Imagine a long river of water vapor in the sky coming into the West Coast. It is how Marty Ralph, the director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography in San Diego, described the storm event threatening California at the moment.
The storms are called “atmospheric rivers,” which are narrow bands of concentrated moisture in the atmosphere emerging from the warm waters of the Pacific Ocean, cruising more than two miles above the sea. An average atmospheric river transports more than 20 times the water the Mississippi River does, as vapor. Throughout the weekend and into next week, parts of the West Coast will go from extreme drought to facing a series of bomb cyclones and an associated atmospheric river. The weather whiplash may unleash rains, flash floods, debris flows, and potential hurricane-force winds, particularly in the Pacific Northwest and Northern California.
“Wherever the storms hit shore on the West Coast is where the heaviest precipitation occurs, and that can be very beneficial in areas that often don’t have enough water — and we have the drought going right now,” Ralph told CNN. “And then there are times when there’s too much and it can create flooding,” he added. “A few of these storms really make the difference over the course of the year.” Human-caused climate change has increased the potential for this weather whiplash, where dramatic shifts in periods of drought and high precipitation can to occur more often. Scientists say the chances of sudden transitions from severe drought to atmospheric river events will become more common in California in the coming decades.
In 2019, Ralph led the development of the system to categorize atmospheric rivers by strength, much like hurricane categories. In the scale, AR4 translates to ‘extreme,’ while AR5 — which is what he projects this storm to be — means ‘exceptional.’ “AR’s 4 and 5 are mostly hazardous, but they can also be very beneficial, as we’re seeing in this case, where it’s coming on the heels of a serious drought,” he said. “And largely the impacts are probably going to be beneficial because it’s moisturizing the soil, restoring some water in the rivers, and a little bit of the lakes.”