News•November 28, 2012

'Atmospheric River' Aims At West Coast; Warmup in Plains

A series of storm systems accompanied by a plume of moisture rich air known as an “atmospheric river” is about to slam into parts of the West Coast, bringing with them the threat for more than a foot of rainfall in some locations, along with flash flooding and mudslides. In addition, heavy mountain snows will fall in the Sierra Nevada Mountains, parts of the Cascades, Bitterroots, and Tetons as the storms spread moisture inland.

Latest Atmospheric River Loop.
Credit: NOAA.

The Pacific storms will play a major part in shifting the jet stream across the U.S., and setting up what is likely to be a remarkably warm start to December, particularly for the Plains and Midwest. Computer model projections show that these areas may see temperatures climb to as high as 20°F above average between Nov. 28 and Dec. 3.

But before that mild weather can take hold, the West must get through this stormy period, which is associated with a big dip, or trough, in the upper-level jet stream winds over the Eastern North Pacific Ocean. The first storm system coming ashore on Wednesday has already been bringing heavy rain and strong winds to northern California, but the atmospheric faucet will really be turned on starting on Thursday, when the moisture plume will be aimed more directly at parts of California and Oregon.

Total rainfall amounts from these storms may exceed a foot in parts of northern and central California, and could cause flooding on some rivers and streams.

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.

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 — as one is forecast to do for a period lasting roughly Thursday through Sunday — major flooding can result. While rivers are not currently running very high in Central California, there is still a risk of some flooding this week.

Computer model projection showing how temperatures in the lower atmosphere may depart from the typical level at this time of year, as of Dec. 3, 2012. The red colors indicate warmer than normal conditions, while the blue indicates cooler than normal temperatures.
Click to enlarge the image. Credit: Weatherbell.

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.

According to NOAA, a strong atmospheric river can transport an amount of water vapor “roughly equivalent to 7.5 to 15 times the average flow of liquid water at the mouth of the Mississippi River.” (Although this year the Mississippi River is running low due to the drought.)

The jet stream configuration that is bringing the storms into the West Coast is also going to allow for a southerly flow of air to take hold in the Plains and Midwest late this week and into the first week of December. Computer model projections show highs may approach 70 degrees in Oklahoma City and Little Rock, Ark., on Friday, with the warm air spreading into Chicago and Detroit as well. The warmth will aggravate longstanding drought conditions across the Plains, where some locations are likely to set records for the driest November since recordkeeping began in the late 19th century.