Natural windslab avalanches were observed today above Leavitt Lake. The colder snow and moderate to strong winds of the second storm wave have probably constructed windslabs in many leeward locations throughout the BWRA's alpine terrain.
A natural storm slab avalanche was observed on Sunday (the first storm wave) on a North aspect at 9000'. Below 10,000', the weaknesses within the first storm wave will probably gain strength quickly due to the warm, consistent temperature profile of this layer (0degC throughout). The colder snow above 10,000' and from the second storm wave is a bit more of a question mark. Watch for lingering storm slab instabilities at higher elevations and within the second storm layer.
Apology: Sorry for the delayed Summary: I had to change out of my ski clothes and into my Nomex on Friday afternoon to help out with the wildlfire racing into the Bridgeport neighborhood of Evans Track. During the wind, wildfire, and precipitation events, Bridgeport lost power, internet, and cell service. Most services are now back up and running.
Discussion: The existing snowpack in most places in the BWRA prior to this weekend's storm was capable of supporting a heavy new load, though the surface in many places was very hard/glazed. The big question was: how would the new snow bond to the old snow surface?
The storm came in two waves: the first was Friday afternoon (Feb 6) through Sunday afternoon (Feb 8) and came in warm with strong winds (we got soaked in rain up to about 9000' on a tour on Sunday afternoon and measured 37cm of settled new snow at Leavitt Lake / 19" new snow reported by the Snotel site). The second was Sunday evening (Feb 8) through Monday morning (Feb 9) and came in cooler with moderate to strong winds (7" new snow reported by the Snotel site).
Evidence suggests the storm snow from the first wave has bonded well to the old snow surface. A snowpit above Leavitt Lake (9700') on Sunday, February 8 showed the old snow surface had turned to slush (possibly due to the precip event starting as rain, even as high as 9700'). The rain then turned to warm, dense snow - the classic Sierra Cement - which mixed with the slush as it froze. A temperature profile of the first wave of new snow showed 0degC throughout the layer and into the frozen slush beneath. This is a near-perfect scenario for the new snow to bond to the old layer.
The second wave of storm snow is a bit more of a question mark, as are the higher elevation avalanche paths: I suspect more windslab development and colder snow, more readily propagating fractures. More observations and data are needed.
Natural avalanches from both storm waves have been observed. Snowmobiling on small test slopes and low-angle terrain at or below 9500' did not produce any cracking or perceptible collapsing/whumpfing.
The Bottom Line for the BWRA: Below 10,000', look for heightened avalanche conditions on specific terrain features and failure planes within storm layers. Above 10,000', snow conditions are somewhat unknown, though look for greater windslab development and colder snow with higher tendency to propagate fractures; use greater caution in the higher terrain and where windslabs exist; careful snowpack evaluation and conservative routefinding and decision-making are essential as these new layers gain strength.
Though the first wave of snow bonded well to the old snow surface, natural avalanche activity has been observed from both waves of the storm event.
On Sunday, we observed one small, shallow slab avalanche on a North aspect at about 9000' that appeared to fracture within the storm snow.
The second storm wave came in cooler, with temps dropping to 26degF at 0200 Monday morning and remaining below freezing through the day at Leavitt Lake Snotel site (9600'). Winds were moderate to strong throughout both waves of the storm, and predominantly out of the S/SW.
On Monday, USFS Snowmobile Patrollers observed two slab avalanches above Leavitt Lake, which presumably ran on the surface of the first wave of storm snow (now glazed/crusted). They reported the second wave of storm snow was colder and lighter density than the first, and was actively forming windslabs with moderate to strong winds.
The second wave of this storm event is on its way out this evening and we are looking at another period of warm, mild weather. A ridge of high pressure moves into the area through the week, centering in Nevada by the weekend. This warm weather will begin to melt away the gains we made from this storm's accumulation.
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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