Avalanche Advisory - Sat, Jan. 07, 2017

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THIS ADVISORY EXPIRED ON January 10, 2017 @ 6:59 am
Avalanche Advisory published on January 7, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

A strong Atmospheric River event is on tap to bring copious amounts of snow and rain to the forecast area from Saturday though Monday.  A Complex mix of dangerous avalanche problems are expected for Saturday, peaking on Sunday and continuing thru Monday. 

Saturday:  Concerns will begin in the morning with wind slabs, both new and lingering, at mid to upper elevations on NW-N-NE-E slopes.  As up to 12” of new snow falls throughout the day above 8,000’ with moderate to strong SW winds, shifting to NW at higher elevations, natural wind slab avalanches will become increasingly possible, and human triggered wind slab avalanches likely.  It is possible that one of these avalanches could step down and trigger a deeper persistent weak layer, resulting in a very large avalanche.  Storm slab avalanches will become increasingly possible in wind-sheltered locations as well.  Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Saturday night through SundayNatural and human triggered avalanches will be CERTAIN!  As temperatures rise and precipitation rates increase dramatically Saturday evening through Sunday, large and destructive natural wet avalanches will become likely on all aspects and slopes that become saturated by the heavy rainfall, which could be up to 9,500’ or even up to 10,000’.  Avalanche danger will increase dramatically as well on slopes above rainfall elevation, as the heavy new snow-load in combination with moderate to strong SW winds will result in natural and human triggered Avalanches becoming very likely especially in exposed terrain, but also in wind sheltered terrain.  Avoid being on or below avalanche terrain!  It is quite possible that avalanche paths that haven’t seen activity in a long time will become active. 

Monday:  Temperatures will drop and bring snow line back down below 8,000’ by mid-morning, resulting in another 6-12” of snow above 8,000’ by days end.  Even though many natural avalanches are likely to have occurred before today, continued strong SW winds will continue to form dangerous windslabs and load slopes. Avalanches on previously avalanched slopes will be likely, and depending on the tipping point, today’s increased load may be enough to trigger a very large slide on a slope that hasn’t yet slid.  Human-triggered avalanches will be likely and natural avalanches possible.  Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. 

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.

Wind Slabs and Storm Slabs!  Wind slabs will be a continuous concern on leeward slopes and terrain features for Saturday, Sunday and Monday.  Moderate to strong SW winds during this period, shifting out of the NW at times at higher elevations, along with significant continuous new snow, will load leeward slopes, especially below ridges, sidewalls of gullies and across exposed slopes.  Observations on Friday have shown that some existing windslabs could still be sensitive to human triggering at mid to upper elevations, and that visible snow transport over upper elevation ridges likely formed some new sensitive windslabs on a variety of leeward slopes.  On Saturday, concerns for new wind slab development will increase above 8,500’, and will dramatically increase thru Saturday night and Sunday at higher elevations above rain-line (which is forecasted to reach above 9,000’, and possibly up to 10,000’).  Intense snow fall amounts Saturday night thru Sunday will mean that natural and human triggered avalanches will be almost certain, and wind-loaded slopes and sheltered slopes with less-dense storm slabs facing all aspects should be avoided.  Wind slab concern and Storm slab concern will return to lower elevations down to 8,500’ on Monday as temperatures cool and snowfall continues at a more modest rate with levels dropping below 8,000’.         

Avalanche Character 2: Wet Slab
Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.

Rain on cold dry snow is a recipe for widespread avalanche activity.  As temperatures rise Saturday night and remain warm thru Sunday night, forecasts are predicting rain up to 9,000’, with potential up to 10,000’.  These warm temps will coincide with intense tropical precipitation.  Slopes >30 degrees facing all aspects that receive heavy rain will become saturated quickly and be at high risk for avalanching.  Greatest risk for large destructive avalanches will be earlier on as these slopes first become saturated, and risk will decrease as slopes already avalanche, or develop water channels thru the snowpack.  Avoid being on or under slopes greater than 30 degrees that are receiving heavy rainfall. 

Avalanche Character 3: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

There are still buried weak layers in the snowpack associated with the rain crust layers from the warm October storms that exist between 9,000-11,200’.  Mammoth mountain patrol caused huge avalanches yesterday (Friday) with small hand charges that caused slope failures that stepped down to these deep persistent October weaknesses and resulted in crowns over 12’ high.  Click here for photos.  In areas that have had shallower snow-packs (south of Mammoth and north of June), basal facets have been found to be weak and in some areas that are failing easily during stability tests.  It is very possible that avalanches from the heavy loading that will come this weekend could step down to one of these weak layers resulting in very large destructive avalanches. 

Snowpack Discussion

A complex mix of Avalanche conditions will exist for the next 3 days.  The bottom line is that very dangerous avalanche conditions will exist, peaking on Sunday, when you should avoid being on or under avalanche terrain all together.  The existing snowpack is very variable throughout our forecast region from Bishop up north through June to Virginia Lakes.  Persistent weak layers have been found.  Their reactivity has varied, with areas of shallower snowpack generally being more reactive.  However, with this intense heavy new load, these deep persistent layers could fail where they exist even in areas with deeper snow packs.  The storm instabilities from the significant storm that occured mid-week (up to 42" are recorded of new snow in the more heavily hit areas including Mammoth and Rock Creek) have been shown to be stabilizing, however upper elevation winds have continued to form sensitive windslabs at upper elevations.   Regardless of what the underlying snowpack looks like, the new intense snow load will on its own will create dangerous avalanche conditions in the form of wind slabs on exposed slopes and less dense storm slabs on sheltered slopes.  Rain on snow is a very dangerous phenomenon.  Snow slopes that recieve heavy rainfall Saturday night thru Sunday night (potentially up to 9,000'+ or even 10,000') will be at high risk for Avalanching, especially as they first become saturated.    

weather

* A very strong and warm Atmospheric River will bring copious amounts of rain and snow to the region Saturday through Monday.

SATURDAY:  Precipitation will begin early Saturday morning as night time temperatures in the teens will increase throughout the day into the upper 30s by late afternoon below 10,000’.  Up to 12 inches of new snow is expected down to 8,000’ feet by 4pm, when snow level is expected to start rising rapidly bringing rain up to 9,000’.  Precipitation rate will increase dramatically overnight, with an additional 12-24” of snow by morning above the rain-level.  Moderate southwest winds will blow 15 to 25 mph at the lower elevations with gusts increasing to 65 mph after midnight. Above 10,000 feet winds will gust up to 95 mph by the afternoon, and shift out of the NW.

SUNDAY:  Intense precipitation is expected throughout the day and into the night.  Heavy rain expected all day below 9,000’, and potentially up to 10,000’, with heavy snow at higher elevations.  4-8 inches of moisture is expected during Sunday and Sunday night, which could mean 2-3ft of additional snow at higher elevations.  Expect highs in the mid to low 40s.  SW winds are expected to blow 35 and 45 mph, with gusts up to 70 mph at low to mid elevations, and gusts over 100 mph at upper elevations.  Sunday night heavy precipitation will continue.

MONDAY:  Snow levels are expected to gradually come down Monday morning into the early afternoon as temperatures cool and precipitation intensity decreases.  Snow level should drop to 8,000’ by mid-morning.  Another 6-12 inches of snow is possible above 8,000’ through the day.  Strong SW winds will continue, with high temperatures in the upper 20s at mid elevations. 

Snow totals above 10,000’ for this 3-day period could easily reach 3 to 8+ feet by Monday night.  Snow will be very heavy and wet.

LONG-TERM - Another Atmospheric River event will follow on the heals of this first one for Tuesday and Wednesday.  This one is projected to be somewhat less intense and cooler, with snow levels staying below 7,000’.  Additional cold storms are expected for the end of the week.

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: CLOUDY. SNOW THROUGH THE DAY. RAIN IN THE AFTERNOON. CLOUDY. SNOW AND RAIN. CLOUDY. RAIN AND SNOW.
Temperatures: 33 TO 38 deg. F. 30 TO 35 deg. F. 38 TO 43 deg. F.
Wind direction: W SW S
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH 15 TO 25 MPH WITH GUSTS TO 40 MPH 20 TO 35 MPH WITH GUSTS TO 70 MPH
Expected snowfall: 4-10 in. 4-8 in. 4-12 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SNOW. CLOUDY. SNOW. CLOUDY. SNOW.
Temperatures: 29 TO 34 deg. F. 25 TO 30 deg. F. 32 TO 40 deg. F.
Wind direction: SW SHIFTING TO NW IN AFTERNOON SW SW
Wind speed: 30 TO 45 MPH WITH GUSTS TO 75 MPH, 35 TO 55 MPH WITH GUSTS TO 95 MPH IN THE AFTERNOON. 30 TO 50 MPH. GUSTS UP TO 80 MPH INCREASING TO 90 MPH AFTER MIDNIGHT. 45 TO 60 MPH INCREASING TO 50 TO 70 MPH IN THE AFTERNOON. GUSTS UP TO 105 MPH.
Expected snowfall: 6 TO 12 in. 12 TO 24 in. 24 TO 36 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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