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Level 5 atmospheric river could trigger flooding in drought-stricken California

2021-10-21T06:41:20.965Z


After almost a year without rain, a series of powerful storms from the Pacific are heading to Northern California this week, which could bring up to 300 millimeters of rain and up to a meter of snow in the Sierra Nevada.


(CNN) -

After almost a year without rain, a series of powerful storms from the Pacific are heading to Northern California this week, which could bring up to 300 millimeters of rain and up to a meter of snow in the Sierra Nevada.

Supercharged by a classic atmospheric river pattern, the storms could trigger flash floods and dangerous debris flows across a wide swath of the region already ravaged by recent wildfires.

With each successive storm, the potential for moisture increases, possibly peaking with a rare Category 5 atmospheric river event on Sunday.

"An atmospheric river marked Category 4 or 5 is capable of producing remarkable rainfall totals over three or more days, probably exceeding 10-15% of a typical year's rainfall in some places," said Marty Ralph, director of the Center for Western Climate and Water Ends at UC San Diego.

Atmospheric rivers are a narrow band of moisture concentrated in the atmosphere, which travels more than 3 km above the ocean;

they can transport more than 20 times the water as vapor as the Mississippi River does in liquid form.

  • California is in a drought emergency in its driest year in nearly a century

By Monday morning, the parade of storms could drop 200 to 300 millimeters of rain in parts of northern California and add another 12 to 1 meter of snow to the high Sierra.

For a drought-stricken area, 300 millimeters of rain is too much, too fast, and too soon and will likely lead to runoff, flash flooding, and debris flow in fire-scarred areas.

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A race to prevent debris flows

Fire scars - the charred landscape - left over from the Dixie Fire near Mount Lassen and the Caldor Fire not far from South Lake Tahoe remain susceptible to flash flooding and debris flows. This deadly, fast-moving mass of water, rocks, dirt, and vegetation can wreak havoc on downstream communities, destroying homes and infrastructure. These geological hazards are a byproduct of burned soil, which can be as water repellent as pavement. Rain that would otherwise be soaked up by the ground can now run off quickly after a forest fire.

The Watershed Emergency Response Team of the Department of Forest Protection and Fire (Cal Fire) has been preparing before the heavy rains, assessing and identifying the areas most susceptible to post-fire hazards, such as debris flows, flooding and rockfalls.

"Erosion is very difficult to treat in the steepest and most severely burned parts of a burned area, and this generally results in the greatest risk to life, safety and property," says Lynnette Round, communications officer for Cal Fire.

"Areas of concern are where values ​​at risk (houses, roads, etc.) are below steep burned areas with moderate to high soil burn severity," Round says. "For the Dixie fire, this is primarily along the Hwy 70 corridor and parts of the Indian Valley and Genesee Valley. For the Caldor fire, this would be along parts of the Hwy 50 corridor and the areas down the Cosumnes River, "Round added.

You don't have to look far to see such a disaster in California.

In January 2018, just weeks after the Thomas Fire burned the hills of Santa Barbara County, people living below the slopes in burned areas were devastated by numerous debris flows after a powerful January storm hit the town. region.

Millions of dollars in damages were incurred and nearly two dozen lives lost.

PHOTOS |

Historic drought and wildfires in California

Not the end of the drought, but maybe the end of the wildfire season

On Monday, Sacramento recorded its first drop of rain since St. Patrick's Day. Registering just 0.02 millimeters of rain, ending a 212-day dry streak, the longest on record. Not surprisingly, more than 92% of the western US and 93% of California are experiencing some level of drought, with the latter largely in the extreme category, according to the US Drought Monitor. last week, a weekly report released by the National Oceanic and Atmospheric Administration (NOAA).

"The good news is that this rain will end pretty well fire weather concerns in northern and central California. However, with La Niña, we are still looking to not get enough rain to end the drought," he said Norm Hoffmann, who served as a meteorologist for the US Air Force and the National Weather Service in California for more than 30 years.

Last week, NOAA announced that La Niña conditions had developed in the Pacific Ocean and are expected to persist through the winter and early spring.

"La Niña tends to be associated with above normal precipitation in the Pacific Northwest and below normal precipitation in the Southwest, with the pivot point in Northern California. However, a couple of major atmospheric rivers hitting the southwest can keep that area from being as dry as it could be, "says Ralph.

Although the rain will greatly benefit large areas of the western US, Ralph points out that it may simply be a drop in the bucket in the broader drought situation.

"The storms represent a strong start to the rainy season, but they do not guarantee recovery from the drought. That would require a series of additional atmospheric river events during the winter," he added.

Ralph is one of the leading experts on atmospheric river events.

He points out that a category 5 north of San Francisco occurs on average once every four years.

These systems can impact the state up to 10 times a year, sometimes accounting for nearly 50% of California's precipitation.

  • A "Pineapple Express" is heading toward the Northwest of the United States, what is it?

Since January 1, the cities of Sacramento and San Francisco have only recorded about 130 millimeters of rain;

the normal to date is closer to 330 millimeters for both cities.

Further north, the city of Redding, California, will also receive the humidity after registering just 270 millimeters of rain since January 1, nearly 300 millimeters below the weather norm by the end of October.

Although rainfall amounts look promising to help alleviate much of the fire weather concerns in Northern California, models have been much less optimistic about their potential impacts for Southern California, where fire weather a it can often last until November and December.

California

Source: cnnespanol

All news articles on 2021-10-21

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