Avalanche Advisory: Friday - Apr 6, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON April 7, 2018 @ 6:53 am
Avalanche Advisory published on April 6, 2018 @ 6:53 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger will rise into this evening as a very warm atmospheric river brings rain and thick snow to the Sierra crest. Loose wet avalanches will be very likely on all aspects and elevations. Wind slab avalanche will become increasingly likely once the upper elevations receive 4” or more of dense new snow for strong SW winds to redistribute onto leeward alpine slopes. It is even possible for a very large wet slab avalanche to be triggered once the snowpack becomes saturated. The complexity of the situation will make for dangerous avalanche conditions. Careful snowpack evaluation, cautious route finding, and conservative decision-making will be essential to your safety as the day goes on.

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Near Treeline

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Below Treeline
Avalanche Problem 1: Loose Wet
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Poor re-freeze overnight, warm humid air and rain up to very high elevations today will make loose wet very likely at all aspects and elevations where there is enough snow coverage. On steep slopes these sluffs could entrain enough snow to bury or injure a person. As more moisture falls throughout the day the problem will only get worse.

Avalanche Problem 2: Wind Slab
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Snow levels are expected to be between 11,000’ and 12,000’ today with thick, wet new snow. Winds are expected to be strong enough to redistribute that snow into dense slabs on high leeward slopes. Once we get up to 4” of snow up high the new wind slabs could be large enough to be dangerous. You will most likely find them below ridgetops and the sidewalls of gullies. If you are traveling in or below alpine terrain today be aware of blowing snow, cornice formation, and dense drifts on steep slopes. Those will be the most likely places you should avoid.

Avalanche Problem 3: Wet Slab
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In the northern area of the forecast zone, winter-like snow has still been found on steep northerly slopes. A few observations have found weaknesses from earlier in the season are still present in the middle and bottom of the snowpack. These layers are not weak enough to fail on their own, but once they become saturated with new water they could become very weak. A large trigger, like a loose wet or wind slab avalanche, rock or cornice fall, might “step-down” and trigger one of these weak layers resulting in a large avalanche. The problem will become more likely into this evening, but is certainly something to keep in mind if you venture out onto E-N-W facing middle and upper elevation slopes today.

advisory discussion

As rain falls today and dissolves the bonds between already wet snow grains loose wet avalanches are expected to be widespread and potentially large. Watch out for wet snow at the surface and boot-top penetration into slushy snow. Rollerballs or pinwheels are a good indication that the snow is losing cohesion. Loose wet avalanches will be the first problem to become likely today with a poor refreeze overnight and rain already falling. Wet snow on steep slopes is a combination to avoid today.

By this evening 4” or more of thick moist snow are expected above 11,000’. Winds will be strong enough out of the SW to redistribute it into dense wind slabs on leeward alpine slopes. Winds can deposit snow at 3 to 5 times the rate it can fall from the sky, so some of the new slabs could be large. Watch out for dense drifts below high elevation cornices and in the tops of steep chutes and gullies where crossloading may deposit slabs further down slope.

The snowpack this season remains complex. Areas at lower and middle elevations, and in the southern part of the forecast area, seem to have transitioned to a moist spring-like regime. But above 10,000’ on northerly slopes, especially in the northern half of the region, the snow is still cold enough to be wintry. These colder and deeper snow will absorb and hold more of the rain and become saturated. This could further weaken any poorly bonded layers in the snowpack and create a wet slab problem. Today this problem will be less likely, but the problem will become more touchy and more possible as we receive more moisture. Steep northerly slopes in the middle and upper elevations may become the starting zone for very large wet slab avalanches by tonight. Especially if a smaller loose wet or wind slab avalanche suddenly overloads one of the weak layers buried in the colder, drier snowpack.

recent observations

4/5- Rock Creek: Warm, Wet Spring Snowpack

4/5- Negatives

4/4- Spring Snow Up To 10,300' on Red Cone

Overnight Low Temps (as of 4am)

Virginia Ridge, 9409’:                37 deg F
Tioga Pass, 9972’:                    28 deg F. 9hrs freezing
Agnew Pass, 9355’:                 31 deg F, 2hrs freezing
June Mt., 9148’:                       33 deg F
Mammoth Pass, 9500’:             30 deg F, 3hr freezing
Sesame Plot, 9014’:                31 deg F, 3hr freezing
Summit-MMSA, 11,053’:           24 deg F, 12hrs freezing
Rock Creek, 9600’:                   33 deg F
South Lake, 9580’:                  34 deg F
Sawmill, 10,200’:                     32 deg F, 1hr freezing

Weather and CURRENT CONDITIONS
weather summary

*Flood Watch in effect from April 6, 05:00 PM until April 8, 11:00 AM*

A strong storm with an impressive deep moisture plume for April will impact our area through Saturday, bringing periods of moderate to heavy rain, very high snow levels, gusty winds, and flooding concerns along some rivers and streams. Precipitation amounts through the daytime hours Firday should become quite significant for early April...but the heavier precipitation looks to be Saturday. Snow levels climb well above 10,000 feet today and stay at that level into the overnight hours, meaning precipitation in most areas will be rain.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Chance of rain in the morning, then rain in the afternoon. 0.30-0.55 in. water. Cloudy. Rain. Up to 0.60 in. water. Cloudy. Rain in the morning, then chance of showers in the afternoon. 0.60-1.10 in. water.
Temperatures: 45 to 53 deg. F. 36 to 42 deg. F. 44 to 50 deg. F.
Wind Direction: SW SW SW
Wind Speed: 15 to 20 mph with gusts to 50 mph. 15 to 25 mph with gusts to 65 mph. 20 to 35 mph with gusts to 70 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Rain and snow. 0.40-0.65 in. water. Cloudy. Rain. Up to 0.70 in. water. Cloudy. Rain and snow in the morning, then snow showers likely in the afternoon. 0.75-1.25 in. water.
Temperatures: 39 to 44 deg. F. 31 to 36 deg. F. 27 to 42 deg. F.
Wind Direction: SW W SW
Wind Speed: 30 to 45 mph. Gusts up to 60 mph increasing to 75 mph in the afternoon. 35 to 55 mph with gusts to 85 mph. 45 to 60 mph with gusts to 100 mph decreasing to 35 to 55 mph with gusts to 85 mph in the afternoon.
Expected snowfall: 60% probability...of 2 to 4 inches. 40% probability...of 1 to 2 in. 30% probability...of 2 to 5 inches. 70% probability...up to 2 in. 3 to 9 in.
Disclaimer
This Avalanche Advisory 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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