Weather whiplash: A series of storms could ease California drought, but also unleash flood hazards

Showers drifted across the drought-stricken and fire-scarred landscape of Northern California on October 20, 2021, followed by a series of stronger storms that are expected to bring significant rain and snow into next week, forecasters said.

(CNN)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.

        Much needed rain, but too much of a good thing?

          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."
          But this storm, according to Ralph, is a rare event. AR5 storms are rare in California, especially during October. Analyzing a 40-year period, his team found only a total of 10 AR5 storms occurred in 40 years, and only one happened in October.
          The impacts also vary depending on the region. Smaller watersheds in urban areas, for instance, may trigger flash floods, while regions where wildfires have left burn scars may experience some debris flows. Transportation, Ralph adds, could also be impacted with the wet roads, cautioning drivers who may travel during the storm event.
          Ultimately, experts like Ralph say the atmospheric river is needed more than ever to replenish the unrelenting drought in the West.
          In California, dry conditions this summer were the most extreme in the entire 120-year record. It drained reservoirs and triggered historic water shortages. Governor Gavin Newsom recently expanded a statewide drought emergency proclamation.
          Climate researchers say two major factors contributed to this summer's severe drought: a lack of precipitation and an increase in evaporative demand, also known as the "thirst of the atmosphere." In this case, the coast did not experience enough storms to quench the thirst of the atmosphere, as well as the drying landscape.
          Low water levels at Lake Oroville in California on July 22, 2021. As  extreme drought persists in California, Lake Oroville's water levels are continuing to drop.
          The historic drought in the West also created the perfect landscape for wildfires to spark and expand.
          In the short term, Julie Kalansky, a climate scientist at Scripps Institution of Oceanography in San Diego, said the storm will help alleviate the dry landscape as well as reduce the potential for fires to spread and ignite.
          "In the long term, it's really hard to tell," Kalansky told CNN. "Just because we have a wet October or have a big event in October, it doesn't necessarily mean the rest of the season is going to be wet, so the rest of the season is still really to be determined."
          California's Mediterranean climate, where dry summers and wet winters provide the perfect conditions for a robust agricultural economy, makes it vulnerable to drastic shifts in weather events. But after a long dry period, Ralph said many farming communities would benefit from a high-precipitation event.
          "It seems to me this is a storm that's going to help return some moisture to the soil in a significant way that should green up the hills and the fields a bit more than otherwise," Ralph said. "And then, a big picture is that this is a storm that's going to set the stage for the watersheds to produce more runoff when the next storm comes, which is going to start filling the depleted reservoirs that they depend upon."

          Climate change is making storms wetter, stronger

          Scientists have linked the climate crisis to an increase in the amount of moisture the atmosphere holds, meaning storms, such as the one impacting the West Coast now, will be able to bring more atmospheric moisture inland than it would without climate change, which in turn leads to an increase in rainfall rates and flash flooding. Meanwhile, the intensity of storms is also increasing.
          This weekend's bomb cyclone, named for the speed at which it intensifies or "bombs out," is expected to near or set records for the lowest pressure for a storm in the Northeast Pacific Ocean. The strong system is forecast to produce winds of hurricane strength (75 mph or greater), though the strongest winds should remain over the ocean, according to the latest forecast.
          And since the storm is coming earlier in the season than usual, Kalansky said preparedness is crucial for an event like this.
          "It is a very extreme event for this early in the season, and people might not be prepared for something like this because it doesn't typically happen," she said. "I always advise people that it is really important that they stay up to date with what the National Weather Service is predicting in terms of hazards."
          These rapid changes in climate extremes — from drought to high precipitation — could make it harder for societies and communities to mitigate and adapt. In some cases, it could be destructive: according to a 2019 study, atmospheric rivers created an annual average of $1.1 billion annually in flood damage across the West.
            But one thing is clear: as the planet warms, extreme weather events such as this will only get worse.
            "The research has shown that atmospheric rivers are projected to become more extreme in the future," Kalansky said. "For California, they're projected to contribute more to the overall annual precipitation. Our climate models are suggesting that atmospheric rivers will become increasingly important as the climate changes."