Salt Lake Avalanche Advisory Friday, December 26, 2008 Created at 7:29 am Updated at 7:37 am Forecaster: bruce tremper AVALANCHE WARNING » Dangerous avalanche conditions are occuring or are imminent. Backcountry travel in avalanche terrain is not recommended. Notice: THIS AVALANCHE WARNING IS FOR THE MOUNTAINS OF NORTHERN AND CENTRAL UTAH INCLUDING THE BEAR RIVER RANGE, WESTERN UINTAS, WASATCH, MANTI SKYLINE. 2-3 FEET OF NEW SNOW COMBINED WITH STRONG WINDS HAS OVERLOADED BURIED LAYERS OF VERY WEAK SNOW NEAR THE GROUND, CREATING LARGE AND VERY DANGEROUS AVALANCHES. BACKCOUNTRY TRAVELERS SHOULD STAY OFF OF AND OUT FROM UNDERNEATH ANY SLOPE STEEPER THAN ABOUT 30 DEGREES. LINGERING AVALANCHE DANGER SHOULD PERSIST THROUGH THE WEEKEND. BOTTOM LINE Danger by aspect and elevation on slopes approaching 35° or steeper. The danger is HIGH today, meaning that conditions are very dangerous and people should avoid all backcountry avalanche terrain. Stay off of and out from underneath any slope steeper than about 30 degrees. You can easily trigger these avalanches from the bottom or from adjacent slopes. If you absolutely have to get onto steeper slopes, do it at the resorts. CURRENT CONDITIONS Storm totals are 1.5 to nearly three feet of new snow with water weight 1.5 to 2.5 inches. The automated station on Ben Lomond Peak near Ogden received an incredible 4 inches of water weight. Places favored by a southwest flow got the most snow including areas near Brighton and Park City, Mt. Timpanogos and especially Ben Lomond Peak. Yesterday, the winds nuked from the southwest for most of the day, averaging 30-40 mph and gusting to 60 or higher. The winds dropped dramatically as the cold front arrived last night and about of foot of low density snow is now draped uniformly over all of yesterday's wind slabs. Ridge top temperatures rose to around 25 degrees yesterday before the front but have plunged down to the single digits this morning. RECENT ACTIVITY Not too many people braved the high wind and poor visibility yesterday. When we finally get some visibility, we will likely see a fairly widespread cycle of natural avalanche activity especially in places hit hardest by new snow and southwest wind. We heard about only a small fraction of these and they include: many large, naturals in the Mary Lake area near Brighton running full width with slabs breaking to the ground, one possibly triggered by a skier from the bottom, but they were not caught. Several other naturals were noticed in Snake Creek near Brighton. In Little Cottonwood Canyon there was a natural noticed in the Y-Couloir and several, significant, explosive-triggered avalanches including south facing chutes above the highway and a west-facing slope at the head of the canyon. There were even a number of wind slabs triggered in town or in the foothills along the steep sides of gullies. I intentionally triggered one yesterday just behind the University of Utah Hospital that swept across a very popular hiking trail. I imagine that the rain yesterday afternoon stabilized most of these low elevation wind slabs. THREAT #1 Yesterday's wallop of wind-drifted snow and new snow have overloaded deep layers of very fragile, faceted snow near the ground. We've had both human triggered and natural avalanches on this layer every day for the past couple weeks and the overloading reached a crescendo yesterday as the nuking winds and warm temperatures drifted dense snow onto lee terrain. The weakest pre-existing snow is on the north through east facing slopes—the exact same slopes loaded most heavily by yesterdays strong winds. Although I suspect many of these slopes have already slid, there are always many that hang in the balance, just waiting for a trigger. These avalanches will be very large and mostly unsurvivable. Anyone who jumps into steep slopes today and through the weekend would be a lot safer clearing roadside bombs in Iraq and Afghanistan. In addition, many of the slopes that avalanched in previous days are filled in again ready for round 2. This problem exists on all slopes above about 8,000' on all but the south facing slopes. THREAT #2 The howling winds from the south and southwest yesterday drifted snow onto lee terrain, especially north through east facing slopes and cross-loaded into other slopes. The resulting wind drifts are dense, heavy and can easily propagate fractures. They are especially dangerous because they are stubborn to trigger and most importantly, they are now all covered up by a foot of light, fluffy snow, so they are invisible as well. Everything will feel solid and safe until the whole slope shatters like glass. MOUNTAIN WEATHER With cold and unstable air drifting in from the northwest, we will see light snow showers for most of the day, adding up to about 5 inches. Ridge top winds will blow 15 from the northwest, calming down to 10 mph later in the day. Ridgetop temperatures will be around 6 degrees and drop to near zero overnight. Snow should taper off overnight and be partly cloudy on Saturday.