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‘Atmospheric River’ Piles Up Massive Rain, Snow & Winds

The West is taking a brief break from storms on Monday after a parade of strong weather systems dumped nearly 2 feet of rain, at least 40 inches of snow, and brought strong winds equivalent to a Category 4 strength hurricane to parts of California, Oregon and Washington through Sunday. Although another storm is forecast to affect the region on Tuesday, it is not expected to be as intense.

The storms knocked out power for many across the region, caused extensive airport delays, and provided a vivid reminder of what a weather phenomenon known as an “atmospheric river” can do. Fortunately, this round of storms was nowhere near a worst-case scenario, which could cause extensive flooding in vulnerable parts of Central and Northern California, such as the Sacramento area.

Atmospheric rivers occur when winds draw moisture together into a narrow region ahead of a cold front, in a region of very strong winds. They can be thought of as tropical connectors that feed tropical moisture into more northern latitudes. In California, one type of atmospheric river is also known as the “Pineapple Express,” since it transports water vapor-laden air from Hawaii into the U.S. mainland.

Radar-estimated accumulated precipitation in the West for the seven day period ending on Dec. 3, 2012.
Credit: NOAA.

Research shows that atmospheric rivers are a key source of water in the West Coast, where 30-to-50 percent of yearly precipitation occurs from a few atmospheric river events, according to the National Oceanic and Atmospheric Administration (NOAA). When atmospheric rivers stall, though, major flooding can result. While streams and rivers rose during this event, most did not overflow their banks.

Atmospheric river events are quite common, although as the planet continues to warm, and more moisture is added to the atmosphere through evaporation, they are expected to carry even more water vapor with them. Research shows that 42 atmospheric river events affected California between 1997 and 2006, and the seven floods that occurred during that period on the Russian River northwest of San Francisco all took place as a result of those events.

The National Weather Service’s Hydrometeorological Prediction Center in Maryland has compiled the most impressive statistics from the storms that struck between Nov. 27 and Dec. 3, which rolled in from the Pacific Ocean and tapped a feed of moisture that stretched from the tropics to San Francisco.

Brandy Creek, Calif., near Mt. Shasta, saw the most rainfall as 23.48 inches fell from Nov. 27 through early morning on Dec. 3. Several other locations recorded rainfall totals of a foot or more, including Oak Mountain, which had 15.16 inches. San Francisco International Airport recorded 4.09 inches of rain during the period.

In the higher elevations much of the precipitation fell in the form of snow, which was a boon to skiers looking to kick off the season. The Mt. Rose ski area in California picked up 45 inches of snow during the six-day period, although it’s likely that higher elevations that do not have weather observing stations greatly exceeded that total. Mammath Lakes, Calif., had 26 inches of snow, and Kirkwood Ski Resort had 22 inches.

The snow also spilled over into Idaho, where Deadwood Summit received 46 inches, Ketchum recorded 42 inches, and North Trinity Lakes had 20 inches. In Montana, West Yellowstone pick up 16 inches, and Big Sky had 9 inches.

The winds associated with these storm systems were extremely strong, especially at higher elevations. Mammoth Mountain, Calif., saw a wind gust to 150 mph, which is equivalent to a Category 4 hurricane, and the winds at Ward Mountain peaked at 139 mph.

Redding, Calif., had a peak wind gust of 71 mph, while San Francisco recorded a 50 mph wind gust at the airport, which is situated at sea level.

Strong winds also buffeted the peaks of Idaho, Nevada, Oregon, Utah, Washington, and Wyoming. In Nevada, Ward Mountain recorded a gust to 139 mph, while Mount Rose Ski Area gusted to 95 mph. There was also a gust to 95 mph near Reno, and 72 mph in Carson City, which is the state capital.

Forecast high temperatures across the U.S. on Dec. 3, 2012, showing the abundance of warm air (orange and red shading) in much of the country.
Credit: NOAA.

The highest recorded wind gust in Utah was 113 mph in the Central Wasatch Peaks, while Sheridan, Wyo., had a peak gust of 68 mph.

More strong winds, although this time not as fierce, are anticipated with the next storm system that will affect the Pacific Northwest and northern California through midweek.

Meanwhile, the Pacific storms have helped to flood the U.S. with relatively mild air, as the jet stream is riding along the northern tier of the country, effectively blocking cold, Arctic air from moving southward. Record high temperatures are expected Monday and Tuesday in the Plains and Midwest, with mild weather also anticipated in the Mid-Atlantic and Southeast. In parts of Illinois, for example, high temperatures may reach the upper 60s to low 70s on Tuesday, which is nearly 30°F above average for this time of year, and more typical of conditions during April.

The warm start to December virtually assures that 2012 will go down in history as the warmest year on record in the lower 48 states, given the extraordinarily warm year the country has seen to date.

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