Posted:
Jan 23, 2015 @ 6:24 am

In the southern Bitterroot Mountains, on slopes steeper than 35 degrees and above 7000 feet, the avalanche danger is rated as considerable. Last weekend’s storm was deposited on a weak surface hoar layer in this area and is still touchy in stability testing. In all other parts of the advisory area, on slopes steeper than 35 degrees and above 6000 feet, the avalanche danger is rated as moderate .

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s advisory for January 23, 2015. The danger ratings do not apply to operating ski areas, are the sole responsibility of the U.S. Forest Service and expire at midnight tonight (1/23/15).

Weather and Snowpack  Discussion

Mountain winds, primarily out of the west, are blowing 16 to 19 mph this morning. Temperatures above 5000 feet are 21 to 28 degrees F across the advisory area. We picked up about an inch of snow last night; expect warming temperatures today with mostly cloudy skies. Some snow with increasing wind is forecast to begin this evening.

There’s been no significant snow since the MLK weekend. We can thank the faceting process for keeping the skiing and riding pretty good. Surface temperature gradients and an inch or so of graupel have kept us in “recycled powder” for the last couple of days.

The snow from last weekend was deposited on a surface hoar layer and it is still reacting to stability tests; notably in the Lost Trail  and southern Bitterroot area (LT pit below). The Montana / Idaho border did pick up the most snow during that storm. Elsewhere in the advisory area the snow appears to be stabilizing. I dug a lot of pits in the Rattlesnakes yesterday with Steve and we were generally happy with what we saw. We did get an interesting result twice in a pit on an E by NE slope (see video below).

The size of the surface slab, our primary avalanche problem, is going to vary depending on where you are riding; i.e. 20″ in the southern Bitterroot, 8-10 inches near Lolo Pass, 8 – 10″ in the Rattlesnakes, 12 – 14″ in the southern Swans.  I’m always going to tell you it’s a good idea to dig a quick pit if recreating on slopes 35 degrees and steeper; it’s a particularly good idea this year when observations and pit tests have been anything but definitive.

Weather and Avalanche Outlook

Light snow, producing a couple of inches and accompanied by west winds gusting up to 30mph, is expected tonight through Saturday. The big story is you might be recreating in spring snow. By Sunday temperatures are expected to rise above freezing; possibly up to 8000 feet. Without significant rain or snow, I would expect these warm temperatures to stabilize the snowpack; and the avalanche danger should decrease.

Ride and ski safe,  have a great weekend. I’ll  issue the next regularly scheduled advisory on Tuesday, January 27.

READ FULL ADVISORY  

Problem 1 - Storm Slabs from the weekend

  • TYPE

    storm-slabs

    Storm Slabs

    Release of a soft cohesive layer (a slab) of new snow which breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    NW - Northwest

Last weekend’s storm slab instabilities have persisted; particularly in the southern Bitterroot.

Problem 2 - Wind Slabs

  • TYPE

    wind-slabs

    Wind Slabs

    Release of a cohesive layer of snow (a slab) formed by the wind.  Wind typically erodes snow from the upwind sides of terrain features and deposits snow on the downwind side.  Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    NW - Northwest

Winds have been strong enough to move snow onto leeward areas. Open exposed ridges above 6000 feet are particularly suspect.

Problem 3 - Persistent slabs

  • TYPE

    persistent-slabs

    Persistent Slabs

    Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks.  Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Persistent, Deep-Slab.

  • SIZE

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

The older layers in the snowpack still show up but are significantly stronger. Again, these layers appear to  be more weak in the southern Bitterroot.

VIDEO

Rattlesnakes 1/22/15 – E by NE slope – 30 degrees – 7800 feet – We got  this result twice; the surface hoar layer propagates when I hit the snow NEXT to it. Not a proper ectp score; but it does show a bit of energy in the surface hoar layer underneath last weekend’s storm.

Lost Trail pit and photos from yesterday.

MAP

southern Bitterroot

The southern Bitterroot appears to have the touchiest snow in the advisory area right now. Last weekend's storm deposited 20" on a weak surface hoar layer.

The southern Bitterroot appears to have the touchiest snow in the advisory area right now. Last weekend's storm deposited 20" on a weak surface hoar layer.

FORECAST & OUTLOOK

Light snow, producing a couple of inches and accompanied by west winds gusting up to 30mph, is expected tonight through Saturday. The big story is you might be recreating in spring snow. By Sunday temperatures are expected to rise above freezing; possibly up to 8000 feet. Without significant rain or snow, I would expect these warm temperatures to stabilize the snowpack; and the avalanche danger should decrease.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.