Eastern Sierra Avalanche Advisory - 3/20/17

Our goals are lofty, but our ask is simple... We need your help... Click Here to Find Out How

THIS AVALANCHE ADVISORY EXPIRED ON March 22, 2017 @ 7:06 am
Avalanche Advisory published on March 20, 2017 @ 7:06 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Monday – one more day of spring like conditions with Loose Wet avalanches being the dominant problem for Monday before winter returns Tuesday. Cloud cover and moderate winds Monday will likely slow surface thawing in the low to mid elevations, while the upper elevations the surface snow will likely not thaw much due to upper elevation winds and cloud cover. Low and mid elevations – as solar aspects warm through the day (Easterly > Southerly > Westerly), natural avalanches are unlikely but small isolated human triggered Loose Wet avalanches are possible as the snow thaws in steeper terrain. Possible “Slide for Life” conditions exist in the AM before the snow surface thaws. Evaluate snow and terrain carefully.

Tuesday – three to eight inches of snow is forecasted through the day with moderate to strong Southwesterly winds, producing sensitive Wind Slabs as the day progresses and new accumulates primarily on NW-N-NE-E-SE aspects, above ~8000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, moderate to strong gusty winds may produce Wind Slabs on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Throughout the day, the avalanche hazard will rise as more snow accumulates and winds persist. Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

How to read the advisory

Bottom Line

Monday – one more day of spring like conditions with Loose Wet avalanches being the dominant problem for Monday before winter returns Tuesday. Cloud cover and moderate winds Monday will likely slow surface thawing in the low to mid elevations, while the upper elevations the surface snow will likely not thaw much due to upper elevation winds and cloud cover. Low and mid elevations – as solar aspects warm through the day (Easterly > Southerly > Westerly), natural avalanches are unlikely but small isolated human triggered Loose Wet avalanches are possible as the snow thaws in steeper terrain. Possible “Slide for Life” conditions exist in the AM before the snow surface thaws. Evaluate snow and terrain carefully.

Tuesday – three to eight inches of snow is forecasted through the day with moderate to strong Southwesterly winds, producing sensitive Wind Slabs as the day progresses and new accumulates primarily on NW-N-NE-E-SE aspects, above ~8000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, moderate to strong gusty winds may produce Wind Slabs on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Throughout the day, the avalanche hazard will rise as more snow accumulates and winds persist. Natural avalanches possible, human triggered avalanches likely, careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Avalanche Problem 1: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Monday - Low to mid elevations, small Loose Wet natural unlikely but small isolated human triggered avalanches are possible in steeper terrain as the snow surface thaws and becomes wet during the day. An approaching storm system with accompanying increasing cloud cover and moderate SW winds will reduce and slow surface snow thawing in the lower and mid elevations despite a weak overnight freeze.The surface snow will warm through the day (Easterly > Southerly > Westerly), increasing near cliffs and rock outcrops. Large pinwheels or snow penetration boot top or greater are signs the snow is loosing strength and becoming unstable, steep terrain on these aspects should be avoided.  Natural avalanches unlikely but small isolated human triggered Loose Wet avalanches possible in steeper terrain as the snow surface softens in the lower and mid elevations.

A number of factors affect the rate of snow thawing: Aspect (Easterly facing slopes begin to thaw first, followed by south and then west in the afternoon); Elevation: lower elevations warm more quickly than higher elevation, Cloud cover: cloud cover can shade the slopes slowing thawing. However, clouds can help to slow or limit overnight cooling limiting how solid the snow sets-up. Thin cloud cover during the day can trap the heating from the sun filtering through producing strong warming, Air temperature: Warmer air temperatures leads to more rapid snowpack warming.  

Caution - small wet slides are dense/heavy and are capable of carrying a rider into hazardous terrain or possibly entrain enough snow to bury a person, especially if a terrain trap is involved.  Early starts are recommended before the snow become saturated and unsupportable. 

Monday - Slide For Life conditions may exist on slopes before they thaw or areas with hard smooth wind board.  Crampons, ice axes, whippets, may be required to travel safely in steeper terrain.

Cornice Warning – Cornices can fail unexpectedly, give them wide berth while traveling along ridgelines, avoid corniced slopes.  

Avalanche Problem 2: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Tuesday - Moderate to strong Southwesterly winds Monday night thru Tuesday with three to eight inches of new snow forecasted during the day Tuesday will result in sensitive Wind Slabs forming primarily on NW-N-NE-E-SE aspects, above ~8000 as the new snow accumulates through the day. However, localized wind channeling and strong gusty winds may produce Wind Slabs on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Wind Slabs will likely be encountered along ridgelines, crossloaded gullies and in shallow depressions, and in and around terrain features that promote drifting and loading. Be wary of hollow, drum-like sounding snow or shooting cracks out. As the day progresses and the new snow accumulates the threat of avalanches will rise through the day, natural avalanches possible, human triggered avalanches become increasing likely through the day (size, depth, and sensitive ). Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

advisory discussion

A change in the weather is forecasted for Monday night when winter will reassert itself through the upcoming week with unsettled conditions dominating with a series of winter-like storms moving through the region bring cooler temperatures and moderate snowfalls. The new snow will be fairly warm and likely bond relatively well to the old snow surface but moderate to strong Southwesterly ridgetop winds Monday night thru Tuesday (30 to 50 mph, gusts up to 95 mph) will form sensitive Wind Slabs throughout the region on NW-N-NE-E-SE aspects. Due to the strong gusty winds and localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual.

During the past two weeks spring has made a brief appearance with above average temperatures and for the most part sunny skies. The last snowfall recorded in the region was on March 5th with up to 7” primarily in the June and Mammoth basins, which was followed by strong winds, producing highly wind-effected and firm snow conditions in the upper elevations. Prior to this temperatures were slightly cooler than seasonable with a winter like snowpack with semi-regular snowfalls. As temperatures began to climb into the 40’s and 50’s (in paces low 60’s) the snow began to transition to a spring snowpack with melt freeze crusts forming at the surface and the snowpack as a whole consolidating and strengthening. With the arrival of Spring’s warming temperatures, Loose Wet avalanches became the dominant problem for the past two weeks. Loose Wet avalanches were observed in the Low to Mid elevations, slowly creeping into the upper elevations as temperatures continued to climb into unseasonably warm territory this last week. During the warm-up, solar aspects saw a significant loss of snowcover with some slopes becoming completely bare below ~8000’.  Creeks below ~8500’ have begun to reemerge from the melting snowpack. The deep snow cover has created some rather big drops to the creek bed below creating a new backcountry hazard to be aware while approach streams and creeks.

Monday - Slide For Life conditions may exist on slopes before they thaw or areas with hard smooth wind board.  Crampons, ice axes, whippets, may be required to travel safely in steeper terrain.

Cornice Warning – Cornices can fail unexpectedly, give them wide berth while traveling along ridgelines, avoid corniced slopes. 

Sunday High / Overnight Low Air Temperatures in Degrees F as of 0600;

                                                            Low     High

Virginia Lakes (9300’):                          32        50

Ellery Lake (9545’):                               33        47

Agnew Pass (9450’):                             35        51

June Mtn Wx Plot (9220’):                     36        49

Sesame Snow Study (9014’):                 35        49

Rock Creek (9700’):                              23        48

South Lake (9600’):                              35        50

 

 

 

weather

Monday - Sunny skies are expected early today as drier air works into the region ahead of an incoming storm. High and mid-level clouds should begin to develop Monday afternoon as low pressure off the coast begins to push moisture into the region. Winds will pick-up Monday with gusts up to 25 to 35 mph during the afternoon with gusts of up to 80 mph at ridgetop.  

Tuesday - Recent model runs have trended towards a splitting storm on Tuesday with the main low pressure moving north towards the Pacific Northwest coast. This continues the trend of higher snow levels as the initial cold front is stretched north and slower to move into the Sierra. This has also lowered total QPF for the storm closer to 1-1.5 inches along the Crest. Recent models have split more of the precipitation towards Mono County. The main brunt of moisture is still expected during the morning and early afternoon on Tuesday. Snow levels during that time are expected to be in the 7000-8000 foot range dropping to around 5500-6000 feet by Tuesday night. Snow amounts below 7500 remain the biggest question depending on how fast snow levels fall, accumulations of 2-4" expected. Above 7500-8000` accumulations could range from 8-16". Gusty winds of 30 to 40  on Tuesday between periods of light spillover. Isolated thunderstorms possible as mid-level instability increases under the cold pool during the afternoon to early evening.

Wednesday - Upper level low moves over CA/NV on Wednesday, with convective showers likely to develop with a mix of rain, snow, and pellet showers are likely with periods of travel disruptions, especially in the Sierra. Isolated thunderstorms possible as mid-level instability increases under the cold pool during the afternoon to early evening.

Thursday - A ridge builds on Thursday, which will give the region a short break in the precipitation before the next storm pushes into the Sierra. Winds will begin to pick up along the ridges late Thursday night ahead of the storm for the weekend with typical winter storm wind gusts in the 80-100 mph range.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy then becoming partly cloudy. Mostly cloudy. Slight chance of showers. Cloudy. Showers through the day. Slight chance of thunderstorms in the afternoon.
Temperatures: 44 to 52 deg. F. 27 to 34 deg. F. 30 to 37 deg. F.
Wind direction: Southwest Southwest South
Wind speed: 15 to 25 mph. Gusts up to 50 mph increasing to 65 mph in the afternoon. 15 to 25 mph with gusts to 75 mph. 20 to 30 mph with gusts to 60 mph.
Expected snowfall: 0 in. up to 1 inch. in. 2 to 6 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy then becoming partly cloudy. Mostly cloudy. Chance of snow showers. Cloudy. Snow showers through the day. Slight chance of thunderstorms in the afternoon.
Temperatures: 39 to 47 deg. F. 23 to 29 deg. F. 25 to 33 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 25 to 35 mph with gusts to 65 mph increasing to 30 to 45 mph with gusts to 85 mph in the afternoon. 35 to 50 mph with gusts to 95 mph. 30 to 50 mph. Gusts up to 95 mph decreasing to 85 mph in the afternoon.
Expected snowfall: 0 in. up to 1 INCH in. 3 to 8 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.

ESAC receives support from ...