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More extreme weather on tap for California as series of atmospheric river events continues




CNN
 — 

California has been battered by heavy snow, damaging winds and flooding this week – and now another round of storms is set to hit the West Coast this weekend.

“Relentless parade of cyclones from the Pacific will bring more flooding rains and mountain snows to the West Coast with main focus across northern California,” the Weather Prediction Center said Saturday.

Multiple storms will reach the West Coast over the next few days. The concern is not just the rain, snow and wind, but there will be not much of a break in between events for the water to recede or cleanup to be completed.

“We do expect an even stronger storm to impact the state Sunday night through Tuesday than the one we will see early on this weekend,” said Matt Solum, Meteorologist with the National Weather Service’s Western Region Headquarters. “We encourage everyone to take the time over the weekend to make any needed preparations for the next storm coming in.”

The next storms come on the heels of a powerful cyclone which flooded roads, toppled trees and knocked out power to most across California. Earlier, a New Year’s weekend storm system also produced flooding.

This weekend the main concerns for the coastal communities will be widespread flooding, gusty winds, and dangerous beach and marine conditions. In the higher elevations it will be heavy snow and strong winds leading to near whiteout conditions for anyone traveling on the roads.

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Winds are forecast to be around 40-50 mph in the valleys and up to 70 mph in the mountains, which is lower than the storm earlier this week, but still nothing to brush off.

“While these winds won’t be on the order of the previous/stronger system it really won’t take much to bring trees down given saturated conditions and weakened trees from the last event,” the weather service in San Francisco posted Friday.

Even a 40 mph wind can do damage when the ground is so saturated from record rainfall earlier this week and the cumulative effect of the new rainfall expected this weekend.

“Impacts to infrastructure include but are not limited to; river flooding, mudslides, power outages & snow load,” the prediction center said in a tweet.

The most widespread concern over the next week will certainly be flooding thanks to several atmospheric river events. Atmospheric rivers are a narrow band of concentrated moisture in the atmosphere.

The Center for Western Weather and Water Extremes, which monitors atmospheric river events, is now forecasting a level 5 atmospheric river event – the highest level possible – in the next several days. While the focal point of this event will be near Monterey and Big Sur, California, intense moisture will also spread into the surrounding areas of San Francisco and San Jose where a level 4 atmospheric river event is forecast.

Earlier this week, San Francisco experienced its wettest 10-day period on record for downtown since 1871. So far they have had more than a foot of rain just since December 1, and the forecast calls for an additional 4-6 inches of rain in the next five days.

Sacramento is also expected to see significant rainfall totals of 4-7 inches in the valleys and 6-12 inches in the foothills.

“Additional rain on already saturated soils will contribute to additional flooding concerns across much of the state,” Solum told CNN. “There will continue to be an increased risk of rock slides and mud slides across much of the state as well.”

More than 15 million people are under flood watches across the state of California this weekend. There is also a slight-to-moderate risk of excessive rainfall across much of northern and central California Saturday and Sunday. It increases to a more widespread moderate risk by Monday.

The rainfall over the weekend will bring renewed concerns for local streams, creeks, and rivers. The Colgan Creek, Berryessa Creek, Mark West Creek, Green Valley Creek, and the Cosumnes River all have gauges either currently above flood stage or expected to be in the next few days.

“Tuesday is probably the day where you’ll likely need to keep a really close eye on the weather as the potential for widespread flooding of rivers, creeks, streams and roadway and urban flooding will be at its highest during the next week as all the runoff and heavy precipitation comes together resulting in a mess,” the weather service office in Sacramento said.

In addition to heavy rain, there will be significant amounts of snow across the higher elevations.

“Snow totals are looking to be 1-2 feet with some of the higher elevations seeing 3 feet or more leading to significant travel impacts,” the weather service office in Sacramento said.

We are currently under a La Niña advisory for the winter months before transitioning back to a more neutral pattern by the spring.

El Niño and La Niña forecast patterns put out by the Climate Prediction Center give guidelines on what the overall forecast can be during a seasonal time period.

“During a La Niña, typically the Pacific Northwest sees wetter than normal conditions and Southern California sees drier than normal conditions,” Marybeth Arcodia, a postdoctoral researcher at Colorado State University said. “This is due to the jet stream being pushed farther north and having a wavier pattern. “

The problem is, Mother Nature hasn’t exactly been following the anticipated norms for a La Niña winter so far this year.

“However, in the past three months, Oregon has been slightly drier than normal and California has been slightly wetter than normal (the opposite of what is expected),” Arcodia told CNN. While El Niño and La Niña patterns typically have a large influence on seasonal conditions in the West Coast, “there are always additional factors at play,” she added.

One such factor has been multiple atmospheric river events pummeling California with intense amounts of moisture.

“Atmospheric rivers typically form during the winter months and can occur during El Niños or La Niñas,” Arcodia said, noting their strength, frequency, and landfall location can be influenced by the larger patterns in the Pacific.

Michael Tippett, a professor of physics and mathematics at Columbia University, points out that the forecast patterns are not meant to be used on a day-to-day forecast scale but rather the entire season as a whole. This is why researching the patterns is so important.

“There is an element of randomness that is not explained by the patterns,” Tippett told CNN. “This might help us understand why one year is different than the other.”




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