Eastern Sierra Avalanche Advisory - 5/16/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 18, 2017 @ 6:56 am
Avalanche Advisory published on May 16, 2017 @ 6:56 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problems for Tues thru Wednesday will focus on isolated Wind Slabs in the Mid and Upper elevations and Loose Wet in the Mid to Lower elevations.

Wind Slabs - Moderate Northwesterly winds (Mon PM) are forecasted to shift to Westerly (Tues & Wed) with the potential for light snowfall (Tues) will form isolated shallow Wind Slabs primarily on exposed N-E-S-SW aspects above ~ 9500’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.

Tues thru Wed – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

Loose Wet - will be a concern in the Low (where snow is present) to Mid elevations, primarily south of Mammoth, as the skies clear and the new snow warms during the day, primarily on NW-W-SW-S-SE-E aspects. North of Mammoth cloud cover will remain thicker and decrease the concern. Wednesday - as skies clear, the concern will rise in the Upper elevation sheltered terrain that promote warming (chutes, couloirs, etc.)

Tues – Low and Mid elevation natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper.

How to read the advisory

Bottom Line

Primary avalanche problems for Tues thru Wednesday will focus on isolated Wind Slabs in the Mid and Upper elevations and Loose Wet in the Mid to Lower elevations.

Wind Slabs - Moderate Northwesterly winds (Mon PM) are forecasted to shift to Westerly (Tues & Wed) with the potential for light snowfall (Tues) will form isolated shallow Wind Slabs primarily on exposed N-E-S-SW aspects above ~ 9500’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.

Tues thru Wed – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

Loose Wet - will be a concern in the Low (where snow is present) to Mid elevations, primarily south of Mammoth, as the skies clear and the new snow warms during the day, primarily on NW-W-SW-S-SE-E aspects. North of Mammoth cloud cover will remain thicker and decrease the concern. Wednesday - as skies clear, the concern will rise in the Upper elevation sheltered terrain that promote warming (chutes, couloirs, etc.)

Tues – Low and Mid elevation natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper.

Avalanche Problem 1: Wind Slab
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Westerly (Mon) to Northwesterly (Tues - Wed) winds are forecasted to be at or above threshold in the Mid and Upper elevations (15 to 25 mph) with Gusts of 60 to 80 mph with 1” to 6” of snow accumulation over the upper elevations will result in drifting on leeward slopes. Anticipate isolated shallow Wind Slabs on N-E-S-SW aspects in the mid to upper elevations above ~ 9500’. Riders will likely encounter these below ridgeline, adjacent to terrain features that promote drifting, crossloaded gullies and depressions. Be wary of hollow, drum-like sounding snow or shooting cracks.​ Changing wind directions can alter windloading and making it difficult to discern previous drifting patterns. 

Tues thru Wed – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

advisory discussion

Spring weather has taken a brief backseat to a quick shot of winterish weather that moved into the region Monday (5/15) with moderate winds, and 1”-6” of new snow over the higher elevations overnight (Monday). Snow amounts varied greatly depending on location due to the squall like convective showers. The new snow has had little time to bond to the underlying snow and will likely peel off the old snow surface easily either as it begins to warm under variable cloud cover or by a rider. Moderate Northwesterly to Westerly winds along ridgelines producing shallow Wind Slabs on N-E-S-SW aspects in the Mid to Upper elevations.

The last significant weather system passed through the region Saturday (5/6) bringing: cooler temperatures, 4” to 8” of snow over the high country, and light to moderate southerly winds, which then shifted to northerly as the system began to move off to the east. The heaviest snowfall was from Mammoth north with a consistent 6” to 8”, south precipitation was a bit more spotty. The new snow bonded relatively well to the underlying melt/freeze crust with little internal weakness and has transitioned rapidly to corn or near corn snow as the storm moved east and sunny skies returned with temperatures rapidly rebounding to well above normal.

Prior to (5/6), spring was the dominant theme with overnight temperatures struggling to fall below freezing and daily highs climbing well above seasonable with upper 50’s and low 60’s recorded in many locations above 9000’. The combination of weak freezes and unseasonably warm temperatures  resulted in the upper snowpack becoming isothermal where the snowpack is shallow. Glide cracks have been observed on solar aspects below 11,000’ on steep slopes with smooth underlying terrain.

A weak spring storm moved through the region Wednesday (4/26) with moderate to strong winds, cooler temperatures, increasing cloud cover (primarily Mammoth north) and light precipitation over the upper elevations forming very isolated and shallow Wind Slabs in the upper elevations primarily on N-E-S-SW aspects.

A significant storm swept though the region Tuesday (4/18/17) with 3” to 12” inches of new snow reported across the forecast area above ~8500’. However, snow levels fluctuated considerably during the storm with many areas receiving rain Monday before turning to snow in the early morning hours on Tuesday. Snow levels rose once again during the day, Tuesday, with rain up to ~ 9,000’ then easing back down to ~8000’ by Tuesday PM. Loose Wet avalanches were prevalent during the storm throughout the mid elevations as the surface snow becoming saturated with water and internal bonds began to dissolve. Moderate to strong SW winds during the storm formed Wind Slabs in exposed locations throughout the mid and upper elevations, primarily above ~ 9000’ on NW-NE-SE aspects with several avalanches observed throughout the forecast region. Mammoth Mountain Ski Patrol reported significant results from avalanche control work on Wednesday morning. Most of these avalanches were triggered in Wind Slabs with small hand charges and ski cutting. Westerly winds continued thru Wednesday and Thursday (4/19-4/20) with snow banners and localized drifting observed from Mammoth south to Rock Creek, forming a new round of Wind Slabs throughout the upper and mid elevations, primarily on N-E-S aspects.

Moderate SW winds in upper elevations Sunday (4/23) formed another round of isolated shallow Wind Slabs in the upper elevations on NW-NE-SE aspects where upwind fetches still had snow available for transport. Since then, there have been several days for the Wind Slabs to strengthen and heal, redirecting the avalanche threat toward typical spring Loose Wet avalanches.

weather

Tuesday - A potent low pressure over Washington will move quickly to the southeast today and drive a cold front southward across the Great basin with gusty winds from mid afternoon through the evening. Later tonight, another shorter burst of stronger gusts is possible with the cold front passage. Precipitation, warm air advection with a push of Pacific moisture will spread a broad area of widespread cloud cover with light rain and snow across most of the region this morning, with snow levels generally around 7000 feet. Precip amounts will be light, generally 0.10 inch or less. Snow amounts for the Sierra above 7000 feet up to a few inches are possible.

Wednesday thru Thursday - An upper ridge will slowly build overhead. With a chilly start Wednesday morning and breezy northwest to north flow, highs Wednesday will remain below average. On Thursday, temperatures will begin a moderating trend although heating will be partially offset by a lingering north to northeast surface flow.

Friday thru Monday - High pressure continues to build over the West Coast into the weekend, with rising heights and warming temperatures. The longwave pattern becomes very amplified, with the ridge reaching well into British Columbia. A weak shortwave trough drops down the through the Northern Rockies over the weekend with moderating northeast flow, helping to keep temperatures only about 5-8 degrees above normal. There are indications that the ridge may slide further east over the Western States by the middle of next week, with temperatures continuing to rise to well above normal with a few afternoon/evening thunderstorms develop over the Eastern Sierra. High temperatures going into the weekend and next week will be in the low 70s in the Sierra. Warming temperatures will increase snowmelt from the Sierra and increase flows into the mainstem rivers, creeks and streams draining out of the Sierra.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy then becoming cloudy. Chance of snow through the day. Chance of rain in the afternoon. Mostly cloudy. Chance of snow showers in the evening, then slight chance of snow showers after midnight. Partly cloudy.
Temperatures: 40 to 48 deg. F. 26 to 32 deg. F. 41 to 47 deg. F.
Wind direction: West West Northwest
Wind speed: 15 to 20 mph. Gusts up to 30 mph increasing to 50 mph in the afternoon. 15 to 25 mph. Gusts up to 60 mph decreasing to 50 mph after midnight. 10 to 15 mph with gusts to 35 mph.
Expected snowfall: up to 1 inch in. up to 1 inch. in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy then becoming cloudy. Chance of snow. Mostly cloudy. Chance of snow showers. Partly cloudy.
Temperatures: 33 to 39 deg. F. 21 to 26 deg. F. 32 to 38 deg. F.
Wind direction: Northwest West West
Wind speed: 15 to 20 mph with gusts to 35 mph shifting to the west 20 to 30 mph with gusts to 65 mph in the afternoon. 30 to 45 mph with gusts to 80 mph. 20 to 30 mph with gusts to 50 mph shifting to the northwest 15 to 20 mph with gusts to 40 mph in the afternoon.
Expected snowfall: up to 1 in. up to 1 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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