Rattlesnake

Evaluate slopes for reactive wind drifts before stepping into steep or exposed terrain. Smoother snow surfaces can be found on protected slopes. If you find cracking on small test drifts or pillow-shaped hollow-sounding snow nearby, steer clear of slopes steeper than 35º. 

On Thursday, Feb, 17th we received observations from Lolo Pass that snowfall was heavier than forecast, up to an inch an hour for several hours. The SNOTEL wasn’t recording this. In addition to new snow from earlier in the week, up to 20” of low-density snow was available for transport. In conjunction with the snowfall, winds were much stronger than forecast reaching 30mph sustained winds at the visitors center, creating wind slabs on multiple aspects. Widespread shooting cracks and shifting of snow on low-angle terrain were observed. The steep road cuts naturally released small slab avalanches—clear signs of dangerous avalanche conditions.

In addition to the observations from Lolo pass, we also received observations from Gash that winds were much stronger than forecast, significant wind transport was taking place, and avalanche conditions were deteriorating. Wind slab was very touchy, easily triggered by a person's weight and propagating long distances. By mid-afternoon, the hazard was approaching high. Winds continued into the evening.

Spot forecasts for the central to northern Bitterroot and The Lolo pass area indicated sustained 30mph wind, critical speed for slab formation, continuing for at least the next 12 hours. With the prospect of further wind transport on top of already very touchy conditions, it was prudent to issue a special avalanche bulletin for the central and northern Bitterroot and Lolo Pass. 

We issued a high hazard rating forecast for elevations above 6000’ in the central and northern Bitterroot and Lolo Pass area. 

Despite the wind forecast for Thursday night yesterday, at Rocky Point we found less wind transport than anticipated and surprisingly well-bonded conditions. Light winds Thursday night gave the snowpack a chance to adjust to the load, and mild temperatures aided bonding This may not have been the case across the entire Bitterroot, areas of higher danger may exist. However, the observations we received, coupled with the drastic increase in stability in the Lolo Pass area indicate moderate avalanche danger. 

The fresh snow was bonding well to the underlying crusts in the Rattlesnake yesterday. Minimal cracking was observed in the top 4 to 6 inches on small test slopes. The westerly wind had scoured open terrain and left small lateral drifts on the treed ridges. Smoother surfaces could be found in protected areas. We observed roller balls on the road cuts below 5,000 feet. Reports from the Southern Bitterroot yesterday had 6 inches of fresh snow transitioning to 10 to 12 inches for wind-loaded terrain. These were stubborn on ski cuts and test slopes. There is more snow in the Seeley zone but recent observations made similar mention stubborn wind drifts. 

In the Lolo Pass area, buried surface hoar layers still exist. We’re taking persistent off the problem list today as these layers appear to have healed over the past week. We have not found propagating results in extended column tests since last weekend (2/12-13) and warmer temperatures will contribute to bonding. It’s still worthwhile to look for a grey stripe in snow pits down 10 to 30 inches (25 to 75 cm) and to be suspicious of sheltered northerly aspects where surface hoar was protected from wind and sun.

There is a winter storm watch in effect from Saturday afternoon through Monday morning in the  Seeley Lake Region. Heavy snow and cold temperatures will arrive sometime late Saturday afternoon and continue through Monday. Total snow accumulations of 3 to 9 inches are possible with winds gusts as high as 45 mph, some lightning is also possible. Avalanche danger will increase with the storm. 

As always, we’d love your observations. If you get out in the mountains today, please submit them here.

Ski and ride safe