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California At Risk Of Life-threatening Flooding From New Atmospheric River

Late this week, a powerful storm with lots of rain will hit California, leaving behind warmer air.

A high-impact storm packed with moisture will drag warmer air behind it as it slams California late this week, and AccuWeather meteorologists warn that damaging and life-threatening flooding and mudslides could occur in the Golden State.

The heavy rain will threaten to wash away the snowpack on intermediate slopes rapidly and spread more snow over the highest elevations, in addition to helping wipe away persistent drought conditions. This storm system will follow a couple of recent colder storms that unloaded several more feet of snow over the Sierra Nevada from last Saturday through early Wednesday.

While a warmer storm compared to the recent onslaught from Old Man Winter may seem comforting to some people, the problems the storm will cause may far outweigh the benefits.

As the storm rolls in from the Pacific and moves inland, it will tap into much warmer air from the south and pull a plume of tropical moisture across a large part of California. This plume of moisture is called an atmospheric river and will likely lead to torrential rainfall in some areas and very heavy snow over the highest elevations of the Sierra Nevada and Siskyous.

On Friday, the atmospheric river will extend for more than 2,400 miles from California to the southwest of Hawaii. This type of potent storm is often referred to as Pineapple Express by meteorologists.

The atmospheric river will travel almost 2,400 miles from California to the southwest of Hawaii on Friday. Meteorologists frequently refer to this kind of strong storm as Pineapple Express. ACCUWEATHER

During storms that pack atmospheric rivers, there is potential for a foot or more of rain to be released.

“Only because the firehose effect of rain will keep moving, rather than hold nearly stationary, will rain be generally limited to several inches,” AccuWeather Chief On-Air Meteorologist Bernie Rayno said, adding that there could be some higher amounts locally.

Because above-freezing temperatures are forecast in higher elevations, the several inches to a couple of feet of snow already on the ground in intermediate elevations from 2,500 to 5,000 feet may not fully absorb the rain and could completely melt instead, Rayno explained.

The equivalent of 8-12 inches of water could be released in a matter of hours when accounting for the moisture already locked up in the snow and the incoming several inches of rain. As this rapid runoff occurs, the smaller rivers and streams from the mountains and hills have the potential to lead to rapid flooding, which can create dangerous and damaging conditions for people, homes, businesses and other structures in the path of the raging waters.

Areas close to Santa Barbara to Redding are expected to have urban flooding incidents. ACCUWEATHER

A general 1-4 inches of rain will cause problems on area streets, highways and rural roads. Incidents of urban flooding are likely in areas from near Santa Barbara to Redding. Motorists should be prepared for road hazards ranging from high water to falling rocks and mudslides. In some severe cases, stretches of roads could be washed away.

“The torrential rain and piles of snow laying at the edges of area streets and rural roads is likely to lead to flooding,” AccuWeather Senior Meteorologist Brett Anderson said. “Where possible, road crews and property owners may want to open up the storm drains to minimize the flooding risk.”

Enough rain and melting snow will occur from the storm to create significant rises on the major rivers, including the Sacramento and the San Joaquin. ACCUWEATHER

Rain will fall much of the time at elevations from 5,000 to 7,500 feet. “While the rain is not likely to wash away the snow at this elevation, it will add considerable weight to the existing snow cover,” AccuWeather Lead Storm Warning Meteorologist Billy Clark said. “Buildings with flat roofs will be at risk for collapse due to the added weight.” The risk of street flooding and roof collapses includes communities such as Tahoe City, California.

While mostly rain will fall from the storm over the high ground along Interstate 5 in Northern California and even over Donner Pass along I-80 near Truckee, California, snow will pile up over the high country. Starting at elevations near 7,500 feet and above, the potential exists for 2-4 feet to fall on top of a snowpack that is over 100 inches in spots.

Most of the storm’s rain will avoid areas from the Los Angeles basin to San Diego, but enough shower activity can occur to make roads slick at times from Friday to Saturday. Because of the warmth involved from the storm, no snow will fall over the mountain passes in Southern California, including over 1-5, I-8, I-10 and I-15.

The long-term drought, which has been ongoing for years, has taken a major hit this winter. The severity has already been rolled back by multiple categories over much of the state, and some locations are no longer dealing with any type of drought.

Although this latest installment in the series of moisture-laden storms this wet season across California will once again bring a variety of life-threatening impacts, the storms will also provide additional drought relief, building on the relief from previous storms, according to AccuWeather Chief Meteorologist Jonathan Porter.

“AccuWeather meteorologists expect that the majority or even all of the short-term drought across California can be wiped out by mid-April,” Porter said.

Most of the residual dryness in Central California will likely be eliminated by the storm’s rain from Thursday night to Friday night. ACCUWEATHER

“The rain from the storm from Thursday night to Friday night will likely wipe out most of the remaining drought across Central California,” Rayno said. As of the last week of February, conditions ranged from adequately moist to severe drought over the Golden State.

San Francisco may end up with close to 30 inches of rain since the start of the rainy season, which began on Oct. 1, Rayno said. As of Wednesday, 24.61 inches of rain has fallen at San Francisco International Airport, where official records are kept, a total that is above the average of 15.57 inches for this time period. Even though the prospect of receiving more than 5 inches of rain from the storm late Thursday to Friday may be a tall order, it is conceivable that the Bay Area could pick up this much rain through next week.

More storms are tracking over the Pacific and will likely make their way toward California next week.

“There is the potential for another atmospheric river to be tied in with a storm next week,” AccuWeather Meteorologist Joseph Bauer said. “It appears that the storm next week will tend to spread the heavy rain throughout much of the state, and some of the downpours will extend all the way into Southern California.”

The intensity, timing and duration of the storm and the next atmospheric river are still being gauged by AccuWeather meteorologists. However, the general timing for the heaviest rainfall is likely to be on Monday in the north, Monday night and Tuesday in the central counties and Tuesday night in southern parts of the state.

The storm next week and any ensuing storms will continue to erase the remaining drought and lingering long-term rainfall deficit across the state.

Produced in association with AccuWeather

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