Our next storm system moves into the Great Basin today. Warm and moist air arrives on south flow and highest snowfall totals look to favor the South San Juan and Sangre de Cristo zones. Warm air already in place makes nailing down the rain/ snow line a challenge. We anticipate rain on snow up to near 10,000 feet at the onset before moving to around 9,000 feet or lower. Localized, heavy snowfall during the day is forecast to become more widespread overnight. Today could be a good day to pack a second jacket if venturing out in the mountains.
The warm nature of our storm will keep snowfall totals from appearing all that impressive. However, dense snow can quickly form into a cohesive slab. Slopes with more than six to eight inches of new snow should be evaluated before blindly jumping in. Pay attention to snow totals as you climb in elevation and look for cracking in the new snow to clue you into storm slab formation. New snow is falling onto a variety of old snow surfaces and can move fast and far downslope where it falls onto a slippery surface – melt-freeze crust or firm wind-board.
Rain on snow will make for messy conditions below treeline. If you observe heavy rain or rain continues for prolong period, you should anticipate an increase in wet snow activity. This will most likely start with loose wet avalanches but could transition to wet slabs if enough rain falls. Rollerballs will clue you into the snow surface becoming wet and loosing cohesion. Wet slabs are more likely in steeper, rocky terrain. If you see signs of loose wet activity, consider moving up in elevation or getting off said slopes. Pay attention to terrain above you if traveling in creek bottoms or above cliffs at lower elevations.
A lower likelihood but much bigger consequence problem is weak layers buried under a 1.5 to 2 meter cohesive slab. These layers have produced many large to very large avalanches this season. During our most recent dry spell the snowpack continued to settle, stiffen, and made triggering an avalanche in deeper weak layers more difficult. When weak layers are buried as deeply as they are, digging into the snow is difficult, time consuming, and snowpack test often don’t give reliable information about instability. A rider is most likely to trigger one of these monster slides from thin rocky areas or near margins of a slab – this is where weak layers are closer to the surface. Avoid travel below large, overhanging cornice. If a piece of a cornice breaks off and goes tumbling down a slope, that could be a large enough trigger to cause one of these Deep Persistent Slab avalanches to fail. Don’t linger in avalanche runout zones and if you have to cross a runout, do so quickly.
It really comes down to the terrain you wish to ride and your risk acceptance. This may be different from person to person – having good communication amongst partners and making a solid plan is important. Many folks will slowly step out into steeper terrain and many will ride away without triggering a massive avalanche. Carefully evaluate terrain above and below you and consider the consequences of not only those in your group but those near you. Good habits save lives so make a plan that fits the current avalanche conditions, stick to it, and only expose one person to a slope.