23 February 2017

FLOOD UPDATE: What is an Atmospheric River? (re Oroville and Nevada)

UPDATE: Watch: Aftermath – Drone video captures the extensive flooding in San Jose, California INNews

(CNN)While floodwaters in the northern California city of San Jose were receding Wednesday, the mayor said they were contaminated and residents of 14,000 households ordered to evacuate should not yet return home. The residences under the mandatory evacuation are near a creek that appears to have been breached, Mayor Sam Liccardo said. Tens of thousands of more homes are under voluntary evacuation notifications. One resident slept in his car instead of going to a shelter.

"We woke up at 1:30 in the morning, cops knocked on the door of our apartment. My wife got the dog out of there and I parked in the parking lot. Yeah, I slept in the car," Santos Aguilar told CNN affiliate KGO.

"This is not about rain, this is about Anderson Dam," Liccardo said. "Obviously, the rain contributed to the overflowing of Anderson Dam but what we have is truly unique. We have evacuation areas that exceeded the 100-year flood zone. This is not a flood we would typically anticipate. What happened at Anderson Dam is the cause."

Anderson Dam, just southeast of the city, is at 103% of capacity and the water level at the reservoir is still 2.2 feet above the top of a spillway where water flows out of the reservoir. Read more: SanJoseFlood

An “atmospheric river” continued to batter parts of Northern California on Tuesday, causing widespread flooding. The heavy rains prompted the National Weather Service to warn of a dam failure outside Carson City, Nevada, saying that it was “not a drill” and that residents should “move to higher ground now.”

But less than 90 minutes after issuing the alert, the agency changed its report to say the retention basin in Dayton, Nev., had not failed. Instead, it was full and overflowing into drainage areas, the weather service said. However, Lyon County, Nev., emergency officials warned that the situation could change in the next few hours.

What are atmospheric rivers?

Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.

Infographic: The science behind atmospheric rivers (NOAA)

Although atmospheric rivers come in many shapes and sizes, those that contain the largest amounts of water vapor and the strongest winds can create extreme rainfall and floods, often by stalling over watersheds vulnerable to flooding. These events can disrupt travel, induce mudslides and cause catastrophic damage to life and property. A well-known example is the "Pineapple Express," a strong atmospheric river that is capable of bringing moisture from the tropics near Hawaii over to the U.S. West Coast.

NOAA research (e.g., NOAA Hydrometeorological Testbed and CalWater) uses satellite, radar, aircraft and other observations, as well as major numerical weather model improvements, to better understand atmospheric rivers and their importance to both weather and climate.
Scientific research yields important data that helps NOAA's National Weather Service forecasters issue warnings for potential heavy rain and flooding in areas prone to the impacts of atmospheric rivers as many as five to seven days in advance.

For more information: noaa

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