Colorado Avalanche Information Center Statewide Avalanche Conditions Issued: 05/20/2011 4:09 PM by Spencer Logan Expires: 05/23/2011 6:00 PM (EXPIRED) 2 2 Highlights The snowfall will not stop. A storm spring storm rolled through the end of the week, leaving behind some impressive May powder. The almost June sun quickly cooks the surface into creamy or gloppy snow. Through the weekend, afternoon snow showers will add more layers of cold snow, and breaks in the clouds will add more layers of wet, sticky slabs. The upper snowpack is a complicated stack of thin slabs and weak layers. Expect them to change rapidly through the day and over short distances. Warming temperatures Saturday night and Sunday will increase the potential for deep wet avalanches near and below treeline. The CAIC will update the Statewide Forecast on Monday, Wednesday and Friday afternoons through the end of May, and more frequently if conditions warrant. Zone forecasts will resume in mid-November. We really appreciate your observations of snow conditions and avalanche activity. Twitter 4:14 PM by Spencer Logan: Lots of triggered storm slabs in Front Range. Problem continues with new slabs from add'l snow showers Mon-Tue Weather Discussion The end-of week storm is pulling off to the northeast. That leaves Colorado under unsettled northwesterly flow. Weak disturbances, favorable orographics, and daytime heating will keep afternoon snow showers in the picture Saturday and Sunday. The potential for deeper snow accumulations, stronger snowfall, and thunder snow increases from the San Juans to the Central Mountains and peaks in the Northern Mountains. Expect periods of heavy snowfall and locally variable accumulations. Temperatures will be increasing through the weekend, with highs Saturday about 5-10 degrees warmer than Friday, and another 5-10 degrees warmer on Sunday. Temperatures near treeline will drop below freezing for most of the night. Elevations below 10,000 feet may get a brief freeze or remain above freezing on Saturday night. A weak ridge builds Sunday. Another storm system quickly squashes the ridge on Monday. It is not as strong as the system that just rolled through, but will drop more snow through Wednesday. Saturday April 30th was the last full weather forecast for the season. Snowpack & Avalanche Discussion Winter returned on Wednesday. A storm spring storm dropped snow across the Colorado high country. Observers reported eight to over 12 inches of snow in the San Juans on Thursday. The Front Range had 8 to 16 inches by Thursday, with some areas close to two feet of storm snow by Friday morning. Automated stations indicated about 6 inches in the Park Range, and 3 to 6 inches across the Gore, Elk, and Sawatch Ranges. Winds were drifting the snow near and above treeline. Moderate winds came from unusual directions, adding small drifts to some unusual spots. Thursday night, winds picked up the south and west. Expect to find windslabs form several inches to a foot or more thick. Most will be soft and not very extensive. Periods of sun quickly cook the snow surface and turn it into wet glop. The glop sticks together better than the cold snow underneath and makes a shallow soft slab. The slabs will pop out in shallow, but potentially wide avalanches. On Friday morning, I saw one of these avalanches on a southeast aspect near treeline. It was less than a foot deep but 250 feet wide, and ran after about 45 minutes of sun. The snow surface can go from dry and powdery to moist slab in minutes—really more like tens of minutes. Roller balls and loose avalanches will be extensive below cliff bands and rock outcrops. Below treeline, warm temperatures quickly consolidated and melted the new snow. Squalls and snow showers rolled though after the brief sunny periods. The showers will continue through the weekend, with deeper accumulations in the Northern Mountains. The showers and sun build thin layers of dense and warm snow with colder weaker snow in between. Add in some variable, intermittent winds and it makes an interesting layer cake. You will need to evaluate the recent snow frequently through the weekend. Tune into the impact the changing weather has throughout the day. Check the difference in layers as you change slopes, or across slopes. There is potential to trigger large, dangerous avalanches near and above treeline. There is a frozen crust under the recent snow. That marks the top of the old snowpack. Below treeline the old snow is mostly large, rounded grains that are poorly bonded. It is not that strong of a snowpack. Near treeline the old snowpack has more layering and is more variable. The variability increases above treeline. You will find some very dense layers of old windslabs, weak rotten snow around rocks, and even some cold, persistent weak layers deep in the snowpack. Weekend temperatures near and above treeline will drop below freezing each night. Overnight temperatures will not be as cold below treeline, and the old snow may not have a chance to freeze. Anticipate an increasing potential for deep wet avalanches Saturday afternoon through Sunday, starting below or near treeline. It looks like temperatures will cool again early next week. The weekend warm up may not push us to a widespread, destructive avalanche cycle. We are worried about a rapid warm up and prolonged period of near or above freezing temperatures. The chance for a rapid warm up increases as we head into June. This would mean an increasing chance for a large wet slab cycle. There are weak layers from this winter under six or eight (or more) feet of snow. Should these layers get wet for the first time with a rapid and/or prolonged warm up, we could see some avalanches run larger than they have in decades. Predicting exactly where these weak layers persist, where the deep snowpack is still waiting to get wet for the first time, and when and if these deep slabs might release is difficult and complicated right now. It may not be an immediate problem, but something to consider. Give yourself an extra margin of safety as you travel in the backcountry, especially near and below treeline. Cornices are GINORMOUS. As temperatures warm, they will start shedding chunks like a glacial serac. They will shed more frequently with warmer temperatures. Couple big cornice chunks falling on slopes with the surface slabs, melt water percolating to deeper layers, and we could see some large avalanches result. Even the chunks, without an accompanying avalanche, would be unpleasant to have tumbling down from above you.