Avalanche Advisory: Monday - Mar 19, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 20, 2018 @ 7:07 am
Avalanche Advisory published on March 19, 2018 @ 7:07 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Monday (3/19) – Overall avalanche hazard to day is Moderate. Loose Wet avalanches will be a concern on solar aspects (SE–S-SW), especially in steep rocky terrain near treeline and below where strongest heating will take place. Potential lingering Wind Slabs on NW-N-NE-E-SE aspects in the Upper Elevations.

There’s a very remote possibility that small to mid size avalanches could trigger or step down into the deep persistent weaknesses in the mid and lower snowpack, primarily a concern outside of the Mammoth/June area where the snowpack remains shallow. 

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Loose Wet
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Below 10,000’, the weather forecast calls for high temperatures to climb into the upper 30’s and mid 40’s with light winds. The combination should allow the surface snow to warm appreciably. As the snow becomes wet and saturated, especially in steep rocky terrain below 10,000’, loose wet avalanches will become increasingly likely on SE-S-SW aspects. Lower elevations will warm more quickly than mid elevations. Use extra caution in and around rock outcrops, below cliff bands, or where the snowpack may heat up more quickly and triggered releases are more likely. Watch for signs of unstable snow: large roller balls, deep ski penetration, and small point releases. Small point releases can be a sign that larger avalanches are increasingly possible. Loose Wet can trigger larger deeper releases. Timing is critical for avoiding Loose Wet releases. Time your travels to be out of steep sunny terrain before the snow becomes saturated from the heat of the day. For near tree line and below, natural avalanches are possible, triggered releases are likely, especially below 10,000’ in sheltered or rocky terrain.

Loose Wet slides are dense / heavy and are capable of carrying a rider into hazardous terrain or lead to possible burial when combined with a terrain trap.

Avalanche Problem 2: Wind Slab
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The latest storm (3/16) to transit through the eastern Sierra brought a nice shot of snow with 9” to 20” (20cm to 50cm +) of new snow throughout the region on top of the 4” to 20” from the storm on Wednesday/Thursday (3/14-15). These two systems were accompanied by moderate to strong Southwesterly winds near and above tree line, which formed Wind Slabs on NW-N-NE-E-SE aspects from near tree line and above. Winds have mostly subsided as the system moved off to the east. Today’s (Monday) forecast calls for light winds for all elevations, which will limit snow transport to very localized saltation in channeled areas and allow recently formed Wind Slabs in the upper and mid elevations to continue to strengthen and bond to the underlying snow, becoming less sensitive to triggered release. Lingering Wind Slabs may be encountered on NW-N-NE-E-SE aspects, primarily in the Alpine regions. Dense snow deposits are a telltale sign and will likely be encountered immediately below ridgelines or corniced slopes, in crossloaded gullies, or near terrain features that encourage drifting. Shooting cracks are a strong sign of instability. Hand shear assessments and pole probing can help identify poorly bonded or suspended slabs. Natural avalanches are unlikely, triggered releases are possible.

 

Avalanche Problem 3: Persistent Slab
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The persistent deep slab weakness … persists throughout the region. The variability of the overlying snow structure, distribution, and depth of burial continues to pose a forecasting challenge. Tests have shown continued clean and energetic propagation, though becoming progressively more stubborn. The layer of weak facets can be found ~50 cm (~20”) south of Mammoth to ~>150cm (~>60”) in the Mammoth/June region, somewhere in between around Virginia Lakes. Though becoming increasingly unlikely, a large trigger (i.e. avalanche, snowmobile landing a jump, or a large cornice failure) or a sensitive sweet spot in a shallow area of the snowpack, could trigger this layer and produce a larger destructive release, especially north and south of the Mammoth /June area where the snowpack is shallower and the layer is not as deeply buried. The persistent facet layer is showing slow signs of strengthening (sintering and rounding) as the temperature gradient in the snowpack has relaxed a bit, especially in the mid and lower elevations. The problem primarily exists in the mid-elevations on E-N-W aspects but may extend into the upper elevations where it may be poorly bridged over by Wind Slabs. Make your own assessments, especially if traveling outside of the Mammoth/ June area where the snowpack is shallower. Natural avalanches are unlikely, triggered releases from shallow pockets are possible, primarily north or south of Mammoth /June where the snowpack is thinner.  

advisory discussion

Loose Wet activity that was expected yesterday was not as pervasive as anticipated. Cloud cover moved in quicker and thicker than expected, which helped keep the surface snow cool, limiting Loose Wet activity to steep rocky terrain.  Today’s (Monday) forecast is calling for Partly Cloudy to Sunny skies and mild temperatures below 10,000’, which will increase the likelihood of generally small Loose Wet releases to on solar aspects (SE-S-SW), primarily tree line and below, as temperatures begin to rebound.

The latest storm to traverse through the region brought anywhere from 9” to 20”+ (SWE 1” + for many locations) of new snow Thursday/Friday with the greatest amounts of new snow concentrated around the Mammoth/June area, lesser amounts north and south. Moderate to strong W-SW winds in the upper elevations throughout the storm formed Wind Slabs in the mid to upper elevations, primarily on NW-N-NE-E-N-SE aspects, which have begun to strengthen and sinter to the underlying snowpack. Winds have eased over the past two days, allowing recently formed Wind Slabs time to bond and strengthen, becoming less sensitive to human triggered release. Lingering sensitive Wind Slabs may still be lurking in the upper elevations but becoming increasingly isolated and stubborn.

The previous Storm Slab problem noted Thursday has generally healed. Sheltered locations from near tree line and below, the recent snowfalls are bonding well to the underlying snowpack.

The graupel layer previously reported (3/14) in the June and Crestview area is still present but is showing signs of strengthening with hand shears now moderate. The graupel layer can be found primarily below 10,000’, ~25cm to ~70+cm (~10” to 25”) deep.  

Loose Dry sluffs may be encountered in isolated steep terrain as the surface snow begins to facet near and below tree line in steep sheltered locations. While these slides are not a concern for burial, they can sweep a rider into potential hazardous terrain or obstacles.

The lingering persistent weak facets from early season continues to linger but are showing signs of sintering and rounding, with some shallow areas becoming wet and refreezing. The facets are strengthening and becoming more deeply buried in many locations, making it more difficult to trigger. The most recent storm (3/16), despite adding the stress of an inch or more of water weight to the snowpack, wasn’t enough to tip the balance and cause this layer to become more reactive, at least based on observations. As the facet layer slowly recedes into the snowpack depths (Mammoth / June) area and the facets continue to round and sinter, the concern is becoming more limited and isolated. However, the storm forecasted for Thursday has a strong AR signiture and is forecasted to drop 3 to 4" of water over the Sierras. This may be enough weight to cause this layer to become stressed and reactive. 

 

 

 

 

 

 

Weather and CURRENT CONDITIONS
weather summary

Monday thru Tuesday – Monday, areas of high clouds, light winds and high temperatures up to mid 30s. Tuesday, warm air advection (WAA) sets-up over northeast California with relatively light snowfall for Sierra.

Wednesday and Thursday (AR Storm): Latest trends have shown a distinct period of a warm storm (higher snow levels) Wed-Thu followed by colder and shorter duration systems heading into the next weekend. Roughly 24-36 hour period of precipitation from Wednesday through Thursday evening with peak precipitation Thursday AM into the early evening. Snow levels look to start around the 7,500-8,000` Wednesday into early Thursday morning in Mono County a bit higher in Inyo. Snow levels will begin to fall to 6,500 to 7,000 by Thursday afternoon and dropping below 6,000` by Thursday evening. The main moisture plume is focused mainly from Mono County southward. Precipitation amounts for Wednesday through Thursday along the Sierra Crest in the 2-3" range and 3-4" possible across the crest     in southern Mono County. Anticipate gusty winds with ridge gusts reaching over 80 mph by Wednesday and continuing into Thursday.

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: Partly cloudy then becoming sunny Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow in the afternoon.
Temperatures: 36 to 42 deg. F. 20 to 25 deg. F. 40 to 46 deg. F.
Wind Direction: Light winds. Light winds. South
Wind Speed: Gusts up to 30 mph after midnight. 10 to 15 mph. Gusts up to 30 mph.
Expected snowfall: 0 in. 0 in. Up to 1 in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Partly cloudy. Partly cloudy then becoming mostly cloudy. Mostly cloudy. Chance of snow through the day.
Temperatures: 29 to 34 deg. F. 20 to 26 deg. F. 33 to 38 deg. F.
Wind Direction: Light winds. South Southwest
Wind Speed: 15 to 20 mph with gusts to 35 mph increasing to 20 to 30 mph with gusts to 45 mph after midnight. 20 to 30 mph. Gusts up to 45 mph.
Expected snowfall: 0 in. 0 in. up to 2 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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