Avalanche Advisory: Wednesday - Mar 28, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON March 29, 2018 @ 6:59 am
Avalanche Advisory published on March 28, 2018 @ 6:59 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Wednesday (3/28) – The overall avalanche hazard is MODERATE. For all elevations, Loose Wet natural avalanches are unlikely, human triggered avalanches are possible on solar aspects (W-S-E). Use extra caution in sheltered or steep rocky snow-covered terrain where strongest heating will take place. Lingering likely stubborn Wind Slabs near and above treeline (~9000 – 10500’) on W-S-E aspects may be encountered, primarily from Mammoth to June.  Natural avalanches are unlikely (but not impossible), triggered releases possible.  

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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A moderate freeze overnight with forecasted daily high temperatures ranging from upper 30’s in the upper elevations to 50’s below 10,000’ will allow the surface snow to warm appreciably and become wet and saturated, especially in steep rocky terrain. Loose Wet avalanches are increasingly possible on E-S-W aspects where there is significant warming of the surface snow. Generally, the snow at the lower elevations thaws more quickly. Use extra caution in and around rock outcroppings, cliff bands, or sheltered areas where the snowpack may heat up more quickly and triggered releases are more likely. Signs of unstable snow: large roller balls, deep ski penetration, and small point releases. Loose Wet can trigger larger deeper releases. Time your travels to be out of steep sunny terrain before the snow becomes saturated from the heat of the day. Large natural avalanches are unlikely (small point releases possible), triggered releases are possible, especially in sheltered or rocky terrain. The trend for the next couple of days will continue to see temperatures rise, maintaining the potential for Loose Wet releases in the near term.  

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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Strong Northerly winds in the Alpine yesterday (3/27) likely formed thin isolated Wind Slabs where there was loose transportable snow in the upper elevations, primarily above ~10,500’, on W-S-E aspects. The few fresh Wind Slabs that formed have begun to bond to the underlying snow and are becoming increasingly stubborn and less reactive. Dense snowdrifts 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. Hand shear assessments and pole probing can help identify poorly bonded or suspended slabs. Evaluate the snowpack and terrain carefully. Natural avalanches are unlikely, triggered releases are unlikely but not impossible, especially in steep and/or complex terrain.

 

 

advisory discussion

 

Loose Wet – Spring is a challenging environment for avalanche forecasting because of the often opposing weather inputs. Strong northerly winds early yesterday helped keep temperatures cool in the Alpine regions while from ~ treeline and below, light winds and mostly clear skies allowed temperatures to climb into mid 30’s to low 50’s (9000’ to 10700’) at a number of reporting stations. This was enough to see the surface snow begin to warm significantly, even up into the Alpine on favored aspects. However, the key element that was lacking in some locations was loose surface snow. Areas where there was loose surface snow (i.e., Mammoth to June), the threat of Loose Wet was more evident. A few Loose Wet releases were noted but not as numerous as expected. Elsewhere, areas with denser surface snow (wind slabs, avalanche scoured slopes, etc.) were slower to thaw because the tightly compacted snow is somewhat resistant to becoming loose and incohesive as it thaws. Today (3/28) overnight lows ranged from upper teens to low 30’s and highs are forecasted to run between upper 30’s to low 50’s, which will drive more significant thawing of the surface snow. For all elevations, anticipate Loose Wet on W-S-E aspects. Today, large natural avalanches of D2 or greater are unlikely (small natural avalanches are possible), triggered releases are possible, trending toward likely in the low to mid elevations, on solar aspects (W-S-E), especially where there is significant loose snow.

Wind Slabs - The recent series of storms were accompanied by strong SW winds forming widespread lingering Wind Slabs on NW-N-NE-E-SE aspects. These Wind Slabs have generally had enough time to strengthen and bond to the underlying snow and as a result, less reactive but a potential lingering concern in steep complex terrain. Strong Northerly winds Tuesday above 10,000’, likely produced small isolated shallow Wind Slabs in favored locations (leeward slopes, under cornices, near rock outcrops, areas with large windward fetches with loose transportable snow). The lingering Wind Slabs have likely had sufficient time to strengthen/bond and are expected to be stubborn to trigger, though not impossible. Stubborn Wind Slabs may be encounter on W-S-E aspects, near and above treeline(~10000’ and above). I don’t expect this to be a widespread problem. More of a concern in favored localized areas.

Persistent Slab - The recent spat of storms brought 4 to 6’ of snow and has helped turn the corner on a rather lean season. The series of storms began to roll into the region in early March and continued through 3/22 with the last storm producing the biggest avalanche cycle of the season with impressive debris fields in many of the runouts throughout the range. The heavy dense snow accumulations have buried the persistent weak layers deep in the snowpack reducing the potential for triggered release. However, we have limited information concerning this problem. We can use your help with obtaining additional information on the distribution and sensitivity of this layer, especially outside of the Mammoth and June region. 

 

 

 

 

 

 

 

 

 

Weather and CURRENT CONDITIONS
weather summary

Wed thru Friday - A ridge of high pressure will remain centered off the California    coast through Thursday before moving inland Friday with occasional high clouds possible. Easterly winds over the Sierra decreasing for this afternoon, increasing tonight with an easterly surface gradient maximizing during the overnight and early morning hours then switching to westerly Thursday night and Friday. Temperatures will continue to warm few degrees each day with highs 10-15     degrees above average by Friday.

Saturday thru Next Week - Easter weekend through Monday, warm and dry conditions to continue. A shortwave approaching the CA coast is now looking even less organized with areas of mid-high level cloud cover on Saturday. Some modest late day west breezes are expected. For early morning on Easter Sunday, light winds, temperatures ranging from upper 20s to lower 30s. Westerly flow aloft is expected with fast moving shortwaves moving across the Pacific Northwest with periods of increased clouds and winds. High temperatures will trend down a few degrees    from the weekend, but should remain near or above average for early April.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny Clear Sunny
Temperatures: 47 to 53 deg. F. 25 to 30 deg. F. 50 to 56 deg. F.
Mid Slope Winds: North to northeast Light winds Light winds
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny Clear Sunny
Temperatures: 39 to 44 deg. F. 23 to 28 deg. F. 43 to 48 deg. F.
Ridge Top Winds: North North Northeast
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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