Lawinenlagebericht

Rattlesnake

West Central Montana Avalanche Center
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Lawinengefahr

Sun
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Kernpunkte

The avalanche warning has been terminated. However, the dangerous avalanche conditions persist. Riders are likely to trigger storm slab avalanches over 2 feet deep in middle and upper-elevation terrain today. Strong to extreme northeast winds are building windslab on west and south aspects. Limiting travel to sheltered terrain under 35º is recommended.

Lawinenprobleme

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    The Stuart SNOTEL is showing an incredible 3.5" snow water equivalent (SWE) since the start of the storm. This translates to roughly 3 feet of snow. Despite 1.8" SWE at the Stuart SNOTEL yesterday, outside of Snowbowl, we found just 10" of dense but dry snow. Other areas of the Rattlesnake may have deeper storm totals.  Winds were also lowered than forecast; while they were blowing stronger at the upper elevations of Point 6, they were light, with moderate gusts elsewhere. Over a foot of low-density snow fell last night, with Snowbowl reporting 16" at the summit. 

    Despite the amount of snow and wind, we did not see the amount of instability that we were anticipating. The new snow is bonding to the old snow. Cracking was isolated, and wind slab was reactive but not touchy. We were able to trigger one slab on a 38º slope that was arrested by the supportive slope below. Snow Bowl Patrol triggered multiple D1 avalanches with 10" crowns using explosives.

    Today it is very likely for riders to trigger avalanches large enough to bury, injure or kill a person in upper elevations. Avalanches are likely at middle elevations and possible at lower elevations.  Where the snow is wind affected, slabs will be deeper and touchier. Last night's snow came in cold and will be light, increasing the chance of loose dry avalanches that could step down to a slab avalanche below the new snow. Yesterday winds were out of the northwest, and today they have switched to northeast, meaning that wind loading has occurred on multiple aspects. Travel in avalanche terrain is not recommended. If you do get out, sheltered mid-elevation slopes are the better choice. Keep slope angles well below 35º, stay off of and well away from the base of avalanche slopes and avoid convexities. Recent avalanches, shooting cracks, and whumphing are all signs telling you to keep it mellow. 

    (1/27/2023)

    This slab propagated and shifted but was supported by the slope below

    Photo: Jeff Carty

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1–2

    Strong northeast winds, with extreme gusts, will build windslab on NW through SE slopes today. There is abundant new snow available for transport, and strong winds will carry it further down slopes than normally encountered and cross-load slopes. The risk of fresh windslab avalanches will increase throughout the day. It is likely for riders to trigger a wind slab avalanche multiple feet thick. Coupled with the storm slab, the potential for buried surface hoar, and frigid temperatures, any slide today could be very consequential. 

    Sheltered treed mid-elevation slopes are your best bet today for avoiding wind slab, and the NE wind. Choosing low-angle terrain,  below 30º is the west way to mitigate the avalanche hazard. Stay alert for signs of wind loading, such as wind lips, drifts, and stiff hollow feeling pillows of snow. Shooting cracks are a clear warning sign that avalanches are possible. 

Avalanche Discussion

Yesterday we issued an avalanche warning with high avalanche hazard at upper and middle elevations due to heavy snowfall and wind. Reports of avalanches Thursday pointed to increasing avalanche hazard and a high likelihood of an avalanche cycle. We certainly got the snow. 3.5" snow water equivalent SWE has fallen at the Stuart SNOTEL in the Rattlesnake, 2.7". However,  outside of Snowbowl, just 10" of dense but dry new snow was found. Higher amounts likely exist elsewhere in the Rattlesnake. The North fork of the Jocko in the Seeley Zone shows 2.7" SWE, which agrees with observations from the west side yesterday afternoon. The east side of the Seeley zone on Richmond peak received much less snow, with a foot of fresh found yesterday. Twin Lakes in the central Bitterroot shows 2' SWE as the storm total. Lolo Pass and the southern Bitterroot were mostly passed over with just 0.6" of SWE. Winds did blow as hard as forecast, and snow transport was not as extensive as anticipated. 

Overall, the new snow was not as reactive as anticipated. In the Rattlesnake and Bitterroot, it was somewhat stubborn to skier triggering, although we were able to get some movement. The snow was touchier where wind loaded. Snowbowl patrol triggered several small avalanches about 10" deep with explosives. Overall the avalanche hazard was Considerable. Today it is likely for skiers and riders to trigger avalanches in terrain over 35º.

In the Seeley zone, a layer of surface hoar, likely developed earlier this week, was found below the new snow. Conditions felt touchier, and there was more potential for human-triggered avalanches. Hazard at upper elevations the hazard approached high. 

Overnight we received up to one foot of new snow. Colder temperatures mean it will be lighter and more likely to create loose dry avalanches than slabs. Based on yesterday's conditions, it likely hasn't increased avalanche hazard dramatically. 

The largest impact to the snowpack and recreationists today is the frigid temperatures and strong northeast winds with extreme gusts. Windslab will develop throughout the day on northwest through southeast slopes at upper and middle elevations. The cold will compound the problem and make any incident more consequential. Staying in low angle, sheltered terrain and in the trees is a good plan today to limit exposure to the developing windslab and cold wind. Enjoy the fresh powder on low-angle slopes, and give the snowpack a few days to adjust and temperatures to warm up. 

Stay alert for signs of instability, such as shooting cracks, whumpfing, and recent avalanches. All are a sign of dangerous avalanche conditions. 

Please submit an observation telling us how much new snow there is and if you see any signs of instability. 

Ski and ride safe

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