Behind the Forecast: Atmospheric Rivers

Behind the Forecast: Atmospheric Rivers
GF Default - Science Behind the Forecast: Atmospheric Rivers

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LOUISVILLE, KY (WAVE) - Atmospheric rivers are narrow areas that transport moisture from the tropics to more northern latitudes.

They are a vital part of the Earth’s ocean water cycle but also play a big role in flood risks in some parts of the country. While they can be dangerous, they also bring much-needed precipitation and are an essential contribution to annual freshwater supplies. They carry the amount of water vapor basically equal to the average amount of water that flows through the mouth of the Mississippi River.

A NASA study found that globally, precipitation from atmospheric rivers contribute 22 percent of the total water that flows across Earth’s land surfaces. In certain areas, such as the west and east coasts of North America; Southeast Asia; and New Zealand, that contribution can top 50 percent. Atmospheric Rivers are around 250 to 375 miles wide on average and can supply the west coast of the United States with 30 to 50 percent of their annual rainfall in just a few events.

There are about 11 atmospheric rivers present on Earth at any time.

This image uses satellite data to show water vapor and precipitation as California deals with an atmospheric river.
This image uses satellite data to show water vapor and precipitation as California deals with an atmospheric river. (Source: NASA)

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The winds that drive the water vapor northward are part of the global conveyor belt, a constantly moving system of deep ocean circulation driven by temperatures and salinity.

Satellites, aircraft and radar, as well as computer models, are used to better understand these atmospheric rivers impact on weather and climate.

One of the best known atmospheric rivers is the “Pineapple Express” - not to be confused with the Seth Rogen and James Franco movie of the same name. It’s called this because it pulls moisture from the Pacific, mainly near Hawaii, and can drench the U.S. and Canadian west coasts with heavy rain and snow; as much as five inches of rain in a day is possible. In December 2010, 11 to 25 inches of rain was dumped in areas from Washington state to Southern California.

It’s not just the west coast that has to deal with atmospheric rivers, the east coast can be affected too. In October 2015, an atmospheric river flooded parts of South Carolina while a Hurricane named Joaquin spun off the coast. Upper-level winds associated with an area of low pressure over Florida drug moisture from the storm over the southeastern U.S. Parts of South Carolina saw more than 20 inches of rain.

Science Behind the Forecast: Atmospheric Rivers

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