Avalanche Advisory: Tuesday - Apr 24, 2018

Avalanche Advisory published on April 24, 2018 @ 6:11 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Loose wet avalanches will be the primary concern for the forecast period. Time your day to be off of steep sunny slopes before they become unsupportable. Pinwheels rolling down the hill, small wet sluffs originating from rocky areas or exposed vegetation, and boot-top penetration into wet snow are direct signs that triggering loose wet avalanches is becoming increasingly possible.

If you are out in the mountains early enough to hit good corn by late morning you may encounter firm snow conditions before the surface melts. Ski and boot crampons, an ice ax, and helmet can help reduce your vulnerability in a slide-for-life scenario where a simple slip and fall could have higher consequences on steep alpine terrain.

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Avalanche Problem 1: Loose Wet
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

The spring warming trend continues with poor overnight re-freezes and above average daily temperatures. Afternoon thunder showers mean very warm temperatures and possible light rain further thawing high elevation snow. The danger of triggering small or even large loose wet avalanches will rise each day as slopes warm. At upper elevations, areas that are still transitioning to good corn skiing are more suspect because snow can become more unstable the first time it thaws. Lower elevations will warm first and most, but avalanches will be isolated to areas that have enough coverage to slide. Time your day to be off of steep slopes before they become too wet and you can avoid the problem all together. Watch for pinwheels rolling down the hill around you and wet snow that is unsupportable, especially near rocky areas. These are signs that it is time to move to cooler slopes with inclines less than about 35 degrees.

advisory discussion

The diurnal melt-freeze pattern has taken hold of almost all the accessible snow on the East Side. Spring has sprung. The snow has warmed and a round of natural loose wet avalanches released as cold snow warmed for the first time throughout the weekend. More recently good corn skiing has been found at practically all elevations by mid-morning to mid-day. Ideal spring conditions involve a thin layer of large wet rounded snow grains on top of a firm and supportable crust layer. With overnight lows barely dipping below 32 degrees and daytime temperatures well above average the thawing has been rapid and early in the day. By afternoon you can find increasingly unstable snow conditions. Once the top of the snowpack thaws deeply enough for the skiing to suck it is also becoming wet enough that it loses cohesion with its surroundings and you can trigger loose wet avalanches. Snow that has gone through the melt-freeze cycle several times tends to resist sliding better than snow that is becoming wet for the first time. That’s why we’re seeing the most recent evidence of loose wet instability on high elevation northerly slopes where the snow has remained wintry the longest. By now almost all aspects and elevations have stronger spring snow. However, areas of colder snow were reported as recently as two days ago. So some higher elevation slopes are still transitioning and may slide under increasingly warm conditions. That doesn’t mean that lower elevation slopes can’t slide once they become wet enough to be unsupportable. Rocks and trees tend to reflect solar radiation back down and warm the snow around them even more, so loose wet avalanches often originate from these features. Under the intense spring sun even open slopes are becoming wet by the afternoon. Chances of afternoon thunder showers means there is also the slight possibility of rain which would further warm and weaken the snow. Wet snow usually needs a steep slope, steeper than about 35 degrees, to slide. Once moving, slushy snow can slide down to very low angled terrain. So regardless of where you're going to travel in the coming days, plan to be off of and out from under steep slopes before they thaw too deeply. Pinwheels coming from rocks and trees, or your skis, are often the first sign that the surface is losing cohesion. Boot-top penetration into wet snow is a good sign of instability. You can avoid the avalanche problem by starting early and moving to less steep, shady slopes away from rock outcrops before conditions deteriorate. If the skiing sucks because your sinking in too far, it’s probably becoming unsafe too.

weather summary

 Warm afternoon temperatures are expected through Thursday, with a cool down to near or possibly below average temperatures by next weekend. Areas for showers and a few thunderstorms increase each afternoon through Friday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny. Isolated thunderstorms in the afternoon. Partly cloudy. Isolated showers and thunderstorms in the evening. Partly cloudy. Slight chance of showers and thunderstorms in the afternoon.
Temperatures: 56 to 66 deg. F. 36 to 41 deg. F. 59 to 67 deg. F.
Mid Slope Winds: Light winds. Light winds. Light winds.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny. Isolated thunderstorms and snow showers in the afternoon. Partly cloudy. Isolated showers and thunderstorms in the evening. Partly cloudy. Slight chance of thunderstorms and snow showers in the afternoon.
Temperatures: 49 to 55 deg. F. 32 to 38 deg. F. 51 to 57 deg. F.
Ridge Top Winds: Light winds. Light winds. Light winds.
Expected snowfall: 0 in. 0 in. 0 in.
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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