Good evening, this is Sue Burak with the Eastern Sierra Avalanche Center with a weather and snow update for Thursday December 20, 2007. MOUNTAIN WEATHER Todays weather started out clear with some gusty SW winds on the ridgetops. High clouds in advance of the incoming storm moved in later in the day. Highs today ranged from 29 F at Tioga Pass, 29 at Main Lodge and Rock Creek Lakes Resort. Lows tonight will be in the mid 20s. A winter storm warning is in effect from 8 PM tonight through 8 PM Thursday. This will be a very windy storm with ridgetop wind gusts exceeding 100 mph above 10,000 ft. Winds at the higher elevations will decrease to around 30-50 mph gusts and shift to the west by tomorrow afternoon. Daytime highs will be in the mid to upper 20s with lows in the low 20s. Snowfall will begin tonight and continue throughout most of the day on Thursday. This storm is splitting with less moisture available for the eastern Sierra than Tuesdays storm. Expect 10-18 above 9,000 ft from the Mammoth area northward. Rock Creek and Bishop Creek will see lesser amounts, perhaps 5-8 of new at Rock Creek Lodge with the higher elevations of Bishop Creek receiving perhaps up to 5 inches. By Friday, an upper ridge will lead to dry conditions. SNOWPACK AND AVALANCHE DISCUSSION Snowfall and water content tallies from Tioga Pass to Cottonwood Lakes show 12 to 23 inches of snowfall and 1 to 2.5 inches of water, depending on location. Top awards for the most snowfall and water content are shared between the Sesame Street study plot at the 9,000 ft elevation on Mammoth Mountain and the Devils Postpile sensor at 7600 ft. Both locations received 23 of snow with the water content at Devils Postpile coming in at 4 compared to 2.7 at Mammoth. Bishop Pass picked up 17 while Charlotte Lake (10,800 ft) received 22 of new. The upper Mammoth Lakes Basin picked up about 2 feet of new snow, some of which fell on dirt and rocks. It is important to note that the Mammoth Mountain ski patrol had an active control day today. Many avalanches went to the ground on 4-8 inches of depth hoar. There were many natural avalanches that occurred during the storm. Backcountry skiers and riders beware! There are places in all our favorite front and backcountry slopes with 4-8 inches of depth hoar underneath all the new snow. However, there are no patrollers to throw bombs and chase away the depth hoar crystals. Tomorrows storm will come with 3 feet of wind. Expect dangerous wind slab formation in most exposed locations. Try to remember that you might be carving turns through a foot of new powder sitting on top of a couple of feet from Tuesdays storm. The surface snow doesnt look any different but the entire snowpack could be sitting on weak sugary crystals. Take the time to stick a ski pole to the bottom and see if there is sudden loss of resistance to the pressure you are placing on the pole. Depth hoar can collapse or it can shear. Depth hoar can also trigger slides from the bottom of a slope. Depth hoar is your basic nightmare. BOTTOM LINE Avalanche danger ratings will be issued once there is enough snow to ski more terrain than is currently available. Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.