Avalanche Advisory - Sat, Feb. 04, 2017

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THIS ADVISORY EXPIRED ON February 7, 2017 @ 7:00 am
Avalanche Advisory published on February 4, 2017 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

Elevated avalanche danger exists for the next 72 hours due to new snowfall and moderate to strong SW winds.  Half to a foot of new dense snow fell Thursday and Friday accompanied by moderate to strong SW winds which will continue into next week.  Up to another foot and a half of new snow is expected Sunday night into Monday.  

Saturday and Sunday natural avalanches will be possible and human triggered avalanches likely due to new fresh wind slabs and some lingering sensitive storm slabs.  Likelihood and size will increase with increasing elevation and exposure to wind.  Careful snowpack evaluation and cautious routefinding is essential.

Monday avalanche danger will increase.  Natural and human triggered avalanches will be likely as new snowfall occurs accompanied by strong SW winds.  Concern will be focused on fresh wind slabs and storm slabs.  Careful snowpack evaluation and cautious routefinding is essential, and for areas that receive greater amounts of new snow-load, it is not recommended to travel in avalanche terrain.

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.

Half to a foot of new dense snow fell Thursday and Friday and was accompanied by moderate to strong SW winds which will continue into next week.  These winds have and will continue to form new sensitive wind slabs at all elevations, which will most likely be found below ridgelines, the leeward side of ridges and across cross-loaded slopes which face NW-N-NE-E-SE.  Size and likelihood will increase with elevation, but remember even small low elevation slopes that end in a terrain trap where snow could pile up deeply can be dangerous.  These wind slabs will become larger and more widespread as up to another foot and a half of new snow falls Sunday night and Monday. Careful snowpack evaluation and cautious routefinding is essential to avoid traveling in areas with new snowload that could be sensitive to human triggering, or traveling below slopes being loaded by wind that could fail naturally.

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Half to a foot of new dense snow fell Thursday and Friday.  This dense new snow immediately formed cohesive slabs.  In areas sheltered from the wind that are 35 degrees and steeper where new snow amounts are closer to the 1 foot range, human triggered avalanches could still be possible where this new snow has not yet bonded to the underlying old snow.  This concern will decrease gradually throughout the day on Saturday and Sunday, before more new snow comes Sunday night and Monday when it will again elevate.  This concern will exist at all aspects and elevations.  Do your own stability assessments to see how well the new snow is bonding and stabilizing before venturing onto steeper terrain.  Be especially aware of steep convex points in the slope where a storm slab avalanche would most easily be triggered. 

Snowpack Discussion

Last week’s stable conditions that allowed safe access to high peaks and more extreme terrain has come to and end with half to one foot of new dense snowfall and high SW winds this past Thursday and Friday.  A break from snowfall Saturday and Sunday will allow storm slabs which formed in sheltered areas time to stabilize, but continued high SW winds which will continue through next week will continue to form sensitive potentially dangerous wind slabs.  As snowfall begins again Sunday night into Monday, storm slab concern will again rise, and wind slab concern will rise even more dramatically. 

weather

Saturday:  Partly cloudy skies with a chance of scattered flurries in the afternoon, with no new accumulation expected for today.  Around 10,000’, high temperatures will reach the low to upper 20s, and winds will be mostly out of the SW in the 25 to 40mph range gusting to 75 mph, decreasing slightly in the afternoon.

Sunday:  Mostly cloudy skies are expected for Sunday, with high temperatures reaching the mid 20s to mid 30s at 10,000’, and SW winds in the 30 to 30mph range gusting to 45mph, and increasing up to 65mph in the afternoon.  Snow is forecasted to begin Sunday night after 10pm with 4-10” of new snow expected by morning, with the greater amounts near Mammoth.  Lows Sunday night will be in the upper teens around 10,000’, and strong winds will continue out of the SW.

Monday:  Snow, heavier in the morning, is expected for Monday, with up to another half a foot of accumulation expected near Mammoth.  Winds are expected to increase even more out of the SW gusting up to 100mph over ridgetops.  Expect high temperatures around 10,000’ in the 20s. 

Long-Term:  More snow on the way thru next week!  At this time 2 more weak to moderate Atmospheric River events look to be on their way for Tue and Wed, with a break Wed night into Thur, then Thur night into Friday.  Snow levels over 8,000’ look probable to begin with before dropping significantly on Friday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE AFTERNOON PARTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE EVENING MOSTLY CLOUDY
Temperatures: 27 TO 35 deg. F. 19 TO 25 deg. F. 30 TO 40 deg. F.
Wind direction: SW SW SW
Wind speed: 15 TO 25 MPH WITH GUSTS TO 55 MPH 10 TO 20 MPH WITH GUSTS TO 40 MPH 15 TO 25 MPH. GUSTS UP TO 35 MPH INCREASING TO 50 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE AFTERNOON PARTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE EVENING MOSTLY CLOUDY
Temperatures: 20 TO 30 deg. F. 16 TO 22 deg. F. 24 TO 34 deg. F.
Wind direction: SW SW SW
Wind speed: 25 TO 40 MPH. GUSTS UP TO 75 MPH DECREASING TO 65 MPH IN THE AFTERNOON 20 TO 30 MPH. GUSTS UP TO 55 MPH DECREASING TO 45 MPH AFTER MIDNIGHT 15 TO 25 MPH WITH GUSTS TO 45 MPH INCREASING TO 20 TO 35 MPH WITH GUSTS TO 65 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This Snowpack Summary 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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