Avalanche Advisory: Thursday - Apr 12, 2018

THIS AVALANCHE ADVISORY EXPIRED ON April 14, 2018 @ 7:22 am
Avalanche Advisory published on April 12, 2018 @ 7:22 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Thursday – Wind Slabsand Loose Wetare the avalanche problems for the day. Three to four inches (~8 to ~10cm) of new snow overnight with moderate to strong SW > W-NW (AM) has likely formed tender Wind Slabsnear and above treeline (~9000’ and above) on all aspects but West. Natural Wind Slabsare possible, triggered Wind Slabsavalanches are likely. As skies clear and temps rebound, the new snow will warming quickly with the potential for Loose Wetavalanches rising primarily on E-S-W aspects on Thursday. Small naturalLoose Wetavalanches are possible, triggered releases are likely. Rollerballs, small wet sluffs, or boot-top snow penetration are signs to move off to shaded or lower-angle slopes. Extra caution near rocky outcrops and vegetated slopes more heat is retained in the snow.  

Friday – The threat of avalanches will most likely decrease Friday as the new snow begins to settle and bond to the underlying snow. Moderate northerly winds will continue to form new Wind Slabson E-S-W aspects but they’ll be more isolated and shallow. Natural Wind Slabswill be unlikely, triggered releases are possible. As temperatures begin to rebound, expect the threat of Loose Wetto rise through the day from ~10500’ (possible higher in sheltered rocky terrain) and below primarily on E-S-W-NW aspects but may creep around the compass to include N-NE aspects as well. Look for wet snow and rollerballs in steep terrain and near rocky areas. Natural avalanches unlikely (but not impossible), trigger releases possible.  

 

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Below Treeline
Avalanche Problem 1: Wind Slab
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The storm system that passed through the region overnight (4/12) depositing 3” to 4” of new snow and accompanied by strong SW winds and likely forming tender Wind Slabson NW-NE-SE aspects, near and above treeline (~9000 and above). Winds are forecasted to swing to the West-Northwest this AM (Thursday), which will redirect wind loading to N-E-S-SE aspects. Thursday, natural Wind Slabare possible, triggered releases are likely. By Friday, expect natural avalanches to become increasingly more unlikely but triggered releases will remain possible in isolated areas that favored drifting (rock outcrops, gullies, leeward side of corniced ridges, etc.) Use additional caution in and around terrain features that promote drifting, under cornices, tops of chutes, and gully sidewalls. 

Avalanche Problem 2: Loose Wet
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As skies clear today (Thursday) and temperatures begin to rebound, the new snow will warm quickly with the potential for Loose Wetavalanches rising primarily on E-S-W aspects, primarily from treeline and below (~10500”). Small Loose Wetavalanches are possible, triggered releases are likely Thursday. 

Friday, temperatures are forecasted to climb into the mid 30’s and upper 40’s, which will increase the potential for Loose Wetavalanches to rise through the day from ~10500’ (possible higher in sheltered rocky terrain) and below, primarily on E-S-W-NW aspects but may creep around the compass to include N-NE aspects as well. Signs of wet snow instabilities include: pinwheels, rollerballs, deep ski or boot penetration, and point releases. Loose Wetavalanches can trigger larger deeper releases, especially in the lower elevations (~10500’ and below). Small natural avalanches will be possible, triggered releases likely, especially in sheltered, steep, or rocky terrain. 

 

advisory discussion

The overnight storm that passed through the region (4/11) left 3” to 4” of new snow in it’s wake. A band of significant graupel occurred with frontal passage, at least in Mammoth. The new snow was accompanied by strong SW winds which likely formed sensitive Wind Slabs above ~10500 on NW-NE-SE aspects overnight. Winds are forecasted to shift to the W-NW, which may form smaller and more isolated but possibly tender Wind Slabs on N-E-S-SW aspects. Generally, the new snow will likely bond well to the surface as temperatures didn’t fall below freezing until after precipitation onset. 

Loose Wet avalanche concerns will rise in the mid and lower elevations (10,500’ and lower) on primarily E-S-W aspects and by Friday including NW and possibly N-NE . The new snow will need a few nights to fully bond and become less responsive to triggered release.  

 

 

recent observations

Red Cone (4/11)

 

Mount Wood (4/11)

 

Mono Pass Tour (4/10)

 

Spring Conditions In Valentine Cirque (4/9)

 

Large Loose Wet Avalanches, Valentine Cirque (4/9)

 

Wet Slab in Fresno Bowl (4/9)

Temperatures @ 0400

Loc                                          New    Highs/Lows                                            

Virginia Ridge, 9409’:             4”        48/18 deg F., 18 hrs below freezing, 
Tioga Pass, 9972’:                 N/A     N/A deg F.,     N/Ahrs below freezing.
Agnew Pass, 9355’:               3”        45/18 deg F., 15 hrs below freezing
June Mt., 9148’:                    3”        49/17 deg F., 16 hrs below freezing
Mammoth Pass, 9500’:          N/A     46/20 deg F., 15 hrs below freezing
Sesame Plot, 9014’:              3”        42/28 deg F., 8 hrs below freezing
Ch. 22-MMSA, 10,067’:                     37/14 deg F., 10 hrs below freezing
Summit-MMSA, 11,053’:                  32/11 deg F., 12 hrs below freezing
Rock Creek, 9600’:                N/A     53/17 deg F., 15 hrs below freezing
South Lake, 9580’:                            N/A     52/20 deg F., 16 hrs below freezing 

Sawmill, 10,200’:                   4”        48/18 deg F., 16 hrs below freezing

Weather and CURRENT CONDITIONS
weather summary

Thurs thru Saturday– today (Thursday), chilly conditions and brisk winds (gusts mainly    35-40 mph) will be the main weather feature as the shortwaveexits to the east. A short period of increased north or northeast winds is likely across Sierra ridges into Friday morning. For Friday and Saturday, a low amplitude ridgeof high pressure will set up overhead then shift to the east with highs in the lower 50s near degrees in the Sierra, then warming further by 6-12 degrees on Saturday as late day west breezes return.

Next Week- Another typical spring stormis expected early in the period for    Sunday into Monday. It will be windy Sunday ahead of the cold frontand also relatively dry. Then the cold frontwill move in with most of the precip again falling behind the frontitself with the core of the low moving through Central CA. This will bring the mid-level cold pooloverhead with more numerous snow/pellet showers. A few thunderstorms are also likely in this pattern with some brief bursts of heavy     snow possible, but not widespread. Snow showers wind down during the evening with dry days expected Tuesday into Wednesday. 

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: Sunny then becoming partly cloudy. Isolated snow showers in the afternoon. Clear Sunny
Temperatures: 26 to 34 deg. F. 13 to 19 deg. F. 41 to 47 deg. F.
Wind Direction: Northwest Northwest North
Wind Speed: 15 to 25 mph. Gusts up to 40 mph. 10 to 15 mph with gusts to 35 mph in the evening becoming light. 10 to 15 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Partly cloudy. Isolated snow showers in the afternoon. Clear Sunny
Temperatures: 20 to 26 deg. F. 11 to 16. deg. F. 35 to 40 deg. F.
Wind Direction: West becoming northwest Northwest North
Wind Speed: 30 to 45 mph with gusts to 75 mph, 20 to 30 mph with gusts to 45 mph in the afternoon. 15 to 25 mph with gusts to 35 mph increasing to 25 to 35 mph with gusts to 55 mph after midnight. 25 to 35 mph with gusts to 50 mph decreasing to 15 to 25 mph with gusts to 40 mph in the afternoon.
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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