Several rounds of heavy snow fell across the East Slopes of the Washington Cascades between December 16th and the 24th.  The most significant accumulations occurred with two storm systems; one which moved through the region during the 16th and 17th and second during the 19th and 20th.  These storm systems were addressed in Episodes 69455 and 70424.  Additionally, a Puget Sound Convergence Zone materialized on the afternoon of the 17th dumping an additional foot of snow over the Lake Wenatchee and Plain areas.  Over the seven day period, cumulative snow amounts were as followed: Plain 39.1 inches, Winthrop 28.3 inches, Mazama 36.7 inches, and Holden Village 49.6 inches.  The weight of the snow combined with gusty winds on the 17th resulted in several downed trees.  As snow continued to fall over the next few days, the weight of the snow brought down hundreds of trees which sporadically fell from the 17th through the 24th.  County officials and utility companies were overwhelmed by the amount of power line damage and tree removal necessary and were forced to delay cleanup and repair operations because of unsafe conditions.  The hundreds of downed trees resulted in 3800 power outages, road closures, damage to vehicles, buildings, and utility infrastructure.  Several trees fell on moving cars resulting in at least nine injuries and two deaths.  A majority of the problems occurred within the communities of Lake Wenatchee, Plain, Leavenworth, and along Highway 2 between Leavenworth and Stevens Pass.  It is not uncommon for these locations to experience several days of heavy snow.  What made this event unusual was a combination of factors.  Most important was the temperatures over the entire period which ranged between 28 and 33 Fahrenheit.  Thermal profiles in this range allowed the snow to efficiently stick to the trees.  Secondly, the area never experienced a deep freeze prior to the event.  The unfrozen ground was uanble to hold the trees in place and left the trees vulnerable to snow loading.  Lastly, the combination of the first storm system and local convergence zone brought over two feet of snow within the first 24 to 48 hours followed by an additional foot over the next few days and the lack of winds following the 17th brought no means to blow the snow off the trees.

Winter Weather in EAST SLOPES NORTHERN CASCADES, WASHINGTON
From 17-DEC-12 00:00:00 to 25-DEC-12 16:00:00 (PST-8)
Property damage: 5.60M
Crop damage: 0.00K
Reported by: Emergency Manager

Roads that were closed due to downed trees included: Cedar Brae, River Road, Shugart Flats, Chiwawa Loop, Baraburn, Cottonwood, and Highway 2 along a 35 mile stretch between Stevens Pass and Leavenworth.  The highway is one of two primary east-west routes through the mountains open during the winter.  About 7,000 to 10,000 drivers cross Stevens Pass during the Christmas holiday.  The 3800 power outages affected the following communities: Plain, Lake Wenatchee, Leavenworth, Mary Canyon, Little Chumstick, Coles's Corner, Kahler Glen, and Fish Lake.  The worst hit locations experienced outages in excess of five days.  Damage costs included 4.6 million dollars from Chelan County PUD, 218 thousand for Chelan County Public Works, and estimated costs for at least fifty houses or cabins damaged and four vehicles.  On December 19th, a tree struck a pickup truck traveling on Chiwawa Loop Road injuring the driver.  On December 21st, a tree struck a car on Highway two thirteen miles east of Stevens Pass killing two adults and injuring four passengers in the SUV.  Two of the four injuries were listed as serious and critical.  On the evening of December 22nd, a 2005 Audi slammed into a newly fallen tree while traveling on Highway 2 roughly 22 miles west of Leavenworth.  All five people inside the vehicle were injured, some seriously.