A potent winter storm slammed into the Pacific Northwest from the evening hours of December 16th through sunset on December 17th.  The surface low reached a peak intensity of 973 millibars while crossing into Northwestern Washington before gradually weakening and tracking east, northeast along the Canadian Border.  Strong isentropic ascent coupled with orographic flow resulted in heavy wet snow across the East Slopes of the Washington Cascades Sunday evening which shifted into Northeastern Washington overnight into Monday morning.  A strong low-level jet on the eastern periphery of the low ushered a tongue of warm air into Eastern Washington overnight.  This lead from a transition of heavy snow to rain for most locations between Grand Coulee and Spokane.  Snowfall amounts along the Highway 2 corridor from Spokane to Wenatchee ranged from two to six inches.  Snowfall totals across the The East Slopes of the Washington Cascades and northern mountains ranged from six to thirteen inches in the valleys to over a foot in the mountains.  Strong winds also accompanied this storm. The combination of gusty winds and heavy wet snow lead to numerous downed trees in the East Slopes of the Cascades resulting in several road closures and numerous power outages.  The strongest winds were experienced across the West Plains, Palouse, Columbia Basin, and Northeast Mountains.  The winds knocked down a pair of large trees in the Spokane Area and snapped numerous tree limbs resulting in 7300 power outages.

Heavy Snow in UPPER COLUMBIA BASIN, WASHINGTON
From 16-DEC-12 16:00:00 to 17-DEC-12 01:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Three to six inches of snow had fallen by 0100 PST on the 17th mainly along and north of the Highway 2 corridor.  The snow quickly melted as gusty winds, rain, and warmer temperatures followed during the morning hours on the 17th.

Heavy Snow in WENATCHEE AREA, WASHINGTON
From 16-DEC-12 16:00:00 to 17-DEC-12 03:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to ten inches of snow fell across the Wenatchee Area.  The highest snow amounts were reported in northern sections of the zone with Entiat and Chelan reporting 9.5 and 8.0 inches respectively.

Heavy Snow in EAST SLOPES NORTHERN CASCADES, WASHINGTON
From 16-DEC-12 16:00:00 to 17-DEC-12 11:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Six to ten inches of heavy wet snow was observed across most valleys within the East Slopes of the Washington Cascades.  Heavier amounts, generally ten to fourteen inches, were reported near the crest.

Heavy Snow in WATERVILLE PLATEAU, WASHINGTON
From 16-DEC-12 16:00:00 to 17-DEC-12 04:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to nine inches of snow fell across the Waterville Plateau. The highest snow amount was reported at Chief Joseph Dam which received nine inches of snow.

Heavy Snow in OKANOGAN VALLEY, WASHINGTON
From 16-DEC-12 16:00:00 to 17-DEC-12 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to ten inches of snow fell across the Okanogan Valley.  Precipitation transitioned to freezing rain briefly before ending 0800 PST on the 17th.

High Wind in WASHINGTON PALOUSE, WASHINGTON
From 17-DEC-12 07:00:00 to 17-DEC-12 16:00:00 (PST-8)
Magnitude: 39.00 MS
Property damage: 7.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter in Pullman measured sustained winds of 45 mph with gusts to 56 mph on a Davis weather station.  The sensor was located on the roof of the house.  Several houses in the neighborhood had shingles blown off.  In Pullman, a tree top snapped off and landed on a house on the 400th block of SE South St.  Utility companies reported that roughly 1200 power outages occurred across the Washington Palouse due to the strong winds.  Several other observation platforms measured gusts between 52 and 56 mph throughout the day.

Strong Wind in NORTHEAST MOUNTAINS, WASHINGTON
From 17-DEC-12 07:00:00 to 17-DEC-12 09:00:00 (PST-8)
Magnitude: 43.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Numerous power outages were observed across Northeastern Washington due to strong winds and tree limbs falling on power-lines.  Students from Columbia and Evergreen School Districts in Stevens County were released from school just after 0900 PST, due to the power outages.

High Wind in SPOKANE AREA, WASHINGTON
From 17-DEC-12 08:00:00 to 17-DEC-12 09:00:00 (PST-8)
Magnitude: 37.00 MS
Property damage: 4.00K
Crop damage: 0.00K
Reported by: ASOS

Four reports of wind damage occurred across Spokane County. A 28 inch diameter tree was blown down and landed on two homes on the South Hill. The homes were located on the 1100th block of East 41st. Another large tree was snapped roughly five feet from the ground, landing in the street and onto power lines on Ninth Avenue between Perry Street and Altamont Street.  Utilizing references in a picture, this tree was estimated to be around 24 inches or larger in diameter.  A NWS employee located in the Deep Creek Area also relayed minor roof damage to a neighbor's pole barn. In Cheney, a metal roof was peeled from the top of a mobile home. The ASOS at Spokane International Airport and RSOIS at the NWS each measured sustained winds of 43 mph coupled with a gust of between 53 and 55 mph.

Heavy Snow in OKANOGAN HIGHLANDS, WASHINGTON
From 16-DEC-12 20:00:00 to 17-DEC-12 07:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to eight inches of snow fell across the Okanogan Highlands.

High Wind in LOWER GARFIELD & ASOTIN, WASHINGTON
From 17-DEC-12 05:30:00 to 17-DEC-12 14:00:00 (PST-8)
Magnitude: 60.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

A DOT site at Alpowa Summit, elevation 1775 feet, measured sustained winds of 52 mph with a peak gust of 69 mph between 0500 and 0700 PST.  The sensor continued to report sustained winds of 35 to 45 mph with gusts to 60 mph through 1400 PST.  Two additional CWOP sites in Pomeroy, WA also recorded wind gusts between 57 and 60 mph.  No damage was reported in the area.

Heavy Snow in NORTHEAST MOUNTAINS, WASHINGTON
From 16-DEC-12 20:00:00 to 17-DEC-12 19:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to ten inches of snow fell across northeastern Washington resulting in snowfall amounts ranging between five to ten inches.  Warmer air eventually found its way into the valleys during the afternoon on the 17th and snow accumulations during the afternoon hours on the 17th shifted mainly into the mountains.

Heavy Snow in SPOKANE AREA, WASHINGTON
From 16-DEC-12 19:30:00 to 17-DEC-12 01:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Moderate to heavy snow fell across the Spokane Area during the evening and early overnight time-frame from December 16th into December 17th.  Three to five inches of snow was observed at multiple observation platforms by midnight on the 17th.  Gusty winds and above freezing temperatures behind an accompanying warm front lead to rapid melting and very little impact on travel by the morning commute.