Last year there were nine back-to-back atmospheric rivers from December 2022 to January 2023. On Wednesday an atmospheric river will bring back memories as extreme weather conditions hit the West Coast. 

According to the NOAA, an atmospheric river is a long pattern in the atmosphere, similar to a river. It moves large amounts of water vapor from the tropics. That water vapor falls as either rain or snow when it reaches land. On average, atmospheric rivers can carry an amount of water equivalent to the flow of water in the mouth of the Mississippi River.

It is not just any atmospheric river, it is a “Pineapple Express.” The NOAA explains that a “Pineapple Express” happens as, “moisture builds up in the tropical Pacific around Hawaii and can wallop the U.S. and Canada’s West Coasts with heavy rainfall and snow. Prevailing winds cross over warm bands of tropical water vapor to form this ‘river,’ which travels across the Pacific as part of the global conveyor belt.”

The New York Times reported that the “substantial” rain could lead to flash flooding. The Weather Prediction Center has defined Los Angeles as a moderate risk, while multiple regions are put on flood alerts. On Wednesday, the storm should bring heavy snowfall in the Sierra Nevada’s as well as heavy downpour from Washington to Northern California. The storm would arch south to bring rain across Southern California.

The Weather Prediction Center expects a second atmospheric river on Sunday or Monday. At worst, the storm system could bring prolonged rain and snow to California.