Eastern Sierra Snowpack Summary - 2015-12-29 07:47

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THIS ADVISORY EXPIRED ON December 31, 2015 @ 7:47 am
Avalanche Advisory published on December 29, 2015 @ 7:47 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Windslab – Pockets of freshly deposited windslabs and drifts on leeward slopes (North-East-Southeast) in the mid to upper elevations aspects. These pockets could trigger larger releases on adjacent slopes or possibly step down into deeper portions of the snowpack, especially from thin areas. 

Snowpack Discussion

The Christmas Day’s storm has moved off to the east over the weekend, a cold Northerly flow followed, which easily transported the low-density snow and redistributed it onto Northeast, East, to Southeast aspects, forming brittle windslabs and variable skiing except in sheltered areas where soft snow could still be found. As temperatures began to slowly moderate over the weekend, slab sensitivity began to ease in the mid to lower elevations while the upper elevations continued to see Northerly winds of 15 to 25 with continued transport snow onto Northeast – East – Southeast aspects.  The weekend wind has reduced the low-density snow into variable ski conditions with intermittent windslabs, unsupportive wind crusts, with patches of soft snow in sheltered areas (forested areas and sheltered lee slopes). Observations over the weekend indicate that the snowpack is adjusting to the recent snow load with the new snow showing signs of bonding and strengthening. The weak system that moved through late Monday has deposited an additional 1” to 4” of new snow and winds are forecast to remain light to moderate, which will help to cover the windslabs and improve the skiing. However, leeward slopes in the mid to upper elevations should be treated with caution where fresh drifts and windslabs have recently accumulated on North – East – Southeast aspects. Lower elevations are still relatively weak and travelers should excise caution around steep terrain features, unsupported slopes, and terrain traps where small slides can have dire consequences. 

recent observations

Observations Monday revealed a couple of reactive layers confined to the mid to upper snowpack with signs of some strengthening of the deeper facet layers with no failures recorded in the basal facets.  Ski Cuts produced only limited results confined to the new snow. During the brief lull between storm systems this weekend temperatures warmed enough to produce a weak unsupportive melt-freeze crust on East - South - Southwest aspects while Northerly aspects remained cool with a mix of soft snow in sheltered locations and variable windslabs in exposed terrain.

Stability Tests: Slope 21 degrees, Elevation 8,758', Aspect N, Location 37.62585, -119.00116.

CTH 23 @ 71cm above ground (Q3)

CTH 26 @ 51cm above ground (Q2), CTH 24 @ 51cm above ground (Q2), CTH 25 @ 51cm above ground (Q2)

ECTN 22 @71cm above ground (Propagated 5 cm beyond shovel blade)

 

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 16 deg. F.
Max. temperature in the last 24 hours: 27 deg. F.
Average wind direction during the last 24 hours: S-SW, N-NE
Average wind speed during the last 24 hours: 3-15, 10-20 mph
Maximum wind gust in the last 24 hours: 42 mph
New snowfall in the last 24 hours: 4, 2 inches
Total snow depth: 30, 57 inches
weather

The light snow and snow showers will gradually diminish this morning as the Low moves off into southern Nevada and Arizona. As the system moves out of the region, the low clouds are expected to break up some this morning although the northwest flow aloft is expected to bring mid and high clouds back overhead by this evening and overnight as a weak upper disturbance pass over CA.

Temperatures will continue to average a solid 10 degrees below normal Thursday with cold nights. A break is expected Tuesday before a weak trough moves into northeastern Nevada Wednesday with shallow moisture. Expect clouds for the Sierra Front with a north to northeast flow developing reinforcing colder air for Thursday as a strong surface high nudges in from eastern Nevada. High temperatures are expected to top out in the upper 20s to mid 30s for most valley locations on Thursday keeping cold temps across the Sierra and western Nevada.

Long-term (New Year`s day through Monday): A Rex block feature is in place for the start of the weekend, with cold low pressure remaining over the Great Basin with high pressure over the northern Rockies. The operational Global Forecast System (GFS) and the European Center for Medium Range Weather Forecasting Integrated Forecast System (ECMWF) are in disagreement in exactly how this feature will evolve. However, models suggest an upper low cut off over central Nevada Friday and Saturday. This low is locked under a ridge near the US/Canadian border but does begin to drift slowly north late Saturday into Sunday as the broad scale flow is under cut by the polar jet pulling an elongated trough toward the west coast. Ahead of this system, conditions will remain dry with temperatures well below average, as easterly flow keeps cold locked into the region. Model solutions diverge late Sunday into Monday with low confidence in any solution. What was once a consolidated negatively tilted trough that would impact much of California and Nevada is now shown as a fractured system with the first bit of energy moving into southern CA/northern Mexico late Sunday into Monday. If the ECMWF verifies we will see virtually nothing from this system while the GFS would support snow in the southern 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 PARTLY CLOUDY, OCCASIONAL FLURRIES IN THE MORNING. CLEAR THEN BECOMING PARTLY CLOUDY. PARTLY CLOUDY.
Temperatures: 19 TO 26 deg. F. 9 TO 15 deg. F. 25 TO 31 deg. F.
Wind direction: NORTHWEST WEST WEST
Wind speed: 15 TO 25 MPH. GUSTS UP TO 30 MPH INCREASING TO 40 MPH IN THE AFTERNOON. 20 TO 30 MPH WITH GUSTS TO 40 MPH. 15 TO 25 MPH WITH GUSTS TO 40 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY, OCCASIONAL FLURRIES IN THE MORNING. CLEAR THEN BECOMING PARTLY CLOUDY. PARTLY CLOUDY.
Temperatures: 16 TO 24 deg. F. 8 TO 14 deg. F. 16 TO 23 deg. F.
Wind direction: NORTHWEST NORTHWEST NORTHWEST
Wind speed: 20 TO 30 MPH. GUSTS UP TO 45 MPH INCREASING TO 50 MPH IN THE AFTERNOON. 20 TO 30 MPH WITH GUSTS TO 50 MPH. 20 TO 30 MPH WITH GUSTS TO 50 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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