Parts of the United States are likely to see the worst flood in 1,000 years, thanks to the weather phenomenon known as atmospheric rivers. The forecast from the Weather Prediction Center of the US National Weather Service (NWS) warned that the storms associated with this phenomenon will move towards northeastern states on Friday and into the weekend. The landfall will hit the middle Mississippi Valley and the Arkansas-Louisiana-Texas region.

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Parts of Tennessee, Kentucky, Arkansas, Missouri, Indiana and Illinois states could come under heavy flooding.

 

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What are atmospheric rivers?

The term atmospheric rivers, abbreviated as ARs, was popularised in the 1990s by researchers Yong Zhu and Reginald E  Newell of the Massachusetts Institute of Technology. Atmospheric rivers are often referred to as ARs.

In simplest terms, atmospheric rivers are a weather phenomenon that appear like rivers in the sky: a narrow, long area of water vapour.

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These 'rivers' take moisture from tropical regions and move them to the mainland US.

Their geographical distribution spans the Pacific to Hawaii. 

Once they form over an ocean, atmospheric rivers flow through the sky and bring water vapour from the tropics to the northern latitudes of the US.

Once the atmospheric river makes landfall, the moisture is released as rain or snow.

Atmospheric rivers are behind nearly half of annual precipitation in the US.

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An average atmospheric river can carry as much water as the mouth of the Mississippi River. How much water can be carried in an atmospheric river is calculated by measuring the vapour and dividing it by the density of liquid water.

The worst kind of atmospheric rivers can lead to unprecedented rain and floods. The rains can cause landslides, disrupt travel and damage both life and property.

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Atmosphere rivers are also responsible for the snowpack in the Sierra mountains.

How are atmospheric rivers formed?

Atmospheric rivers are generally a consequence of cyclones and storms. They can extend up to 600 kilometres in width. Weather conditions affect the direction of their movement, as well as the intensity of rain or snow that they bring in.

What is a Pineapple Express?

Pinapple Express is the nickname given to the atmospheric river that brings vapour from the tropics near Hawaii to the West Coast states of the US.

Are all atmospheric rivers dangerous?

No. Most atmospheric rivers are weak, and could bring much-needed rain that fills aquifers. They are in fact a crucial aspect of water supply in the US.

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But an excess supply of rain due to atmospheric rivers can lead to floods. Past cases of flooding by atmospheric rivers include the December 2010 winter storm that hit the US West Coast. Rainfall of up to 25 inches was recorded. 

How are atmospheric rivers monitored?

In the US, the National Oceanic and Atmospheric Administration or NOAA, a bureau of the commerce department, is the main government agency responsible for monitoring and researching atmospheric rivers.

It forecasts and warns about the dangers of atmospheric rivers, using satellite imagery, radar and weather aircraft.

Based on NOAA research, the National Weather Service forecasters warn about rain and flooding.

They can make the predictions up to seven days ahead of such events.