Eastern Sierra Avalanche Advisory - 4/15/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 16, 2017 @ 7:25 am
Avalanche Advisory published on April 15, 2017 @ 7:25 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Saturday:  The primary problems will be Wet Loose and isolated Wind Slabs avalanches.

Loose Wet - As temperatures begin to climb into the 40’s and 50’s today (8000’ to 10,000’ elevation), the threat of Loose Wet avalanches will rise in the mid to lower elevations. Heightened avalanche conditions exists on solar aspects (E-S-W aspects), NW and NE aspects included below ~8500’, especially near rock outcroppings and cliff bands.

Loose Wet - Natural avalanches possible, triggered releases likely. Careful snowpack evaluation, cautious route finding, and conservative decision-making are essential for your safety.

Wind Slab - A fast moving, weak, spring storm moved through the area Wednesday thru Thursday depositing up to 9” of new snow and accompanied by strong southwest winds forming shallow Wind Slabs primarily on NW-N-NE-E-SE aspects. The strong winds may have formed Wind Slabs on unexpected aspects, further downslope than anticipated, normally sheltered locations, and on the leeward sides of terrain features. 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.

 

How to read the advisory

Bottom Line

Saturday:  The primary problems will be Wet Loose and isolated Wind Slabs avalanches.

Loose Wet - As temperatures begin to climb into the 40’s and 50’s today (8000’ to 10,000’ elevation), the threat of Loose Wet avalanches will rise in the mid to lower elevations. Heightened avalanche conditions exists on solar aspects (E-S-W aspects), NW and NE aspects included below ~8500’, especially near rock outcroppings and cliff bands.

Loose Wet - Natural avalanches possible, triggered releases likely. Careful snowpack evaluation, cautious route finding, and conservative decision-making are essential for your safety.

Wind Slab - A fast moving, weak, spring storm moved through the area Wednesday thru Thursday depositing up to 9” of new snow and accompanied by strong southwest winds forming shallow Wind Slabs primarily on NW-N-NE-E-SE aspects. The strong winds may have formed Wind Slabs on unexpected aspects, further downslope than anticipated, normally sheltered locations, and on the leeward sides of terrain features. 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.

 

Avalanche Problem 1: Loose Wet
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Highs are forecast to be in the mid 40s to 50s in the mid elevations (8000’ to 10,000’elevation) today with sunny to partly cloudy skies. Loose Wet avalanches are likely 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. Extra heat from cliffs, rock faces, and trees can result in additional localized thawing. Lower elevations warm more quickly than higher elevations. Watch out for pinwheels, 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.

 

Avalanche Problem 2: Wind Slab
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Moderate to strong winds associated with Thursday’s storm combined with up to 9 inches of new snow has formed wind slabs primarily on NW-N-NE-E-SE aspects in the mid to upper elevations. These will likely be relatively shallow but numerous and may be encountered in unexpected aspects, further downslope than anticipated, normally sheltered locations, and on the leeward sides of convexities in areas where the most snow accumulates. 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 cracks shooting out from your feet.

advisory discussion

Spring has made a resurgence after the last unusual 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 post-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” at Tioga Pass to 37” at 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 100 at ridgetops. 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 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 begin strengthen and stabilize. However, with the sun’s return, temperatures began to quickly rebound post-storm, resulting in the usual spring time Loose Wet avalanches cycle in the mid to lower elevations on W-S-E aspects in the mid elevations, expanding to NW-W-S-E-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 fresh Wind Slabs on NW-NE-SE aspects. This primarily affected slopes above ~9000’ that had extended upwind fetches with plenty of fresh snow available for transport. Thursday, the storm system had moved in land with moderate to strong winds and a quick shot of moisture with up to 9” of snow before moving off to the east overnight. The moderate to strong winds associated with this system has redistributed the new snow forming Wind Slabs predominately on NW-N-NE-E-SE aspects in the mid to upper elevations. Friday (4/14), sunny skies returned with warming temperatures with some Wet activity being reported in the Lower elevations. Saturday, as temperatures climb into the 40’s and 50’s (below 10,000’) another round of Loose Wet releases can be expected on solar aspects in the mid to lower elevations.

 

 

 

weather

Saturday - Quiet weather continues today before our next spring system reaches the northern Sierra Sunday afternoon and evening. Temperatures are expected to increase 8-10 degrees above yesterday’s highs with Highs this afternoon will reach low to mid 50s for Sierra valleys. High clouds will increase thorough the day ahead of Sunday`s approaching shortwave trough. Winds at ridgetops will become breezy this afternoon and strengthen through the evening.
 
Sunday - Gusty afternoon winds as jet winds drop south across the northern Sierra, peaking Sunday evening. Gusts across the Sierra could see gusts in the 40-50 mph range. This system has been trending a bit slower with the cold core also a bit farther north. Precipitation onset to occur in the 2-4pm timeframe across the Sierra crest with the showers continuing through the evening. Snow levels will start near 7,000-7,500` Sunday afternoon and then fall near 6,500` by Monday morning with 2-5"possible.
 
Monday thru Tues - Weather will remain active heading into early next week as a better organized and wetter system moves through on Monday afternoon and Tuesday. This system does have a subtropical moisture connection and it will raise snow levels back to around 7,000-7,500` Monday afternoon before dropping near 6000 to 6500 feet after the cold front Tuesday afternoon. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny then becoming partly cloudy. Mostly cloudy. Mostly cloudy. Slight chance of snow in the afternoon.
Temperatures: 45 to 53 deg. F. 27 to 33 deg. F. 43 to 51 deg. F.
Wind direction: Light winds becoming southwest Southwest Southwest
Wind speed: 10 to 15 mph. 10 to 20 mph in the evening becoming light. Gusts up to 30 mph. 10 to 15 mph with gusts to 45 mph increasing to 20 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny then becoming partly cloudy. Mostly cloudy. Mostly cloudy. Slight chance of snow in the afternoon.
Temperatures: 40 to 46 deg. F. 22 to 28 deg. F. 37 to 45 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 10 to 15 mph. 15 to 25 mph with gusts to 35 mph. 20 to 30 mph with gusts to 55 mph increasing to 35 to 45 mph with gusts to 85 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 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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