Avalanche Advisory: Wednesday - Feb 7, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON February 8, 2018 @ 6:22 am
Avalanche Advisory published on February 7, 2018 @ 6:22 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Avalanche danger remains LOW today. Small, loose wet avalanches may still be possible on steep solar aspects. Watch for rollerballs and fan-shaped debris by midday on sunny slopes. Isolated wind slabs are not out of the question at higher elevations. Be aware of new drifts near the tops of gullies and just below ridgelines.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Loose Wet
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The relatively calm and clear conditions, and especially the warm temperatures, that dominate during these prolonged periods of high pressure force solar aspects into a melt-freeze cycle. As the sun moves across the sky it heats E to S to W facing slopes and thaws the bonds between previously frozen snow grains. Rocks and vegetation absorb the heat quickly and reflect it back down to the snow which intensifies the warming at the local level. All of this will make loose wet avalanches possible again today, especially as the day goes on. Note rollerballs coming down the hill around you and how deeply you can sink in to wet surface snow. These are the best signs that the slope around is becoming unstable. If you time your trip to be off of sunny slopes before they thaw too deeply you can avoid the problem.

Avalanche Problem 2: Wind Slab
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Isolated small wind slabs may have formed on steep upper elevation slopes in the few places where moderate wind have still had soft winter snow to move around. Winds were out of the north yesterday and a few plumes of blowing snow were seen across ridgelines heading towards the southeast. Overnight and this morning, Mammoth Mountain wind sensors are recording steady winds from the southwest above about 9500’ and so a few new wind deposits may have formed on northeasterly slopes as well. There is not a lot of snow available for transport at the moment, but new, round, smooth wind slabs are worth noting at upper elevations today since they may not have had the chance to bond to the old snow surface. The tops of gullies and just below the leeward side of ridgelines are your best bet to find new wind slabs. These slabs will probably be most touchy where they sit atop hard icy surfaces.

advisory discussion

High pressure continues to deflect any storms heading for the California coast. Temperatures have been near 20 degrees above normal for this time of year on occasion. It feels like April and if you time your day right you can ski spring snow on southeasterly slopes. But it isn’t April, it’s February and while the air temperature is mild, the sun is still too low in the sky to create good corn skiing on northerly aspects. The picture in the mountains is one of two snowpacks coexisting just a ridgeline apart. Cold winter snow ranging from surface facets to wind blown has been found on cooler northerly slopes while the melt-freeze process has been transforming and thawing solar aspects. It’s a complicated situation.

Test results targeting persistent weak layers of sugary facet snow on northern slopes still show some potential for failure. We haven’t seen any avalanches on these layers this season, primarily due to the lack of snow. Slope-wide failure would most likely require a heavy slab on top that exceeds the strength of the underlying snowpack. We have been monitoring these layers all season and their strength has waxed and waned. They will continue to be of interest if you want to play on north-facing wintry snow because while triggering these layers is very unlikely the resulting avalanche could be large. We will have to wait and see if we ever get a big enough dump to overpower these persistent weaknesses.

Meanwhile, solar radiation will continue to thaw southerly slopes as the sun climbs from E to S to W throughout the day. Clear skies and below freezing temperatures overnight have allowed the snowpack to get a good re-freeze in most areas. Clear skies and calm winds will allow the snowpack to heat up quickly and the thawing process will begin all over again. No new wet instabilities have been reported in the forecast area, but that does not mean that getting caught in the wrong place at the wrong time is impossible. If you find yourself on steep solar slopes, especially near rock outcrops by midday you may find loose wet avalanche activity. Avoiding slopes that are warm enough for large rollerballs or to sink in past your boot tops is the best way to keep yourself out of trouble.

Coverage remains thin and obstacles abound. Be wary of rocks and logs just below the snow surface.

Early in the day, and as slopes re-freeze in the afternoon, hard and fast conditions could turn a simple fall into a dangerous slide.

Weather and CURRENT CONDITIONS
weather summary

Quiet and mild weather continues into the start of the weekend as high pressure remains in control. High temperatures more typical of mid April persist with highs reaching the mid to upper 50s in the Sierra.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny. Clear. Sunny.
Temperatures: 47 to 55 deg. F. 27 to 32 deg. F. 51 to 59 deg. F.
Wind Direction: Light winds. Light winds. Light winds.
Wind Speed: Light winds. Light winds. Light winds.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny. Clear. Sunny.
Temperatures: 40 to 46 deg. F. 25 to 31 deg. F. 44 to 50 deg. F.
Wind Direction: Light winds. Light winds. Light winds.
Wind Speed: Light winds. Light winds. Light winds.
Expected snowfall: 0 in. 0 in. 0 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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