Avalanche Advisory: Thursday - Dec 12, 2019

THIS AVALANCHE ADVISORY EXPIRED ON December 14, 2019 @ 6:57 am
Avalanche Advisory published on December 12, 2019 @ 6:57 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Strong SW to W winds continuing thru Friday, with ridge top gusts up to 85mph, will continue to transport snow into pockets of fresh dense wind slab sensitive to human triggering. Warm overnight conditions could lead to loose-wet instabilities in steep terrain at lower elevations. 

 

*Daily avalanche advisories with danger ratings will begin this Saturday, December 14th!! 

 

*Come to Mammoth Brewery 6pm tonight (Thursday) for an ESAC season introduction, film night and raffle!  Save the date for ESAC’s season kick-off extravaganza on Jan 11th! 

 
 

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Below Treeline
Avalanche Problem 1: Wind Slab
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Strong SW winds yesterday will continue thru Friday, shifting out of the West for higher elevations with gusts up to 85mph.  Plenty of snow could be seen being transported by these winds on Wednesday, and at least one 3ft deep fresh pocket of dense wind slab was triggered by ski cut.  Fresh wind slabs sensitive to human triggering will continue to form thru Friday, mostly on northerly to easterly terrain at tree line and above, but could form at lower elevations as well.  Use cues such as blowing snow and cornice formation to help identify and avoid potential areas where smooth fresh dense wind slab may lie. These slabs may be found in more unusual areas and further downslope than expected due to the strength of the winds.  Do your own localized assessments in inconsequential terrain and realize the high degree of variability that can exist across even a small slope. A resulting avalanche could be large enough to bury a person especially if a terrain trap is involved.

Avalanche Problem 2: Loose Wet
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Overnight temperatures remained above freezing Wednesday night up to ~9000’, and are expected to be even warmer on Thursday night. This combined with cloud cover and even some light drizzle will keep lower elevation snow surfaces wet and susceptible to loose wet point releases and sloughing in steep terrain.  This concern will be greatest for non-southerly aspects where cold wintery snow is transitioning for the first time.  Be aware of wet snow surfaces as you travel, and the implications of small loose wet slides entraining more snow as they flow downslope especially when confined in a feature like a gully.

advisory discussion

The sierra snowpack is off to a great start.  With virtually no early season snow before Thanksgiving, the underlying snowpack has been consistently observed to be right-side-up and well bonded.  The avalanche concern currently remains focused on short-lived surface instabilities mostly in the form of wind slabs.  With the warm weather system upon us at the moment however, lower elevation loose-wet instabilities could be a concern.  

 

Although the underlying snowpack is mostly stable, and the coverage is quite impressive for this time of year, the snowpack remains shallow in many areas!  Beware of the rocks, logs, and stumps that lurk not far under the surface.

recent observations
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-12/11 - Mammoth Area - 3 foot thick ski cut triggered wind slab pocket

-12/11 - Lee Vining: Winds and shooting cracks

-12/11 - Mammoth Lakes Basin - Red Cone: Winds!

-12/10 - Convict Lake - Mono Jim: Conditions

-12/10 - McGee Creek - Wineglass Couloir: Isolated fresh wind slabs, conditions

-12/9 - VA Lakes - South Peak: Sensitive wind slabs

-12/9 - June area - Chicken Wing: Evidence of recent storm avalanches, surface hoar, conditions

-12/9 - Mammoth - Sherwins: Recent storm cornice failures, conditions

 

Weather and CURRENT CONDITIONS
weather summary

For Thursday a weak warm system has moved in bringing warm temperatures, strong W to SW winds with ridge top gusts up to 85mph, light flurries and drizzle at lower elevations, and up to an inch of new snow for northern parts of the forecast area toward VA Lakes. 

 

For Friday a cooler system will move in bringing continued high winds and up to a few inches of new snow in the afternoon and evening particularly for northern areas of the forecast area.  Higher snowfall rates are expected Friday night with 3-5” expected.  

 

Light snowfall is possible thru Saturday night.  

 
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow and rain. Snow levels 9000 feet. Chance of precipitation is 25%. Mostly cloudy. Slight chance of snow and rain. Snow levels 8500 feet decreasing to 7500 feet after midnight. Chance of precipitation is 15%. Mostly cloudy. Chance of snow. Snow levels 7000 feet increasing to 8000 feet in the afternoon. Chance of precipitation is 45%.
Temperatures: 37 to 43. deg. F. 27 to 32. deg. F. 34 to 40. deg. F.
Mid Slope Winds: Southwest 15 to 30 mph with gusts to 45 mph. West 15 to 25 mph with gusts to 45 mph. West 15 to 25 mph with gusts to 55 mph.
Expected snowfall: up to 1 inch. | SWE = less than 0.10 inch. in. up to 1 inch. | SWE = less than 0.10 inch. in. 30% probability of 1 to 3 inches. 70% probability up to 1 inch. | SWE = up to 0.15 inch. in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow. Snow levels 9000 feet. Chance of precipitation is 35%. Mostly cloudy. Slight chance of snow. Snow levels 8500 feet. Chance of precipitation is 20%. Mostly cloudy. Snow likely. Snow levels 8000 feet. Chance of precipitation is 60%.
Temperatures: 31 to 37. deg. F. 22 to 27. deg. F. 27 to 33. deg. F.
Ridge Top Winds: West 30 to 50 mph with gusts to 80 mph. West 35 to 55 mph with gusts to 85 mph. West 30 to 50 mph with gusts to 75 mph.
Expected snowfall: up to 1 inch. | SWE = less than 0.10 inch. in. up to 1 inch. | SWE = less than 0.10 inch. in. 60% probability of up to 2 inches. 40% probability of 2 to 4 inches. | SWE = up to 0.25 inch. 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 Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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