Miracle or mirage? Atmospheric rivers end California drought year with heavy snow and rain

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After the driest start of any year, California will end 2022 with snow-capped mountains, wet roads and, in some places, flood warnings.

The wet end of an otherwise bone-dry year came as a surprise. Just weeks ago, officials sounded the alarm about the third rare occurrence of La Niña, a climate pattern in the tropical Pacific often associated with dry conditions in the state. On Thursday, skiers at Mammoth enjoyed the deepest snow in the country, while a steady drizzle in Los Angeles signaled stronger storms to come.

Officials said the atmospheric river parade soaking up the state will likely continue in the coming days, providing a glimmer of optimism after a year of water restrictions, drying up wells and dangerous lows on the Colorado River. But while California’s wet season has so far lived up to expectations, this model really needs to undo the significant rain deficits that have persisted for several years.

“The humidity we’re getting right now is a big help, but we need more, much more, to really drastically reduce drought,” said Richard Heim, a meteorologist at the National Centers for Environmental Information. Authors of the US Drought Monitor.

Still, humid December came as a welcome change. While the drought monitor shows that about 81% of the state is under severe, extreme or exceptional drought, this is a notable improvement from three months ago when about 94% of the state was classified in the three worst categories. Heim said next Thursday’s update should show even more gains.

“As we deal with drought in the West, we have to be slow to recover in some ways because reservoirs take forever to refill, and you really need a good mountain snowpack,” he said. “And we don’t know until April 1 if we have a good mountain snowpack for the snow season.”

The storms could signal the decay of La Niña, which came as expected but began to weaken around the winter solstice on December 21, when Earth stopped moving away from the sun in the Northern Hemisphere, said state climatologist Mike Anderson of the Department of Water Resources. . Around the same time, regional high-pressure systems were weakening, which allowed some storms to pass, he said.

“We’re seeing things that are more in line with what we would expect climatologically, and a lot of that has to do with the high pressure in power,” Anderson said. “In previous winters, it hung strongly there, preventing storms from entering California.”

Storms in late December also brought some improvements when it comes to the state’s snow cover and reservoirs. California’s equivalent of snow water, or the amount of water contained in the Sierra Nevada snowpack, was 156% of normal for Thursday.

The state’s two largest reservoirs also saw gains, with storage at Lake Shasta from 1.4 million acres to 1.47 million acres in early December and 1.12 million acres in Lake Oroville storage at 965,000 in early December. said Anderson.

But he warned that more moisture is needed. Although high for the date, snow water equivalent is only 51% of the April 1 average, meaning that if there is no more rain and snow, the rainy season will end with about half of what is needed. Similarly, although Shasta and Oroville are improving, both remain well below normal for the time of year.

“It just needs to sustain itself, because we still have two more wettest months of the year, and where they were record-dry this year, we really need them to be wet, too,” Anderson said.

But while the storms brought some pleasant humidity, they also created instances of havoc across the state. The National Weather Service said winter hazards such as snow, ice and fog are causing roads to be closed in parts of central and northern California, and travel may be “almost impossible” in some places over the weekend.

Hannah Chandler-Cooley, a meteorologist with the meteorological service in Sacramento, said atmospheric rivers come from the tropics, not the Arctic, so they are warm systems that can bring rain instead of snow up to 7,000 feet high. Flood monitoring and warnings have been issued in several areas, including Lake Tahoe, Hanford, and Sacramento, where several inches of rain are expected.

He said officials in the region are particularly concerned about flooding in communities along the Cosumnes, Mokelumne and Sacramento rivers, as well as possible urban flooding in low-lying areas and roads with poor drainage.

“There will be small towns, homes, roads, and farms that could be affected, but it will be a little more local to just these few river points, not all of Northern California’s river systems,” he said.

Despite the potential dangers, storms are undoubtedly beneficial to the parched state. The latest view from the National Oceanic and Atmospheric Administration’s Center for Climate Prediction shows the probability of an equal above or below average precipitation in Northern California in January, but that’s not a guarantee.

Heim recalled that 2021 saw a similarly wet December, followed in 2022 by California’s driest period from January to March. He feared that a similar model would appear next year.

“A few months of really wet weather won’t have much of an impact on these deficits, which have accumulated over the years and are reflected in lower reservoirs,” Heim said. He added that Lake Mead, the largest reservoir on the Colorado River, has a more than 20-year rainfall deficit that must make up for it.

But such harsh conditions seemed far from the scene on Thursday at Mount Mammoth as officials prepared for 5 feet of snow on top of the 2 to 3 feet of snow received earlier this week.

“This has been an incredible start to the season at Mammoth,” facility spokesperson Lauren Burke told The Times. “This is a real winter wonderland.”

2022 Los Angeles Times.

It is distributed by Tribune Content Agency, LLC.

Quotation: Miracle or mirage? Atmospheric rivers end California drought year with heavy snow and rain (Dec 2022, 30 December), retrieved December 30, 2022 from https://phys.org/news/2022-12-miracle-mirage-atmospheric-rivers-california.html .

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