Major flooding in Oregon, heavy snows in Washington and California

A major atmospheric pattern change is underway, and the Pacific storm door is open. A major winter storm supported by an atmospheric river of high moisture is pounding Oregon, California, and Washington today, bringing record flooding, heavy snows, freezing rain, and damaging winds. The ridge of high pressure that had brought much of Northern California its driest winter period on record during the past two months has retreated to the northwest towards Alaska, allowing the subtropical jet stream to dive underneath the ridge. The subtropical jet is bringing a plume of moisture called an "atmospheric river" to the coast between Northern California and Southwestern Washington, resulting in heavy rains near the coast and snow measured in feet to the mountains. Heavy rains of 2 - 5 inches have affected much of Western Oregon over the past 24 hours, and the Marys River in Philomath has risen to its highest flood on record. Additional rains of 1 - 3 inches are expected today over the region, and damaging major flooding is expected along several Oregon rivers.


Figure 1. Radar-estimated precipitation for Oregon shows a wide swath of greater than 8 inches of rain has fallen since January 16 over Western Oregon.


Figure 2. The Marys River in Philomath, Oregon crested at its highest flood height on record this morning. Image credit: NOAA.

Heavy snow, ice storm hit Seattle
Yesterday in Seattle, Washington, 6.8" of snow fell, making it their greatest 24-hour snowfall since November 1985. Winds gusting to 85 mph in the mountains to the east of Seattle closed some ski areas, but heavy snows of 38" in 36 hours fell at White Pass, and the this weekend will see the best skiing of winter in Washington. The problem will be getting to the ski areas--an ice storm this morning has closed the Sea-Tac airport and snarled traffic.

Record dry spell ends for California and Nevada
Skiiers, take heart: Tahoe's Ski Season From Hell will end this weekend. A series of three snowstorms will pound the Sierra Mountains of California and Nevada, bringing the first heavy snows since November. According to Tahoe weather historian Mark McLaughlin, December 2011 was the second driest in the Sierra Mountains since record keeping began in 1920. He based this on an aggregate of data from eight locations from Highway 50 in the south to Mt. Shasta in the north in the central and northern Sierra Nevada. This winter has seen the longest winter dry spell in history in Reno, Nevada (56 days). The dry spell ended on January 16, when .03" of precipitation fell. The previous longest dry spell was 54 days, ending on January 24, 1961. Farther to the south in Minden, Nevada, the record dry spell ended at 72 days on January 16. Nevada's Carson River Basin snowpack is at 8% of normal, compared to 224% last year at this time. In San Francisco, the first significant rains since November 20 are expected to begin later today. The past two months have seen just 0.26" of rain in the city, making it the longest two-month winter dry period in the city since records began in 1850, according to wunderground's weather historian, Christopher C. Burt. Fresno, Califonia has seen no rain at all since November 20, and that dry spell should end today or Friday. Today's storm in Northern California will be followed by a new storm on Friday, which will bring 100 mph wind gusts and 1 - 2 feet of snow to the Sierras (2 - 3 inches of rain equivalent). A third storm with more snow is expected on Sunday night.


Figure 3. 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 California/Oregon border, bringing heavy rains and snows. Image credit: University of Wisconsin SSEC.

I'll have a new post on Friday.

Jeff Masters