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

Supercharged by a classic atmospheric river model, the storms could cause flash floods and dangerous debris flows across a wide swath of the area already devastated by recent wildfires.

With each successive storm, the potential for humidity increases, culminating with perhaps a rare Category 5 atmospheric riverine event on Sunday.

“An atmospheric river marked as a Category 4 or 5 is capable of producing remarkable precipitation totals over three or more days, likely to exceed 10 to 15% of a typical year’s precipitation in some places,” said Marty Ralph, director of the Center. for Western Weather and Water Extremes at UC San Diego.

Atmospheric rivers are a narrow band of moisture concentrated in the atmosphere, navigating more than three kilometers above the ocean; they can carry in vapor form, more than 20 times the water that the Mississippi River does, in liquid form.

By the time Monday morning arrives, the storm parade could drop as much as 8 to 12 inches of precipitation in parts of northern California and add an additional 1 to 3 feet of snow in the High Sierra. For a drought-stricken region, a foot of rain is too much, too fast, and too early and will likely result in runoff, flash floods, and debris flows in the burn areas.

A race to prevent debris flows

The scars of burns – the charred landscape – left after the Dixie Fire near Mount Lassen and the Caldor Fire near South Lake Tahoe remain susceptible to flash floods and debris flows. This deadly and swift mass of water, rocks, soil and vegetation can wreak havoc in communities downstream, destroying homes and infrastructure. These geological hazards are a by-product of the burnt soil, which can be as water repellent as the pavement. Precipitation that would otherwise be absorbed by the soil can now run off quickly after a forest fire.

The Cal Fire-led Watershed Emergency Response Team prepared for the heavy rains, assessing and identifying areas most susceptible to post-fire hazards, such as debris flows, flooding and falling rocks.

“Erosion is very difficult to deal with on the steeper and more severely scorched parts of a burnt area, and these usually pose the greatest risk to life, safety and property,” says Lynnette Round , communications manager for Cal Fire.

“Areas of concern are those where values ​​at risk (houses, roads, etc.)” For the Dixie fire, this was primarily along the corridor of Highway 70 and portions of Indian Valley and the valley of Genesee. For the Caldor fire, it would be along portions of the Highway 50 corridor and lower areas of the Cosumnes River, “Round adds.

You don’t have to look far to see such a disaster unfold in California. In January 2018, just weeks after the Thomas Fire burned down the hills of Santa Barbara County, people living under the hills in the scorched areas were devastated by numerous debris flows after a powerful snowstorm. January hit the region. Millions of dollars in damage have been incurred and nearly two dozen lives have been lost.

It’s not the end of the drought, but maybe the season of the wildfires

On Monday, Sacramento recorded its first drop of rain since St. Patrick’s Day. Recording only 0.01 “of precipitation, ending a Dry run of 212 days, the longest ever recorded. Unsurprisingly, over 92% of the western United States and 93% of California experience some level of drought, the latter largely in the extreme category, according to last week’s US Drought Monitor, a weekly report released. by the National Oceanic and Atmospheric Administration. .

“The good news is that this rain will put an end enough to fire concerns in northern and central California. However, with La Niña, we still consider not having enough precipitation to end the drought, ”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 winter and into early spring.

“La Nina 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 few large atmospheric rivers that hit the southwest can prevent this region from being as dry as possible, ”says Ralph.

While the precipitation will greatly benefit large areas of the western United States, Ralph points out that it may only be a drop in the bucket in the larger drought situation.

“The storms represent a good start to the rainy season but do not guarantee a recovery from the drought. This would require a series of additional atmospheric river events throughout the winter,” he added.

Ralph is one of the foremost experts on atmospheric river events. He notes 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 rainfall.

As of January 1, the cities of Sacramento and San Francisco have recorded only about 5 inches of precipitation; the normal to date is closer to 13 inches for the two cities. Further north, the city of Redding, Calif., Will also welcome humidity after registering just 10.66 inches of rain since Jan. 1, nearly a foot below the weather norm for late October.

While the precipitation amounts have shown promise in helping alleviate much of the fire concerns in northern California, models have been much less optimistic about its potential impacts for southern California, where the fires can often persist into November and December.

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