Avalanche Advisory: Saturday - Feb 17, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON February 18, 2018 @ 7:01 am
Avalanche Advisory published on February 17, 2018 @ 7:01 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Saturday (2/17) - The avalanche danger for today is LOW, small avalanches in isolated areas or extreme terrain possible. Small Loose Wet avalanches may be possible at lower elevations as the snow thaws and becomes wet. Rollerballs originating from rocks and trees on steep sunny slopes can be an indicator the snow is loosing strength. Above treeline, possible lingering isolated wind slabs where terrain features favor drifting. Watch for shooting cracks, hollow sounding drifts just under cornices, on convexities, and in steep chutes.

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

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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Above freezing temperatures, clear skies, and light southwesterly winds will allow the snow from earlier in the week to thaw in the mid to lower elevations and by afternoon becoming wet and less stable. Lower elevations generally thaw more quickly than higher elevations. Anticipate Easterly aspects thawing first, followed by South, then West facing slopes. Rocky outcrops and vegetation can store and impart tremendous amounts of heat into the snow, thawing the adjacent snow more quickly. Use extra caution in and around rock outcrops, below cliff bands, or where the snowpack is exceptionally shallow where triggered releases are more likely. Watch for signs of unstable snow: large roller balls, deep ski penetration, and small point releases. Small point releases can be a sign that larger avalanches are increasingly possible. Loose Wet can trigger larger deeper releases. Timing is critical for avoiding Loose Wet releases. Time your travels to be out of steep sunny terrain before the snow thaws from the heat of the day.

Loose Wet slides are dense and heavy and are capable of carrying a rider into hazardous terrain or lead to possible burial when combined with a terrain trap.

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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The cold front that passed through the region last week (2/12-2/13) brought a brief shot of moisture, depositing a nice coating of low-density snow.  Winds veered widely with the storm system from Northerly to Southwesterly, then back to North. The low-density snow was easily picked up by the wind and transported onto leeward slopes forming Wind Slabs from treeline and above. Cold temperatures through most of the week have slowed the normal strengthening and bonding process. As a result, isolated Wind Slabs may still linger in steep or complex terrain, especially below cornices, crossloaded gullies, and below drifted ridgelines.

advisory discussion

The cold front that passed through the region last week (2/12-2/13) brought 4” to 8” of low-density, very transportable snow primarily from Mammoth Lakes north to Virginia Lakes. The storm system was accompanied by periods of moderate to strong northerly, veering to southwesterly, then swinging back toward northerly winds, which have redistributed the snow into Wind Slabs and drifts primarily treeline and above, especially near favored features on many aspects throughout the alpine. The cold temperatures post-frontal passage slowed the normal strengthening and bonding process but enough time has lapsed where tests and observations indicate they have begun to strengthen and more difficult to trigger. Use caution additional caution in steep or complex terrain above treeline where recent drifting is evident. Wind Slabs will most likely be found in the upper elevations under cornices, below ridgelines and at the top of chutes and couloirs, and on steep convexities. Hollow sounding drifts, or dense cracking snow may indicate rider triggered fail is still possible.

From treeline and below, the snow in sheltered areas remains largely undisturbed and lacks sufficient cohesion to form a well-developed slab. However, as temperatures rise above freezing during the day, sunny slopes will begin to thaw and become less stable. Rollerballs and small point releases originating from rocky outcrops or cliff bands and under trees are an indication the surface snow is loosing strength and the threat of Loose Wet avalanches is rising. Though the danger is Low for Loose Wet avalanches, isolated releases are possible on Easterly aspects during the morning, followed by South, then Westerly facing slopes by late afternoon. Hard snow surfaces (i.e. wind board, crusts, and ice) can be dangerous in the event of a fall. Ice axes and crampons are recommended in steep or complex terrain.

The patchy persistent weaknesses from early season that was confined to NE-N-NW aspects above ~9500’ is showing signs of sintering and strengthening with test results show improving cohesion. The approaching cold front is forecasted to produce trace to 2”” of low-density snow. This is unlikely enough load to stress the underlying persistent weakness enough for it to become reactive. However, near treeline where the persistent weakness exists, there is the potential that an large isolated Wind Slab failure could trigger an avalanche on these deeper weaknesses, although unlikely.

Caution – the mid to lower elevation snow coverage remains thin with plenty of hazards lurking below the snow surface, such as rocks, logs and stumps. The new snow is hiding plenty of hazards while providing little protection.

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 Monday - Last day of warm sunny weather today before winter makes its return Sunday into next week. The incoming system’s is taking a slightly farther westward track digging into California with accumulations of Trace -2" expected. Winds will begin to pick up late Saturday afternoon with the strongest winds expected Sunday as the strong cold front moves through the region. Wind advisories have been issued for the area with wind gusts expected to be 40-50 mph and up to 65 mph in wind prone areas. Low temperatures will plummet by Monday night with Tuesday morning lows in the teens and single digits for most areas.

Tues thru Friday - Cold trough of low pressure settles into the western U.S. next week with a shortwave just offshore, reinforcing the cold air over the area. Temperatures will remain below normal Tuesday & Wednesday. Overnight and early morning temperatures will be in the teens and single digits Tuesday & Wednesday morning, with a few valleys seeing below zero. Cold temperatures will be here through the week. Another pair of sliders will push down into the Sierra Wednesday night into Thursday, and then another on Friday bringing light snow across the area. 

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. Slight chance of snow showers through the day. Areas of blowing dust in the afternoon.
Temperatures: 44 to 52 deg. F. 24 to 29 deg. F. 32 to 40 deg. F.
Wind Direction: Light winds becoming southwest Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 25 mph in the afternoon. 15 to 25 mph. Gusts up to 35 mph increasing to 50 mph after midnight. 20 to 30 mph increasing to 25 to 40 mph in the afternoon. Gusts up to 75 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny. Clear. Sunny then becoming partly cloudy. Slight chance of snow showers through the day.
Temperatures: 38 to 44 deg. F. 20 to 25 deg. F. 26 to 32 deg. F.
Wind Direction: Light winds becoming southwest. Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 30 mph in the afternoon. 15 to 25 mph with gusts to 40 mph increasing to 25 to 35 mph with gusts to 50 mph after midnight. 45 to 65 mph. Gusts up to 85 mph increasing to 95 mph in the afternoon.
Expected snowfall: 0 in. 0 in. up to 1 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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