Snowpack Summary - Sat, Dec. 31, 2016

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THIS ADVISORY EXPIRED ON January 3, 2017 @ 7:01 am
Avalanche Advisory published on December 31, 2016 @ 7:01 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 72 hours –

SaturdaySmall isolated Wind Slabs on N-W-S-SW aspects along ridgetops and in and around exposed terrain features that promote drifting or extreme terrain in the upper elevations, generally limited to areas where loose snow is still available for transport. Natural and triggered avalanches are unlikely. Usual caution in terrain that 35 degrees and steeper.

Sunday thru Monday - light snowfall combined with moderate Westerly winds will form sensitive Wind Slabs on N-E-S aspects in the mid to upper elevations along ridgetops and in and around exposed terrain features that promote drifting in the mid to upper elevations. Natural avalanches will be possible human-triggered avalanches likely in the mid to upper elevations, especially along ridgetops and in and around terrain features that promote drifting, especially along ridgetops and in and around terrain features that promote drifting. 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.

Primary avalanche focus for Sat thru Monday:

Saturday – Moderate Easterly winds thru Saturday night will likely form isolated Wind Slabs on N-W-S-SW aspects in the mid to upper elevations.  Wind Slabs will primarily be encountered along ridgetops and in and around exposed terrain features that promote drifting. Wind Slabs will likely be relatively small in size but they could easily capture an unsuspecting rider and drag them into hazardous terrain or if combined with a terrain trap, possibly result in burial. Avoid hollow sounding slabs or freshly formed drifts.

Sunday thru Monday - winds will veer to the West (20 to 30 MPH increasing to 35 TO 50 MPH) ahead of the approaching storm system with light snowfall forecasted along the Crest. The combination will likely form sensitive Wind Slabs on N-E-S aspects in the mid to upper elevations. Natural avalanches will be possible human-triggered avalanches likely in the mid to upper elevations, especially along ridgetops and in and around terrain features that promote drifting. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. Even though expected Wind Slabs will likely be relatively small in size, they could easily capture an unsuspecting rider and drag them into hazardous terrain or if combined with a terrain trap, possibly result in burial. Lookout for smooth firm hollow sounding snow and do your own quick assessments throughout the day.

Because of the widely varying conditions, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.   

Snowpack Discussion

Clear conditions have dominated the weather since the Christmas Eve storm, which dumped 1’-2’ of new snow. Since then, the snowpack has adjusted to the added load with good bonding at the old/new snow interface and sensitive Wind Slabs, which formed earlier in the week, have strengthened and healed. The post-Christmas cold snap helped drive facet formation in the upper snowpack, which makes for good skiing but may form a weak layer as it becomes buried this coming week. Sunday thru Monday, moderate Westerly winds and light snowfall are forecasted which is expected to form small sensitive Wind Slabs in the mid to upper elevations primarily on N-E-S aspects along ridgetops and in and around terrain features that promote drifting.  Small natural avalanches will be possible human-triggered avalanches likely in the mid to upper elevations, especially along ridgetops and in and around terrain features that promote drifting. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential. Even though expected Wind Slabs will likely be relatively small in size, they could easily capture an unsuspecting rider and drag them into hazardous terrain or if combined with a terrain trap, possibly result in burial. Lookout for smooth firm hollow sounding snow and do your own quick assessments throughout the day.Though natural avalanches will be unlikely, human- triggered avalanches will be possible. Evaluate snow and terrain carefully and identify terrain features of concern.

Early season October storms formed a layer of fairly loose facets over a thick basal ice layer at ground, which is now capped by a Melt/Freeze crust that formed during the Dec 9th storm between the elevations of ~9,500’ and ~11,000’ on NE-N-NW facing slopes. This deep persistent weak layer is not currently active but is worth noting as we move into another round of active weather, which could re-awaken it.  This is likely more of a concern in areas where the snowpack is especially thin (South of Mammoth and North of June Lake). Additionally, Surface Hoar (3-6mm) has been reported in many locations, even along the Crest. Pre-frontal winds will likely breakup much of the Surface Hoar in exposed areas but sheltered areas may see the Surface Hoar become buried as the next storm system moves into the region. This will need to be monitored as the weather week develops.

Because of the widely varying conditions along the length of the Range, spend the extra time to investigate the snowpack for yourself and perform your own stability tests.   

 

recent observations

Mammoth Rock, Mammoth Basin (12/30/16) - SMG Avy 1 course toured up from the end of Old Mammoth Rd to Mammoth Rock area. Moderate to Light winds, little to no blowing snow. Temps: above freezing. Snow pit tests inconclusive: ECTN X2, CTH X2. HS just west of Mammoth Rock at around 8600' 65cm.

Sherwins, Mammoth Basin (12/30/16) - Up from end of Old Mammoth Rd, past Mammoth Rock to Ridgeline and back. Steady moderate Southerly winds at ridgetop, some light gusts below on NE facing slope, a little more closer to exposed westerly ridge above mammoth rock.  No visible snow being transported near ridgetop or on any surrounding peaks.  Winds are definitely strong enough, but not much loose snow left in wind exposed areas after the strong southerly and westerly winds that occurred after the last storm.  Perhaps at higher elevations further from Mammoth there could be some small new wind deposits .....? No signs of instability today.  Surprisingly great skiing snow still on the NE slopes of the Sherwins!  (The upper snowpack has still been getting faceted at night) 

Negatives, June Lake (12/29/16) - Standard route off the back of June, up Hour Glass to the ridge, along ridge top to far lookers right chute which seemed to get the most sunlight, down and skinned back into June Mtn. Aspects from E-S-W sun affected, Northerly aspects wintery snow: soft powder to very firm windboard and everything in between. First quick pit dug a few hundred yards past the top of Hour Glass on East facing slope that seemed not too wind effected.  29deg slope, 99cm total snow depth.  1cm melt-freeze crust present 23cm down from top, ECTN26 just below this crust in thin slightly faceted layer.  No discernable crusts or distinct layers below this upper crust.  Air temp at 11:00am at 9,500' = +8.0 deg C.  No clouds, no wind.  Top 5cm of snowpack here was very moist. Another pit dug just below ridgeline (11,100') in one of first main Negative Chutes (NE facing, 49deg slope).  Upper 10cm firm windcrust failed and propagated cleanly during ECT test with moderate force.  Modified ECT/Deep Tap test to check deep Facet layer above thick basal ice layer failed roughly Q3 with hard force, and did not propagate. (removed snow except for ~15cm above this thick faceted layer.)  Of note, these kind of weak layers are considered more heads-up when they are less than 10cm thick (this one was ~24cm thick). Skied far SE facing line, very variable, still quite thin snowpack here.  ~1pm.  At many points it was moist to ground, and in shaded spots it was beginning to re-freeze.  Some wet sloughing, and some rollerballs as a result of upper ski cuts, and lower down with skiing, but none of these point releases entrained much snow.  Punchy, manky, would have been better two hours earlier.   

weather

Sat thru Sunday - A shortwave trough is currently dropping south along the northern California coast with increasing clouds today with most precipitation remaining west of the Sierra. A few very light showers may graze the southern extent of Mono County this afternoon but not much more than a few hundredths of precipitation expected from this system. Overall, cooler Saturday with light northerly breezes and highs in the upper 30s to lower 40s across much of the region. Sunday we will transition to a colder pattern as a strong cold front drops south from the Pacific Northwest. This system will create gusty winds ahead of the front with wind gusts of 40-50 mph possible.

Mon thru Tuesday - The cold unstable airmass remains in place through at least Tuesday morning, with continued scattered snow showers across the area. The pattern looks to remain active and wet with a series of system pushing across the Sierra. A vigorous shortwave will move into the area late Tuesday on top of all the cold air with the potential for widespread snowfall with 1 to 2 feet in the Sierra as a deep moist and unstable atmospheric profile allows for easy spillover. 

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: MOSTLY CLOUDY. ISOLATED SNOW SHOWERS. PARTLY CLOUDY THEN BECOMING CLEAR. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 31 TO 36. deg. F. 14 TO 19. deg. F. 30 TO 36. deg. F.
Wind direction: LIGHT WINDS LIGHT WINDS BECOMING SOUTHWEST WEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 30 MPH AFTER MIDNIGHT. 10 TO 15 MPH WITH GUSTS TO 45 MPH INCREASING TO 20 TO 30 MPH WITH GUSTS TO 65 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: MOSTLY CLOUDY. ISOLATED SNOW SHOWERS. PARTLY CLOUDY THEN BECOMING CLEAR. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 26 TO 32. deg. F. 11 TO 16. deg. F. 24 TO 32. deg. F.
Wind direction: SOUTHEAST EAST SHIFTING TO THE WEST AFTER MIDNIGHT. WEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 30 MPH. 10 TO 15 MPH. GUSTS UP TO 35 MPH. 20 TO 30 MPH WITH GUSTS TO 55 MPH INCREASING TO 35 TO 50 MPH WITH GUSTS TO 85 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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