Aspen ===== Current Keywords: LOW, MODERATE, avalanche danger, cold front, crossloading, depth hoar, drifts, faceted, facets, human triggered, loose snow avalanches, low pressure, slabs, sluffs, trough, weak layer, wind slab, wind slabs, Mountain Weather and Avalanche Forecast for the Aspen Zone Forecast Issued on 12/21/2007 4:01:33 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) -5 to -10 2-7 -5 to 0 Wind Speed(mph) 10-20 G30 15-25 20-30 Wind Direction NNW NW WNW Sky Cover Overcast Mostly Cloudy Partly Cloudy Snow(in) 0-3 0 0 Snowpack Discussion It looks like the snow totals from Friday's storm will end up around a foot. South to southwest winds accompanied the snow. Winds will swing to the northwest Saturday. That means loading and crossloading on a variety of aspects near and above treeline. The new slabs will be sitting on a wind-worked surface. Strong winds above treeline created many pockets of wind slab like this one. You will find a snowpack that is highly variable and made up of varying depths of wind slab at the surface. East and northeast aspects are the most heavily loaded and have the potential for some larger triggered avalanches in steeper terrain. Keep any eye out for areas where the recent slabs are resting on weaker faceted snow below. We have also had several reports of human triggered loose snow avalanche activity (sluffs) near and below treeline around the zone, too. The upper layer of the snowpack has lost a lot of strength recently and resulted in these loose snow avalanches on steeper slopes. New snow on top will increase the size and speed of the sluffs. It may sound like a broken record, but we can't forget about that weak layer of facets and depth hoar at the base of the snowpack on higher elevations shady aspects. You will find this layer mostly on slopes facing NW-N-NE-E but keep an eye out for it in shady pockets and gullies on other aspects as well. Avalanches triggered on this layer will involve the whole season's snowpack. Observers have still noted some easy failures on this layer in some areas of the zone, including some tests that failed on the first tap or while cutting the back of the column. Avalanche Danger Above treeline the avalanche danger is CONSDIERABLE on north, northeast, and east aspects above treeline, and MODERATE on other aspects. Near and below treeline, danger is MODERATE on slopes facing west, northwest, north, northeast, and east. The danger is LOW on other aspects. Unstable wind slabs and drifts are likely on most steep slopes above treeline, and possible on steep slopes or below near treeline. High elevation shady slopes have a weak layer at the bottom of the snowpack. If triggered, this layer could result in some large avalanches. We have been receiving many good observations from the Aspen Zone. Many thanks to those of you who have sent them in. The information is a great help to the forecasts. Please drop us a quick e-mail with your avalanche, snowpack, or weather observations from this area or any other. Front Range =========== Current Keywords: CONSIDERABLE, LOW, MODERATE, avalanche danger, cold front, denser, depth hoar, drift, human triggered avalanches, low pressure, powder. , slab, slabs, trough, wind loaded, Mountain Weather and Avalanche Forecast for the Front Range Zone Forecast Issued on 12/21/2007 3:54:03 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) -5 to 0 2-7 -3 to 2 Wind Speed(mph) 10-20 G40 20-30 G40 20-30 G50 Wind Direction NW WNW W Sky Cover Mostly Cloudy Mostly Cloudy Partly Cloudy Snow(in) 2-5 0-1 0 Snowpack Discussion By itself, Friday's new snow load is probably insufficient to push the snowpack to avalanche. The snow is not by itself, though. We have to add wind and people into the mix. Winds have ranged from the southwest, west, to northeast, and strong enough to drift snow near and above treeline. Fresh slabs will grow to a foot or more thick. There are older slabs, too. Many of the slabs are sitting on top of old powder. This strong over weak configuration can be triggered with the additional weight of a backcountry traveler in the wrong spot. Some recent reports from around the state hint that the wrong spots might be further down slope than usual, where the denser slabs thin and soften. The wrong spot could also be in a thinner area of snow cover, like rock outcrops, or shallow covered cliff bands. Keep the deep slab instability tucked into the back of your mind. It's the same problem we have been discussing all month: pockets of weak depth hoar just above the ground. Knowing that deep hard slabs like these are possible is a bit unnerving. Hard slab releases can be very challenging to predict: the snow underfoot can feel firm and "bomber," giving you a sense of security. However these slabs have the potential to be triggered from afar and often fracture above the point of initiation. Backcountry travelers are tracking up a variety of aspects and slope angles without reported incidents. This is encouraging, but don't take our snowpack for granted! Avalanche Danger The avalanche danger for the Front Range zone is CONSIDERABLE on northwest, north, northeast, and east aspects near and above treeline. Human triggered avalanches are probable on steep, wind loaded slopes. On southeast and south aspects near and above treeline the danger is MODERATE. Below treeline the danger is LOW. Grand Mesa ========== Current Keywords: avalanche danger, cold front, control work, crusts, drift, faceted, low pressure, melt-freeze, settled, shear, slab, slabs, sluffs, stabilized, surface hoar, trough, Mountain Weather and Avalanche Forecast for the Grand Mesa Zone Forecast Issued on 12/21/2007 4:04:43 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) -2 to 3 12-17 0-5 Wind Speed(mph) 5-15 5-15 0-10 Wind Direction N NW WNW Sky Cover Mostly Cloudy Mostly Cloudy Partly Cloudy Snow(in) 0-3 0 0 Snowpack Discussion The snowpack in the Grand Mesa Zone has settled and stabilized since the last storm. Friday morning, CDOT control work triggered two small soft slab avalanches and several loose snow sluffs. The slides were in the recent snow, and not in the older snowpack was not reactive. Snow Friday, then again next week, will add a soft slab to the weak surface. Our forecaster in the area noted widespread 2 mm surface hoar covering the snow on Tuesday, and it is now buried under several inches of snow. This is a bad combination. Surface hoar is notoriously weak in shear and will not support a very large load before failing. Lower elevation sunny slopes likely have melt-freeze crusts, which will not bond well with new snow either. The bottom line: as the snow piles up, the avalanche danger is likely to rise as these weak snows are loaded. Most avalanche activity will be limited to surface and new snow slabs. The forecast snow amounts will lead to relative shallow avalanches on steep slopes out of the wind. Even light winds can drift snow several times deeper than the new snow depth, so higher elevation east and northeast facing slopes could have a new slabs more than a foot thick. Be aware of changing conditions during the storms. Avalanche Danger We have insufficient data to rate the avalanche danger in the Grand Mesa Zone. The most problematic areas are probably steep slopes that have weak faceted snow under new snow. If you have any observations or data to share, please send them to [57]caic@qwest.net. San Juan North ============== Current Keywords: CONSIDERABLE, LOW, MODERATE, avalanche danger, cold front, control work, cross-loaded, density, drifts, faceted, facets, low pressure, powder , slab, slabs, trough, windloaded, windloading, Mountain Weather and Avalanche Forecast for the Northern San Juan Mountain Zone Forecast Issued on 12/21/2007 4:05:42 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) 0 to -5 12-17 2-7 Wind Speed(mph) 10-20 10-20 10-20 Wind Direction NNW NW NW Sky Cover Mostly Cloudy Mostly Cloudy Partly Cloudy Snow(in) 0-3 0 0 Snowpack Discussion The storm total is headed towards 8-10 inches at treeline. Recent winds worked their way around the compass from south to west to northeast and everywhere in between. The gusty winds are easily transporting the new snow and forming slabs, particularly on northerly and easterly aspects near and above treeline. Expect to find wind slabs, drifts, and cross-loaded terrain features on a variety of aspects on higher elevation slopes, as well as on exposed areas below treeline. These slabs can be thick and deep, and they may be reactive on steep slopes where they are sitting on top of low-density powder or facets. Keep on top of changing conditions and windloading, particularly on northerly and easterly aspects near and above treeline. Mid-week, control work at Telluride triggered several impressive avalanches. Their efforts pulled out three significant hard slabs from west-northwest terrain near and above treeline. Two of the slabs ran to the ground, and one dislodged a huge boulder (3 meter diameter) and carried it over 400 vertical feet. Knowing that deep hard slabs like these are possible is a bit unnerving. Use extra caution when you venture into high alpine terrain. Hard slab releases can be very challenging to predict: the snow underfoot can feel firm and "bomber," giving you a sense of security. However, because these slabs are so cohesive, and in many areas are sitting on top of weak faceted snow, they have the potential to be triggered from afar and often fracture above the point of initiation. Backcountry travelers are tracking up a variety of aspects and slope angles without reported incidents. This is encouraging, but don't take our snowpack for granted! Avalanche Danger The avalanche danger for the Northern San Juan is CONSIDERABLE on northwest, north, northeast, and east aspects above treeline. You will find unstable, windloaded slabs on steep slopes. On southeast, south, southwest, and west aspects above treeline the danger is MODERATE. Near and below treeline the danger is MODERATE on all aspects except SE-S-SW aspects, where the danger is LOW. San Juan South ============== Current Keywords: LOW, MODERATE, avalanche danger, cold front, crusts, drift, facets, low pressure, powdery , slab, slabs, surface hoar, trough, Mountain Weather and Avalanche Forecast for the Southern San Juan Mountain Zone Forecast Issued on 12/21/2007 4:06:52 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) 0-5 15-20 8-13 Wind Speed(mph) 10-20 15-25 5-15 Wind Direction SW>NW NW W Sky Cover Mostly Cloudy Partly Cloudy Partly Cloudy Snow(in) 1-4 0 0 Snowpack Discussion Another round of storms will move through over the next few days bringing new avalanche problems. The new snow is falling on near-surface facets and surface hoar--the classic recycled powder--on shadier aspects. These small, angular, surface grains do not pose a problem by themselves. However, these grains tend to bond poorly and will become reactive with the addition of new snow load in the future. Deeper in the snowpack, observers report some crusts and layers of buried surface hoar on east and north aspects. The avalanche danger will rise as the snow piles up. Winds can drift snow several times deeper than the new snow depth. Friday winds were west southwest, and will swing to the northwest on Saturday. Higher elevation north, northeast, east, and souteast facing slopes could easily have new slabs more than a foot thick. The snow can be soft and powdery and still act like a slab if the snow underneath is even weaker. Be aware of changing conditions as the winds continue. Avalanche Danger The avalanche danger is MODERATE on all aspects near and above treeline. New snow will form unstable slabs on steep slopes, especially where the winds drift and load. On other aspects and below treeline the danger is LOW. Sangre de Christo ================= Current Keywords: avalanche danger, cold front, cross loaded, density, drifts, facets, low pressure, powder , settle, slab, slabs, stabilize, trough, wind slabs, Mountain Weather and Avalanche Forecast for the Sangre De Cristo Zone Forecast Issued on 12/21/2007 4:07:42 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) 2-7 17-22 5-10 Wind Speed(mph) 15-25 15-25 15-25 Wind Direction N NW WSW Sky Cover Mostly Cloudy Partly Cloudy Partly Cloudy Snow(in) 1-3 0 0 Snowpack Discussion Friday's storm could bring some good snows to the Sangres. With northeasterly winds during at least part of the storm, westerly aspects could see some slab development by Saturday. Recent mild weather helped the snowpack to settle and stabilize. Don't let this make you complacent though. Weak layers still lurk on north and easterly aspects. Strong and persistant winds have changed the snowpack, stripping snow from some areas while depositing it in others. Winds have blown from the southwest to west to north and back again. This means that you can expect to find fresh wind slabs, drifts, and cross loaded terrain features on a variety of aspects on higher elevation slopes as well as on exposed areas below treeline. These slabs may be reactive on steep slopes where they are sitting on low-density powder or facets. Work into steeper terrain gradually, reevaluating the snowpack often. Avalanche Danger We have insufficient data to rate the avalanche danger in the Sangre de Cristo Zone. Please send observations from this or any other zone to [57]caic@qwest.net Sawatch ======= Current Keywords: CONSIDERABLE, LOW, MODERATE, avalanche danger, cold front, cross-loaded, crossloading, density, depth hoar, drifted, drifting, drifts, faceting, facets, low pressure, paths, powder , settled, slabs, trough, wind drifting, wind slabs, windloading, windslab, Mountain Weather and Avalanche Forecast for the Sawatch Range Zone Forecast Issued on 12/21/2007 3:50:56 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) -5 to 0 10-15 0-5 Wind Speed(mph) 10-20 15-25 G40 20-30 G50 Wind Direction NW NW WNW Sky Cover Mostly Cloudy Mostly Cloudy Partly Cloudy Snow(in) 1-4 0 0 Snowpack Discussion Recent winds worked their way around the compass from south to west to northeast and everywhere in between. The gusty winds easily transported snow and formed slabs, particularly on northerly and easterly aspects near and above treeline. Friday's snow added yet another layer of new slabs. It was very light, and drifted easily. Expect to find wind slabs, drifts, and cross-loaded terrain features on a variety of aspects near and above treeline. The stack of slabs can be thick and deep, and they may be reactive on steep slopes where they are sitting on top of low-density powder or facets. Keep on top of changing conditions and windloading, particularly on northerly and easterly aspects near and above treeline. The older snowpack has settled down and no one has reported recent avalanche activity in the older layers. The older snow is faceting, but that poses little problem until the snow is topped by a windslab. Faceting is much more rapid in shallow snow, like the stating zones of avalanches that have already run this year. We often see paths reload and run on depth hoar again. Other suspect terrain would be areas where there are fresh, soft wind slabs. Avalanche Danger The avalanche danger is MODERATE with pockets of CONSIDERABLE on northwest, north, northeast, east, and southeast aspects near and above treeline. You will find unstable slabs on steep slopes that have recent wind drifting and loading. The danger is MODERATE on all aspects at treeline and other aspects above treeline. Again, watch for steep slopes with crossloading or drifting. Below treeline the danger is LOW on all aspects. Steamboat ========= Current Keywords: avalanche danger, cold front, collapses, cross loading, crossloading, crusts, drift, faceted, facets, low pressure, slabs, surface hoar, trough, weak layers, Mountain Weather and Avalanche Forecast for the Steamboat and Parks Range Zone Forecast Issued on 12/21/2007 3:52:31 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) 0 to -5 8-12 -5 to 0 Wind Speed(mph) 10-20 G30 10-20 G40 15-25 G40 Wind Direction NNW WNW W Sky Cover Overcast Mostly Cloudy Partly Cloudy Snow(in) 1-4 0-2 0 Snowpack Discussion Friday kicked off another round of storms. As the storm clears Friday night, it will likely leave about a foot of new snow. What did the storm snow fall on? Surface crusts have formed on lower elevation sunny slopes. Near-surface facets and surface hoar--the classic recycled powder--have likely formed on shadier aspects. These small, angular, surface grains do not pose a problem by themselves. However, these grains tend to bond poorly and will become reactive with the addition of new snow load in the future. The avalanche danger will rise as the snow piles up. Winds can drift snow several times deeper than the new snow depth, so higher elevation north, northeast, and east facing slopes could have new slabs several feet thick. Winds were west to southwest on Friday, but will swing to the northwest on Saturday. Expect slabs and lots of cross loading. Be aware of changing conditions during the storms. We received second hand reports of continued collapses on south and west aspects. Do not forget that the old snowpack has some weak layers near the ground. They might be stubborn and hard to trigger, but run big. Avalanche Danger We have insufficient data to rate the avalanche danger in the Steamboat Zone. The most problematic areas are probably steep northwest, north, northeast, and east facing slopes that have weak faceted snow under new snow slabs. Expect crossloading and soft slabs on most high elevation slopes. If you have any observations or data to share, please send them to [57]caic@qwest.net. Vail ==== Current Keywords: CONSIDERABLE, LOW, MODERATE, avalanche danger, cold front, depth hoar, low pressure, natural, slab, slabs, trough, wind loaded, wind slab, windloaded, windslabs, windward, Mountain Weather and Avalanche Forecast for the Vail and Summit County Zone Forecast Issued on 12/21/2007 3:55:36 PM Forecaster: Spencer Logan Weather Synopsis A fast moving trough of low pressure in moving through the southwest portion of the U.S. this morning. The leading edge of this system is bringing clouds and snow showers to the western portion of the state, which will spread through the mountains as the cold front moves through today. Expect moderate to strong winds to high elevation areas. Snowfall will be heaviest in the Steamboat and Flat Tops this morning, but gradually shift into central and then southern mountains during the day. There is a decent jet max (area of strong upper level winds) associated with this system, which could produce isolated areas of heaviest snow. The trough axis is forecast to swing into the state this evening, which should put an end to the snowfall. A second system is forecast to move through late Sunday and into Monday. It looks like this system could spit upstream of Colorado. Weather Friday Night Saturday Saturday Night Temperature(°F) 0 to -5 5-10 -3 to 2 Wind Speed(mph) 10-20 G30 10-20 G30 15-25 G40 Wind Direction NNW WNW W Sky Cover Mostly Cloudy Mostly Cloudy Partly Cloudy Snow(in) 1-4 0-1 0 Snowpack Discussion News of triggered slab avalanche activity is slowing down in the Vail Summit zone. On Tuesday a skier triggered a good sized slab on a NE aspect just below treeline near Vail. The slab fractured above the rider, up to 4' deep, and flushed the skier down nearly 800 vertical feet. This was a wind slab created from recent moderate to sometimes strong westerly winds. Other reports of natural and triggered activity have come in from the Eastern side of the Ten-Mile Range, Hoosier Pass and Loveland Pass. There seems to be a re-occuring theme with the avalanche activity; near or at treeline, northeast through southeast aspect, and wind loaded. Those are the only aspects above treeline that have snow on them. Most of the fetches or windward aspects are devoid of snow. It's the only snow to ride, so folks are drawn to them. Keep the deep slab instability tucked into the back of your mind. It's the same problem we have been discussing all month: pockets of weak depth hoar just above the ground. Knowing that deep hard slabs like these are possible is a bit unnerving. Hard slab releases can be very challenging to predict: the snow underfoot can feel firm and "bomber," giving you a sense of security. However these slabs have the potential to be triggered from afar and often fracture above the point of initiation. Don't take our snowpack for granted! Avalanche Danger The avalanche danger for the Vail Summit zone is CONSIDERABLE on north through east through south aspects above treeline. Unstable slabs are probable on steep, windloaded terrain. The slabs are a mix of new and slightly older slabs. The danger is MODERATE on other aspects above treeline and on all aspects at treeline, where windslabs are possible on steep slopes. The danger is LOW below treeline.