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Avalanche Advisory for 2018-03-03 06:02

  • EXPIRED ON March 4, 2018 @ 6:02 am
    Published on March 3, 2018 @ 6:02 am
  • Issued by Nick Meyers - Shasta-Trinity National Forest

A WINTER WEATHER ADVISORY is in effect from 4 AM until 10 PM PST this evening for Mount Shasta.  CONSIDERABLE avalanche danger exists near and above treeline today due to touchy wind slab and/or cornice fall avalanche problems. An additional round of snowfall will continue to impact mountinous terrain. Avoid slopes steeper than 30 degrees without exposure to steeper terrain above.

 

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Avalanche Problem 1: Wind Slab

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Wind slabs, primarily above treeline, remain our greatest concern today. Evidence of previous wind loading was observed yesterday and drifts were observed 3-5 feet deep. A ton of snow remains available for new wind transport and existing wind loaded areas are abundant. Triggering a wind slab could produce a large avalanche. Wind Slabs are most reactive during a wind/storm event and the first day or two after it has ended. Look for convex pillows of wind-drifted snow on the NW-N-NE-E-SE aspects of ridges and other terrain features, near and above treeline. The wind slab may have a chalky look and feel. Wind slabs can be very hard, and may present a hollow drum like sound as you traverse across slope.  Avoid by sticking to sheltered or wind-scoured areas.

     

    Avalanche Problem 2: Cornice

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    Cornices develop whenever snow is available for transport and the wind is sufficiently strong to move the snow. Cornices form on the downwind (leeward) side of sharp terrain features. Ridges above treeline are the most common locations. Weather and terrain awareness are the best tools for recognizing potential cornice problems. Fresh cornices can never be trusted; avoiding them is necessary for safe backcountry travel. While moving along ridge lines, stay well back from areas with cornice formation.

    Avalanche Problem 3: Storm Slab

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    Evidence of the storm slab avalanche problem has been hard to come by. However, with an additional round of snowfall today, small storm slabs below and near treeline in sheltered areas are possible. Storm slabs are best managed by conservative terrain choices during and after storms until the storm snow has stabilized.

    Advisory Discussion

    Mount Shasta has just received 2 to 3 feet of fresh snow over the past two days. Additional snowfall is expected today. While no glaring signs of instability have been observed, the simple data points to cautious route finding and decision making in the backcountry. The skiing and riding conditions are excellent, but routes should be chosen to minimize, or better yet, avoid steep terrain as an avalanche could be large. Enjoy the powder, but be careful out there.

     

     

    Recent Observations

    • The Everitt Memorial Highway and Castle Lake roads are OPEN, as well as the Mt Shasta Ski Park. Grooming has begun at Pilgrim Creek Snowmobile Park also.
    • Several backcountry skiers observed cracking and whoomfing in numerous areas along Green Butte Ridge, near treeline yesterday.
    • The Mount Shasta Ski Park ski patrol was able to get easy failures within storm snow layers, 20 inches down from surface, backside of Coyote Butte.
    • Last wave of snow deposited 5-10 inches of dry, low density snow on top of existing storm layer. 
    • Storm totals equal 2 to 3 feet on top of old snow surfaces. Areas below 6,500 feet, storm snow fell on mostly bare ground. 
    • New snow is right side up. I could not find good evidence of the storm slab problem below and near treeline.
    • Evidence of previous wind loading was observed near and above treeline. Wind drifts were 3 to 5 feet deep. Copius amounts of fresh, low density snow is available for new wind transport.
    • Visibility was and has been very poor above 8,000 feet. Above treeline and alpine terrain, conditions and stability is uncertain. 
    • No natural avalanches have been observed yet. 
    • Winds were eerily calm. A light wind could be felt out of the south and picked up in the late afternoon below treeline. Numerous showers brought periods of intense snowfall throughout the day.

     

    Weather and Current Conditions

    Weather Summary

    A WINTER WEATHER ADVISORY is in effect from 4 AM until 10 PM PST this evening for Mount Shasta... Additional snow accumulations of 3 to 6 inches over the mountains with localized amounts of up to 9 inches are possible today. A cold upper level trough is centered along the Oregon coast and will linger today with isolated to numerous snow showers. Favorable upslope southerly flow will veer to westerly throughout the day. Showers will taper off later tonight and a few light flurries are possible Sunday, mainly over higher terrain. Dry and warmer weather develops next week. By Wednesday, a broad trough will approach from the W/SW and a shift back to wet weather is likely.

     

     

     

     

    24 Hour Weather Station Data @ 3:00 AM

    Weather Station Temp (°F) Wind (mi/hr) Snow (in) Comments
    Cur Min Max Avg Avg Max Gust Dir Depth New Water Equivalent Settlement
    Mt. Shasta City (3540 ft) 32 28 36 31.5 4 ESE
    Sand Flat (6750 ft) station down
    Ski Bowl (7600 ft) 8.5 4.5 33 14.5 66.5 8 0.24 1.5
    Gray Butte (8000 ft) 9.5 5.5 19 12 8 25 SSE
    Castle Lake (5870 ft) 17 12.5 22.5 18 35 5.5 1
    Mount Eddy (6509 ft) 10 10 19.5 15.5 1 12 SW 50 6.5 1.5
    Ash Creek Bowl (7250 ft) station down
    Ash Creek Ridge (7895 ft) station down

    Two Day Mountain Weather Forecast

    Produced in partnership with the Medford NWS

    For 7000 ft to 9000 ft
      Saturday Saturday Night Sunday
    Weather Widespread snow showers, chance of snow 80% Mostly cloudy, show showers. Chance of snow 70% Partly sunny with a 30% chance of snow showers.
    Temperature (°F) 25 16 27
    Wind Direction S S S
    Wind Speed (mi/hr) 5-10 mi/hr 0-5 mi/hr 0-5 mi/hr
    Expected Snowfall (in) 3-6 1-2 0-1
    For 9000 ft to 11000 ft
      Saturday Saturday Night Sunday
    Weather Widespread snow showers. Breezy. Chance of precipitation is 80%. Snow showers. Chance of precipitation is 90%. Partly sunny and cold. Chance of precipitation is 30%.
    Temperature (°F) 7 5 11
    Wind Direction SW W W
    Wind Speed (mi/hr) 10-20 mi/hr 5-10 mi/hr 5-10 mi/hr
    Expected Snowfall (in) 4-8 1-3 0-.5

    Season Precipitation for Mount Shasta City

    Period Measured (in) Normal (in) Percent of Normal (%)
    From Oct 1, 2017 (the wet season) 12.20 30.03 41
    Month to Date (since Dec 1, 2018) 1.31 0.53 247
    Year to Date (since Jan 1, 2018) 6.37 14.82 43

    Disclaimer

    This advisory does not apply to Ski Areas or Highways and is for the Mount Shasta, Castle Lake and Mount Eddy backcountry. Use this information for guidance only. You may find different conditions in the backcountry and should travel accordingly. This advisory expires on midnight of the date it was posted unless otherwise noted.