Special Report: Western Drought Watch: California’s ‘atmospheric rivers’ and ‘bomb cyclone’ bring scant long-term rain relief — a look at the Western drought by the numbers
A “bomb cyclone” was behind the drenching rains and strong winds this weekend in drought-stricken California, and more storms were headed for southern parts of the state on Monday.
Rainfall records were shattered and heavy snow blanketed high elevations of the Sierra Nevada, the Associated Press reported. The National Weather Service issued numerous flash flood warnings.
A bomb cyclone forms when air pressure rapidly drops as the storm strengthens, said AccuWeather. The phenomenon was pulling tropical moisture from the Pacific Ocean in a long plume, creating an “atmospheric river.”
Still, the sudden rain won’t bring much long-term relief to the drought-ravaged state, which is among the hardest hit as the U.S. West continues to strain under dry conditions, given the deficit California faces.
Sudden precipitation brings its own worries — the inability for hard ground to soak up rainfall, causing flash floods and mud slides. That’s especially true for vast areas where vegetation has been scorched by wildfires.
For the still-smoldering fires, however, rain and in the mountains, snow, bring a welcome assist to firefighters trying to keep blazes under control after another high-toll season for wildfires.
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Here are some of the latest updates on drought conditions in California, the world’s fifth-largest economy and the largest U.S. state, and across the broader Western U.S.
As of Oct. 19, 38.9% of the U.S. and 46.5% of the lower 48 states are in drought, according to the National Integrated Drought Information System, which is part of the National Oceanic and Atmospheric Administration (NOAA).
Current drought conditions, extreme conditions in red, from the U.S. Drought Monitor and NOAA.
That means 78.3 million people in the lower 48 states are affected by drought this week, a gain of more than 8% since last month.
Some 208 million acres of crops in U.S. are experiencing drought conditions this week, a gain of 4.7% since last month.
The drought map is changing for some states: rain continues to improve dry conditions in the Northern Plains and Minnesota, according to the U.S. Drought Monitor.
What’s an atmospheric river?
In California, but true for the wider West as well, attention remains on a series of atmospheric rivers hitting the region, which could bring beneficial precipitation for the drought deficit. An atmospheric river is a narrow corridor of concentrated moisture in the atmosphere.
These conditions have other names: tropical plume, tropical connection, moisture plume, water vapor surge and cloud band. Their intensity can be dangerous: invisible rivers in the Earth’s atmosphere move up to 25 times more water in a day than the Mississippi River.
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Drought emergency holds
Even with rain falling this week, California Gov. Gavin Newsom reupped the statewide drought emergency and gave regulators permission to enact mandatory statewide water restrictions if they choose. The move was a reminder that drought conditions are the product of climate change, and less impacted by short-term weather changes.
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For decades, California has relied on rain and snow in the winter to fill the state’s major rivers and streams in the spring, which then feed a massive system of lakes that store water for drinking, farming and energy production.
Shifting drought conditions in early next year show some relief for pockets of the West but issues for Texas and Oklahoma, according to the U.S. Drought Monitor and NOAA.
The annual runoff from the mountains is getting smaller, mostly because it’s getting hotter and drier, not just because it’s raining less. For instance, California’s snowpack in the Sierra Nevada mountains earlier this year was 60% of its historical average. But the amount of water that made it to the reservoirs was similar to 2015, when the snowpack was just 5% of its historical average. This past year, California had its warmest ever statewide monthly average temperatures in June, July and October 2020. Water is evaporating into the hotter air or was absorbed into the drier soil, which is true across the arid Western U.S.
California’s 2021 “water year,” which just ended, was the second driest on record. A year earlier was the fifth driest on record. The cumulative effect leaves reservoirs at record low levels. State officials are already predicting that the Lake Mendocino reservoir could be dry by next summer.
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The Associated Press contributed to this article.