Major winter storm pounds Pacific Northwest

A ferocious winter storm is pounding the Pacific Northwest, bringing heavy snow, flooding rains, and winds near hurricane force over coastal waters. The snow is falling in earnest in Seattle this morning, where 3 - 5 inches are expected. The storm's heaviest snows have fallen in Southwest Washington, just north of Portland, Oregon, where Toutle, Washington received 18" as of 6 am PST this morning. The storm also brought 2 inches of snow to Portland, Oregon overnight. The wet, heavy snow brought down numerous trees and power lines, causing power outages to 30,000 people. Portland averages just 6.5 inches of snow per year. Very heavy snows of 1 - 3 feet have fallen in the Cascade Mountains east of Seattle, causing extreme avalanche danger. From the latest NWS Seattle forecast discussion:

Strong winds and heavy snow have overloaded fragile snow layers weakly attached to old crusts and produced increasingly large and sensitive avalanches. Field reports late Tuesday already indicated lots of natural and human triggered slides ranging from about 1 to 3 feet deep. Avalanche warnings already in effect for high danger...and with warming...further winds and additional heavy to very heavy snow...some quite dense...avalanche activity should become larger and more severe on Wednesday. This should lead to extreme danger above about 5000 feet with increasing high danger below. Due to very dangerous conditions getting worse...back country travel should be avoided Wednesday.

At sea, the National Weather Service has issued a hurricane force wind warning for the Southern Oregon coastal waters. Hurricane force winds of 70 - 75 mph, gusting to 90 mph, are expected today, with waves of 22 - 25 feet.


Figure 1. Heavy snow in Mt. Vernon, Washington on January 17, 2012. Image credit: wunderphotographer spiritjoy.

Seattle snow history
Seattle only averages 8 inches of snow each winter, and so today's storm is a major event for them. According to wunderground's weather historian, Christopher C. Burt, the last major snowfall for Seattle (Sea-Tac Airport location) was 12.0" over the week of Dec. 18 - 25, 2008. It appears the max. 24-hour fall during that event was 6.0" on Dec. 20 - 21. Prior to 2008, it appears Nov. 1985 had 17.5", but what the biggest storm 24-hour total then is not clear. Seattle's record 24-hour snowfall was 21.5" on Feb. 2, 1916, and 32.5" fell as a storm total between Jan. 31 - Feb. 2, 1916. Much deeper snow was reported during the famous Storm King event of January 9 - 12, 1880 when "four to six feet" accumulated according to press reports.


Figure 2. Satellite estimated water vapor in the atmospheric expressed as how much rain would fall if the entire amount of vapor were condensed in a vertical column. Where this "Total Precipitable Water" exceeds about 25.4 mm (1 inch), heavy precipitation can occur. An "Atmospheric River" extending from Hawaii with very moist air harboring precipitable water values near 1.5 inches (38 mm, light blue colors) is impacting the coast near the Washington/Oregon border, bringing heavy rains and snows. Image credit: University of Wisconsin SSEC.

I'll have a new post on Thursday.

Jeff Masters