Eastern Sierra Snowpack Summary - 2016-03-19 07:11

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THIS ADVISORY EXPIRED ON March 21, 2016 @ 7:11 am
Avalanche Advisory published on March 19, 2016 @ 7:11 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

The current melt-freeze cycle will continue thru Sunday on solar aspects at mid to upper elevations and below ~8,500’ where snow is still present. Clear skies and cool nightly temperatures have promoted good refreezing at the mid to upper elevations. Lower elevations (below ~8,500), freezes have been weaker and tend to thaw more rapidly. The melt-freeze surface will soften through the day, with east and southeast facing slopes in the morning, southerly facing slopes by mid-day, and more southwesterly and west facing slopes by afternoon. Lower elevation slopes will soften more quickly and to a greater depth and potential become unsupportable. Temperatures will cool few more degrees by Sunday, slowing the thaw cycle.  Signs of slopes potentially becoming unstable are fresh rollerball activity, boot penetration greater than boot-top, and fresh wet-loose avalanche activity on similar aspects. Plan to be off these slopes early, and be willing to adjust you plan/route/turn-around time.  Natural and human triggered wet-loose avalanches are possible on E-SE-S-SE-W slopes >35 degrees as the melt-freeze crust thaws and becomes unsupportable. Wet-loose avalanches have the potential to trigger larger Wet Slab avalanches. Northerly facing steep slopes below ~8500’ have the potential to become isothermal, especially where snowcover is exceptionally thin, increasing the likelihood of wet releases. Wet avalanches are relatively slow moving but can be very difficult to get out of once entrained.  Small releases can be dangerous, especially when they involve terrain traps, cliffs, rocks, and/or trees.  Be especially cautious when traveling in complex terrain around rock bands, and terraces where the snowpack may be inherently thin and thawing rapidly. These areas may not freeze thoroughly and may become isothermal early during the day.    

 

Avalanche Character 2: 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 Slab potential from the last storm system (Sun-Mon) has declined as the new snow and Wind Slabs bonded to the underlying snowpack and have become less sensitive to triggering. However, winds are forecast to increase over the course of the weekend to 25 to 30 mph with gusts to 45 mph increasing to 40 to 55 mph with gusts to 85 mph by Sunday afternoon, which could produce new Wind Slabs on NW-NE-SE aspects in the mid to upper elevations. With little new snow for transport, these should be isolated and in favored areas.

 

Snowpack Discussion

Avalanche concerns for this Snow Summary (valid 48 hours) will be focused on loose-wet activity on solar aspects for slopes that face E–SE-S-SW-W as they soften throughout the day as a result of clear sunny skies, above average air temperatures, and low winds.

The current melt-freeze cycle will continue thru Sunday on solar aspects at mid to upper elevations and below ~8,500’ where snow is still present. Clear skies and cool nightly temperatures have promoted good refreezing at the mid to upper elevations. Lower elevations (below ~8,500), freezes have been weaker and tend to thaw more rapidly. The melt-freeze surface will soften through the day, with east and southeast facing slopes in the morning, southerly facing slopes by mid-day, and more southwesterly and west facing slopes by afternoon. Lower elevation slopes will soften more quickly and to a greater depth and potential become unsupportable. Temperatures will cool few more degrees by Sunday, slowing the thaw cycle.  Signs of slopes potentially becoming unstable are fresh rollerball activity, boot penetration greater than boot-top, and fresh wet-loose avalanche activity on similar aspects. Plan to be off these slopes early, and be willing to adjust you plan/route/turn-around time.  Natural and human triggered wet-loose avalanches are possible on E-SE-S-SE-W slopes >35 degrees as the melt-freeze crust thaws and becomes unsupportable. Wet-loose avalanches have the potential to trigger larger Wet Slab avalanches. Northerly facing steep slopes below ~8500’ have the potential to become isothermal, especially where snowcover is exceptionally thin, increasing the likelihood of wet releases. Wet avalanches are relatively slow moving but can be very difficult to get out of once entrained.  Small releases can be dangerous, especially when they involve terrain traps, cliffs, rocks, and/or trees.  Be especially cautious when traveling in complex terrain around rock bands, and terraces where the snowpack may be inherently thin and thawing rapidly. These areas may not freeze thoroughly and may become isothermal early during the day.     

Storm slab and Wind Slab potential from the last storm system (Sun-Mon) has declined as the new snow and Wind Slabs bonded to the underlying snowpack and have become less sensitive to triggering. However, winds are forecast to increase over the course of the weekend to 25 to 30 mph with gusts to 45 mph increasing to 40 to 55 mph with gusts to 85 mph by Sunday afternoon, which could produce new wind slabs on NW-NE-SE aspects in the mid to upper elevations. With little new snow for transport, these should be isolated and in favored areas. 

recent observations

Mt Morgan North, SE Ridgeline (3/18/16) -SE aspect, 12,000', boot top boot penetration at 11am.  

-ESE aspect, 12,000', halfway to boot top boot penetration at 11am.  

- Isolated sheltered N aspect, 10,200', 12:30pm, boot top wintery dry soft powder.  

- Began descent at 12:00pm from 12,700', ESE aspect, <10cm ski penetration, fairly perfect smooth creamy almost corn skiing down to 10,500', with very minor rollerball activity in isolated spots.

- No evidence of natural avalanche activity, minimal rollerball activity, especially above 10,000' until at least 1pm.  Below 9,700' at 1pm, ESE facing slope, moderate rollerballs as a result of skiing.   

Negatives (3/18/16) – On approach sun affected snow (thawed melt freeze crust) on W-S-E aspects to at least 10,00’. Cold snow can be found only on a narrow, ever shrink aspect spectrum of NW - N- NE aspects above ~ 9,700’. Some older roller balls on East and South aspects. One wet loose released from a rock band and triggered a small wet slab on the ledge below and ran about 200’ vertical, some rocks mixed in with debris (State Zone Elevation 9,900’, East Aspect, slope angle ~ 40 º). No other activity noted, stability good overall depending on time of day and terrain. Near treeline and below, many southerly aspects and lower elevations are melting-out fast, thin areas are semi supportable and receding quickly, below ~7.600’ snow is nearly non-existent except in favored areas (i.e. avalanche path runouts, wind loaded gullies and ridgelines) Creeks are opening up and snow bridges are suspect. Above treeline, old wind slabs settling and softening in places on Northerly aspects, Southerly aspects are softening, early in the corn cycle the more you ascend.

Lamarck Col (3/16/16) - 168 closed at winter closure at Aspendell. North Lake Road was snow covered above switchbacks and beyond. Travelled through some soft snow with obviously temperature-affected surface on the climb from North Lake to Grass Lake. Very sheltered, shady spots below treeline held powder. Above treeline most if not all snow surfaces we travelled on were supportable windboard with some navigating through sections of sastrugi all the way to Lamarck Col. Generally easy travel and we were able to keep our skis on all the way to the Col with some creative route finding through talus in a few locations. Probing on steeper wind affected slopes revealed 100-130 cm snow with mostly consistent, firm layering throughout, with some weakness felt near the ground. Ski penetration was minimal for the most part. On our descent, only southerly features softened slightly from about 12400' to treeline. Good smooth skiing if you followed the right ribbon. We observed no evidence of recent avalanches, mostly wind. There was no evidence of loose wet activity above treeline, although below treeline is getting close; some minor entrainment of snow with ski cuts on steep sun affected slopes. If visiting this zone, be prepared for a bit of walking to snowline. West of the crest appears to be fat with snow, in particular along the LeConte Divide!

weather

Sat – Sunday: mild temperatures (Saturday highs, 50’s 60’s) and light winds will give way to a moderate Pacific storm due to enter the region Sunday. Mid to upper level flow begins to increase out of the southwest Saturday night as the high-pressure ridge axis shifts east. The moisture associated with the next system will push into northern California by early Sunday. This system is comprised of two main waves. The first will bring enough forcing to produce precipitation across the northern California Sunday afternoon, sliding south into the northern Sierra by Sunday evening. Sunday, snow levels will start quite high, above 8000’, as the upper level jet passes farther north, gusty winds increasing as the lead wave moves off the Pacific into northwest California and Oregon. Many areas could see wind gusts 30-40 mph Sunday afternoon and evening.

Mon-Tuesday: The second wave pushes a bit further south Monday with snow levels falling as colder air begins to push into the region late in the day. The upper level winds will be perpendicular to the Sierra, which may generate some enhanced orographic precipitation by early Monday morning with gusty winds east of the Sierra Monday afternoon/evening with gusts 35 to 45 mph in the valleys, possibly up to 85 mph along the Sierra ridges. Snow levels should drop below 6500 feet by Monday evening. Showers linger through Monday night. Total precipitation amounts between Sunday and Monday night between 0.10" and 0.25" water for the immediate lee of the Sierra, which should be good for a few inches of snow.

 

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 PARTLY CLOUDY THEN BECOMING CLEAR PARTLY CLOUDY
Temperatures: 52 TO 60 deg. F. 31 TO 36 deg. F. 48 TO 58 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 25 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH INCREASING TO 20 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY PARTLY CLOUDY THEN BECOMING CLEAR PARTLY CLOUDY
Temperatures: 44 TO 52 deg. F. 25 TO 31 deg. F. 38 TO 48 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 25 MPH WITH GUSTS TO 35 MPH. 20 TO 30 MPH WITH GUSTS TO 40 MPH. 25 TO 30 MPH WITH GUSTS TO 45 MPH INCREASING TO 40 TO 55 MPH WITH GUSTS TO 85 MPH IN THE AFTERNOON.
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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