A so-called atmospheric river is bringing huge amounts of tropical moisture to California, prompting weather forecasters to issue alerts for high risks of flooding.Â
Atmospheric rivers are channels that form in Earth’s atmosphere and funnel water vapor from the typically humid tropical regions to drier areas farther away from the equator. These channels can transport huge amounts of water within a short period of time, causing deluges when they make landfall.Â
The type of atmospheric river threatening California on Wednesday and Thursday (Jan. 4 and Jan. 5) is a phenomenon that appears regularly. It’s sometimes called the Pineapple Express, a reference to the fruit commonly grown in Hawai’i where this river originates.Â
The current atmospheric river has combined with a region of low air pressure that swirls above the Pacific Ocean, together forming a powerful storm that threatens the usually sunny state.Â
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Satellites of the U.S. National Oceanic and Atmospheric Administration (NOAA) are monitoring the weather system, revealing its various aspects including wind speeds and expected amount of rain.Â
The NOAA Weather Prediction Center (WPC) has issued a moderate risk alert for excessive rain over parts of the Californian coast for Wednesday and Thursday morning.
“The associated heavy rain will create numerous areas of flash flooding,” the WPC said in a statement (opens in new tab). “Furthermore, many streams may flood, potentially affecting larger rivers.”
In higher altitude regions, such as in the Sierra Nevada Mountains, the rain will turn into heavy snow, WPC said.Â
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This is not the first atmospheric river to hit California  recently. On the last day of December, a similar channel dumped a record-breaking 5.46 inches (14 centimeters) of rain on downtown San Francisco.Â
The current storm system threatens to pour over 1 inch (2.5 cm) per hour on the already soaked ground, causing concerns that mudslides may occur in hilly landscapes in the region, WPC said.Â
According to the Washington Post (opens in new tab), over 11.6 inches (29 cm) of rain fell in San Francisco in December. That’s more than twice the December average for the city, which is 4.76 inches (12 cm).
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