below ~8,500’. Watch out for rollerballs, which indicates the snow is loosing cohesion and instability is increasing. Loose Wet releases are dense and heavy, which can make it difficult to escape if entrained and can carry riders into hazardous terrain.
Monday/Tuesday - natural avalanches unlikely, trigger releases possible.
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.
Monday ~0400 Observations Low High (Sun) Snow SWE
Virginia Lakes (Elev. 9445’) 38 50 2” .2”
Ellery Lake (Elev. 9645’) 32 46 N/A .2”
Agnew Pass (Elev. 9355’) 36 46 0 N/A
June Mountain (Elev. 9148’) 35 48 2” .1”
Mammoth Pass (Elev. 9500’) 32 50 N/A .2”
Sesame Study Site (Elev. 9014’) 33 49 5” .9”
Rock Creek Lakes (Elev. 9600’) 35 48 1.3” N/A
Sawmill (Elev.10200’) 32 45 1” N/A
South Lake Cabin (Elev. 9580’) 33 50 N/A 0
Mon thru Tuesday - An upper level low continues to push into the Pacific Northwest, expect moist upslope flow to produce light showers through the morning across the Sierra with snow levels around 7,500` to 8,000’. A more pronounced moisture tap arrives in the northern Sierra this afternoon into Tuesday with 0.50-0.75" in northern Mono county, .25 -.5” south of Mammoth. Tuesday, the frontal boundary moves through the northern Sierra with snow levels between 7,000 – 7,500’. The heaviest precipitation will occur late Tuesday morning into early afternoon with frontal passage. Most shower activity will diminish Tuesday evening.
Wednesday thru Thursday - The last in a series of shortwaves is forecast to move through northern CA Wednesday night and Thursday. Snow levels will fall to 5000-6000 feet, temperatures may stay at or slightly above freezing and limit snow accumulation to above 6500 feet. Snow amounts are expected to be light and spotty as the track of this system stays mostly north of that area. The peak period for showers will be from midnight to noon Thursday with activity tapering fairly quickly by Thursday evening.