The so-called atmospheric river brings huge amounts of tropical moisture to California, prompting forecasters to warn of a high risk of flooding.
Atmospheric rivers are channels that form in the Earth’s atmosphere and channel water vapor from typically humid tropical regions to drier regions further from the equator. These canals can transport massive amounts of water in a short period of time, causing floods when they reach land.
The type of atmospheric river threatening California on Wednesday and Thursday (January 4 and 5) is a phenomenon that appears regularly. It is sometimes referred to as the Pineapple Express, a reference to the fruit commonly grown in Hawaii where the river originates.
The current atmospheric river has merged with an area of low air pressure that swirls over the Pacific Ocean, together forming a powerful storm that threatens the normally sunny nation.
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Satellites from the US National Oceanic and Atmospheric Administration (NOAA) monitor the weather system, revealing various aspects of it, including wind speed and expected rainfall.
The NOAA Weather Prediction Center (WPC) issued a moderate risk of heavy rain over parts of the California coast on Wednesday and Thursday morning.
“Associated heavy rainfall will lead to numerous areas of flash flooding,” the WPC said in a statement. (will open in a new tab). “In addition, many streams can flood, which can affect larger rivers.”
In high mountain regions such as the Sierra Nevada mountains, the rain will turn into heavy snow, according to the WPC.
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This is not the first atmospheric river to hit California recently. On the last day of December, a similar canal brought down a record 5.46 inches (14 centimeters) of rain on downtown San Francisco.
The current storm system threatens to pour more than 1 inch (2.5 cm) per hour onto already wet ground, raising concerns that landslides could hit the region’s hilly landscapes, according to the WPC.
According to The Washington Post (will open in a new tab), San Francisco received over 11.6 inches (29 cm) of rain in December. That’s more than double the December citywide average of 4.76 inches (12 cm).
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