The BWRA could see higher than average snow totals for this storm event due to upslope precip enhancement predicted for Mono County Saturday pm through Sunday. Look for weaknesses within the Storm Slab itself - especially due to the showery nature of the storm and intervals between precip waves. Also pay attention to the bond between new snow and old snow surfaces.
Moderate to strong westerly winds will form Wind Slabs in many locations in the BWRA, predominantly on easterly aspects. The complex terrain of the BWRA combined with high winds can build windslabs far downslope from ridgetops and in irregular locations.
The Bottom Line: Almost all signs indicate widespread stability in the existing snowpack in the BWRA. Watch for storm-related instabilities with the incoming weather event later today through Sunday (Storm Slabs and Wind Slabs).
Discussion: A very strong melt/freeze crust (7cm - 40cm thick) at or near the surface is prevalent across all aspects in the BWRA. Some pockets of soft windslab and faceting snow (particularly on northerly aspects) persist on top of this crust. These pockets of near-surface faceting and lighter density snow, when sandwiched between new slabs and the crust below, could become active failure zones, resulting in Storm Slab and Wind Slab avalanche problems.
Also observed in many places are facet layers developing below this very strong surface crust, as well as below crusts buried deeper in the snowpack.
Persistent weak layers (PWL's) forming the base of the snowpack are widespread on north, east, and west aspects, adding the potential for storm-related avalanches to "step-down" to these Deep Slab instabilities. These PWL's did not react to the Feb 6-9 storm event, and have reacted only occasionally to column tests. Isolated terrain features such as steep, rocky, north aspects may be suspect for this "step-down" avalanche behavior.
No natural or triggered avalanches, shooting cracks, or collapsing/whumpfing have been observed in the BWRA recently. Ski cuts, jumping on small test slopes, kicking cornice, and stomping out small slabs isolated by stacked skin tracks have yielded almost no results (no propagating results, occasional failures directly under skis).
Multiple snowpits and column tests - including Rutschblocks - have shown few results. Some planar fractures have been identified in near-crust facet layers, though most have been hard to trigger, broke through the column, and/or did not show propagation. One column test (CT) on a NW aspect at 9,000' yielded a planar fracture upon excavation in the facets below the surface crust. All Rutschblock tests showed no results from standard loading procedures; "edge-of-block" failures were possible to trigger via non-standard (excessive) loading procedures.
A cold front and low combination progresses south today from the Pacific Northwest Coast. Moisture wrapped into the low will impact the Sierra Crest and Eastern Sierra Front starting this afternoon. The Tahoe Basin is favored early, then the Eastern Sierra of Mono County as the flow turns Northeast through Sunday afternoon, adding upslope enhancement to the precip. The overall showery nature of the storm means snow totals will be highly variable. For Mono County, 6"-10" of snow is expected in the mountains, 2"-4" of snow in the valleys.
The storm also brings lower temps and moderate to high winds, so expect cold mountain snowfall to actively redistribute to form Wind Slabs.
The National Weather Service in Reno is predicting another precip event for the Eastern Sierra early next week, after this weekend's event clears out of the area. Details are minimal at this time.
This snowpack summary applies only to backcountry areas in the Bridgeport Winter Recreation Area. Click here for a map of the area. This snowpack summary describes general avalanche conditions and local variations always occur. This snowpack summary expires in 48 hours unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.
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