Avalanche Advisory: Saturday - Feb 10, 2018

THIS AVALANCHE ADVISORY EXPIRED ON February 11, 2018 @ 6:43 am
Avalanche Advisory published on February 10, 2018 @ 6:43 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Saturday (2/10) - The avalanche danger remains LOW overall. Weak overnight freeze, combined with seasonable temperatures during the day will increase the potential for Loose Wet avalanches during the day on E - S - W facing slopes. Limited snow coverage on solar aspects will limit the problem to areas where snow is present. Use caution in steep, rocky terrain as temperatures climb through the day and the snow becomes wet. Forecasted moderate to strong Westerly then veering to Northeasterly winds could form small isolated Wind Slabs, near and above treeline, initially on N-E-S aspects. As winds switch to the Northeast, anticipate possible Wind Slabs on NW-SW-SE aspects, near and above treeline.

Below ~9,000’, natural and triggered releases are unlikely due to thin snow coverage (below threshold).​

Caution – Possible Slide for Life conditions in the AM or as slopes begin to refreeze on southerly aspects. A minor slide into hazardous terrain can have serious consequences. Crampons and Ice Axe recommended in exposed terrain.

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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Today (Saturday) – The weather forecast calls for sunny skies and high temperatures (mid 30’s to 40’s) with increasing winds developing through the day, anticipate isolated Loose Wet releases on solar aspects, especially sheltered terrain. Due to more seasonable temperatures and windy conditions, the snowpack will thaw more slowly today than previously on solar aspects (SE-S-W) in the mid and upper elevations (where snow is present). Rocky terrain may thaw more quickly due to residual heat from the prior day limiting overnight freezing while enhancing daily warming. Use extra caution where triggered releases are more likely - in and around rock outcrops, below cliff bands, or where the snowpack is exceptionally shallow and insufficiently frozen. As the sun arcs across the sky and temperatures begin to rise, anticipate easterly aspects thawing first, then southerly, followed by westerly aspects. Expect lower elevations thawing more quickly than higher elevations, especially sheltered locations. With Loose Wet avalanches, timing is critical for avoidance. Time your travels to be out of steep sunny terrain before the snow thaws from the heat of the day. Signs of unstable snow: large roller balls, deep ski penetration, and small point releases. Point releases can be an indicator that larger avalanches are possible. Loose Wet avalanches typically involve the snow near the surface of the snowpack but can trigger larger deeper releases

Loose Wet slides are dense and heavy making it difficult to extract yourself, if entrained, and are capable of carrying a rider into hazardous terrain or lead to possible burial when combined with terrain traps that favors deep burial.

Below ~9,000’, natural and triggered releases are unlikely due to thin and well-anchored snow coverage (below threshold).​

Avalanche Problem 2: Wind Slab
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Today’s forecast is calling for moderate to strong Westerly winds then veering to the Northeast later in the PM hours. Though westerly fetches are mostly free of snow, in a few favored locations, it may be possible to transport enough snow to form small isolated Wind Slabs near and above treeline on N-E-S aspects primarily during the the day. As the winds veer to the Northeast, northeasterly aspects still have plenty of snow available for transport; anticipate isolated Wind Slabs forming on NW-W-SW-SE aspects near and above treeline in the late PM thru early AM tomorow. 

advisory discussion

Solar Aspects – The high pressure that has remained parked off the coast of California is forecasted to retrograde to the west somewhat allowing weak “slider” storms to drop into the Great Basin from the north with the potential for light snowfall Monday/Tuesday. In the mean time, this will tighten the pressure gradient over the Sierras, resulting in windy conditions as cooler air moves into the forecast region. This is a welcomed break in the pattern that has dominated the weather since late January (1/27).  Overnight temperatures were once again quite mild with few locations report bellowing freezing temps. However, the cooler air mass moving into the region combined with moderate to strong West to Northeast winds will slow the normal thawing somewhat but isolated wet instabilities on solar aspects (E-S-W) are possible. With the limited snow coverage on southerly aspects, Loose Wet activity will be isolated to terrain that continues to retain sufficient snow, especially east to southeast facing gullies and cirques. Exposed rocky terrain features can introduce tremendous amounts of heat into the snowpack causing rapid localized thawing. Time your travels to be out of steep sunny terrain before the snow thaws excessively is key to avoiding wet instabilities. Snow coverage continues to be thin with plenty of hazards lurking below the snow surface, such as rocks, logs and stumps.

Caution – On southerly aspects, possible Slide for Life conditions in the AM or as slopes begin to refreeze. A minor slide into hazardous terrain can have serious consequences. Crampons and Ice Axe recommended in exposed terrain.

Northerly aspects - Patchy persistent weakness continues to dwell within the snowpack with weak faceted sugar layers down about ~30 to ~60cm (12” to 24”) from the surface in most locations. Distribution is primarily confined to NE-N-NW aspects above ~9500’. Recent test results continue to show the potential for failure but relatively non-reactive. The northerly aspects remain somewhat cool but are showing signs of warming with the surface snow in mid-elevations warming and developing a melt/freeze crust. The crust maturity depends on solar exposure, duration, steepness, and shading. The snow is still somewhat soft in sheltered areas near treeline but the aspect window has narrowed to N to NE aspects and shading is key. Near treeline, where the persistent weakness exists, there is the potential to encounter small isolated older Wind Slabs that were deposited over facets in steep terrain, which could fail if a sufficient trigger is applied.   

Below ~9,000’, natural and triggered releases are unlikely due to thin and well-anchored snow coverage (below threshold).​

 

Weather and CURRENT CONDITIONS
weather summary

Sat thru Sunday - A cold front currently near the California, Nevada, and Oregon border will push through northeast California today, bringing much colder temperatures and breezy northerly winds to the region and some mid and high clouds this morning. North to East winds will increase along the Sierra Crest as high pressure builds into the Great Basin. By Sunday morning, winds should drop off substantially as the flow aloft turns westerly ahead of the next upper disturbance. Sunday afternoon, winds begin increasing as the next, more potent system nears the northwest California/southwest Oregon border. The storm resembles an inside slider with the low passing near the California-Nevada border. Small changes in the track can make a huge difference in precipitation depending on the ultimate track.

Monday - Upper level low moves over the region Monday bringing significant cooling along with breezy northeast winds behind Sunday night`s cold front. High temperatures will be 4-8 degrees below normal. The surface gradient begins to relax into Monday into Tuesday. However, upper level winds will increase due to the jet on the backside of the low, producing gusty winds across the Sierra Crest. A few spotty snow showers possible Monday, especially as the trend for the track of the upper low has been westward. An enhanced area of deformation along the tail end of the frontal boundary brings better chances for snow in Mono Counties Monday evening, tracking south into Tuesday. Localized areas could see a few inches of snow, especially with the upslope flow in the eastern Sierra.

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: Sunny. Clear. Sunny then becoming partly cloudy.
Temperatures: 41 to 47 deg. F. 16 to 22 deg. F. 43 to 49 deg. F.
Wind Direction: Northwest Northeast Light winds becoming southwest
Wind Speed: 15 to 25 mph with gusts to 45 mph. 15 to 20 mph in the evening becoming light. Gusts up to 40 mph. 15 to 25 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Partly cloudy then becoming clear. Sunny then becoming partly cloudy.
Temperatures: 35 to 40 deg. F. 15 to 20 deg. F. 35 to 40 deg. F.
Wind Direction: West shifting to the northwest Northeast West to southwest
Wind Speed: 25 to 40 mph with gusts to 65 mph, 20 to 30 mph with gusts to 50 mph in the afternoon. 25 to 40 mph becoming east 15 to 25 mph after midnight. Gusts up to 60 mph. 10 to 15 mph increasing to 25 to 35mph with gusts to 65 mph in the afternoon.
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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