Eastern Sierra Avalanche Advisory - 4/16/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 17, 2017 @ 6:44 am
Avalanche Advisory published on April 16, 2017 @ 6:44 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Sunday:  The primary avalanche problems are isolated Wind Slabs and possible Wet Loose avalanches.

Wind Slab - Southwesterly winds are forecasted to reach threshold in the mid elevations (15 to 25 mph) and above threshold in the upper elevation (20 to 35 mph) during the day (Sunday) resulting in drifting on leeward slopes where there is sufficient new snow in upwind fetches, which will possibly form isolated Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations above ~ 8500’. Be wary of hollow, drum-like sounding snow or shooting cracks.

Wind Slabs – Natural avalanches are unlikely, triggered releases possible on slopes of 35 degrees and steeper in the mid to upper elevations.

Loose Wet – A weak overnight freeze combined with temperatures forecasted to climb into the 40’s and 50’s today (8000’ to 10,000’ elevation) will increase the concern for Loose Wet avalanches, especially in the mid to lower elevations, despite the cloud cover. Heightened avalanche conditions exists on solar aspects (E-S-W aspects), including NW and NE aspects below ~8500’, especially near rock outcroppings and cliff bands.

Loose Wet - Natural avalanches unlikely, triggered releases possible.

 

 

How to read the advisory

Bottom Line

Sunday:  The primary avalanche problems are isolated Wind Slabs and possible Wet Loose avalanches.

Wind Slab - Southwesterly winds are forecasted to reach threshold in the mid elevations (15 to 25 mph) and above threshold in the upper elevation (20 to 35 mph) during the day (Sunday) resulting in drifting on leeward slopes where there is sufficient new snow in upwind fetches, which will possibly form isolated Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations above ~ 8500’. Be wary of hollow, drum-like sounding snow or shooting cracks.

Wind Slabs – Natural avalanches are unlikely, triggered releases possible on slopes of 35 degrees and steeper in the mid to upper elevations.

Loose Wet – A weak overnight freeze combined with temperatures forecasted to climb into the 40’s and 50’s today (8000’ to 10,000’ elevation) will increase the concern for Loose Wet avalanches, especially in the mid to lower elevations, despite the cloud cover. Heightened avalanche conditions exists on solar aspects (E-S-W aspects), including NW and NE aspects below ~8500’, especially near rock outcroppings and cliff bands.

Loose Wet - Natural avalanches unlikely, triggered releases possible.

 

 

Avalanche Problem 1: Wind Slab
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Southwesterly winds are forecasted to reach threshold in the mid elevations (15 to 25 mph) and above threshold in the upper elevation (20 to 35 mph) during the day (Sunday) resulting in drifting on leeward slopes where there is sufficient new snow in upwind fetches, which will possibly form isolated Wind Slabs on NW-N-NE-E-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.

Avalanche Problem 2: Loose Wet
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Overnight lows were very mild, barely below freezing in many locations. Highs today (Sunday) are forecast to be in the mid 40s to 50s in the mid elevations (8000’ to 10,000’elevation) with cloudy skies. This combination has likely created a weak semi-supportive Melt/Freeze crust at the surface that will breakdown through the day as temperatures climb. Depending on the thickness of the cloud cover during the day, enough warming and re-radiation may thaw the surface snow sufficiently for Loose Wet avalanches, especially in the mid to lower elevations. Loose Wet avalanches are possible as the snow surface thaws, especially around lower elevation rock outcrops, below cliff bands, and in open bowls. Easterly aspects will thaw first, then southerly, then westerly as the sun moves across the sky. Additional heat from cliffs, rock faces, and trees can result in additional localized thawing. Lower elevations warm more quickly than higher elevations. 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. 

advisory discussion

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.

 

 

weather

Sunday - A shortwave trough will begin to ease into the region with gusty winds this afternoon and evening along with scattered snow showers over the higher elevations by late afternoon and evening. Wind gusts this afternoon up to 50-55 mph possible in the late afternoon to early evening. Light snow showers along the crest in the early evening with a brief burst of moderate shower activity mid evening. Snowfall amounts will only be in the 2 - 4" range.

Mon thru Tuesday - Showers will lighten up early Monday morning but a reinforcing wetter system will quickly follow starting as early Monday morning. Snow levels on Monday afternoon will rise to near 7,000-7,500` with the arrival of subtropical moisture, falling back to around 6,000-6,500` behind the front Tuesday morning. Total precipitation amounts Monday morning through Tuesday could see about 0.75-1.25" of liquid equivalent with localized higher amounts near 1.5-1.75" along the Crest and in northern Mono county.

Wednesday - Shortwave ridge will shift east on Wednesday as the next system approaches the west coast. Breezy and mild conditions will develop by afternoon with only a slight chance for showers reaching the Sierra. Models have backed off on the QPF as they keep trough progressive and its overall track a bit farther to the north than previous runs with precipitation chances dropping off rapidly further south.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Sunday Sunday Night Monday
Weather: Cloudy. Slight chance of snow in the afternoon. Cloudy. Chance of snow. Mostly cloudy. Slight chance of snow showers in the morning. Chance of showers in the afternoon.
Temperatures: 42 to 50 deg. F. 28 to 33 deg. F. 43 to 51 deg. F.
Wind direction: Light winds becoming southwest Southwest Southwest
Wind speed: 15 to 25 mph with gusts to 45 mph in the afternoon. 20 to 30 mph with gusts to 50 mph. 15 to 20 mph. Gusts up to 35 mph increasing to 60 mph in the afternoon.
Expected snowfall: 0 in. up to 2" in. up to 1" in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Cloudy. Slight chance of snow in the afternoon. Cloudy. Snow likely in the evening. Chance of snow after midnight. Mostly cloudy. Slight chance of snow showers in the morning. Chance of snow showers in the afternoon.
Temperatures: 35 to 43 deg. F. 23 to 29 deg. F. 35 to 43 deg. F.
Wind direction: Southwest West Southwest
Wind speed: 20 to 35 mph. Gusts up to 50 mph increasing to around 85 mph in the afternoon. 35 to 55 mph with gusts to 90 mph becoming southwest 25 to 35 mph with gusts to 60 mph after midnight. 15 to 20 mph with gusts to 45 mph increasing to 20 to 30 mph with gusts to 65 mph in the afternoon.
Expected snowfall: 0 in. up to 2 in. up to 1 in.
Disclaimer
This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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