Atmospheric river could bring 'dangerous' conditions to Southern California

Starting Saturday evening, Southern California could see days of rain, possibly in record amounts, creating conditions for what the National Weather Service described as a “very dangerous situation'Sunday to Monday.

The storm system is also expected to bring several feet of snow to the Sierra Nevada, and strong onshore winds and intense, damaging surf along the California coast.

Here's what you need to know.

• The effects of this system will be felt in California on Saturday evening and will last through Tuesday.

• The heaviest rainfall is likely south of the Bay Area, with excessive rainfall most likely from Santa Barbara to Los Angeles.

• Dangerous flooding may occur in some places. The Weather Service warned that the Los Angeles River “will quickly fill up” starting Sunday become a raging river and a very dangerous place to be.

• There is an extremely high chance – more than 90 percent – ​​of at least 2 feet of snow, especially above 6,000 feet in the Sierra Nevada, causing difficult to impossible driving conditions on Sunday and Monday.

This atmospheric river will be stronger than the last two.

This storm will join an atmospheric river, a stream of moisture in the air that is typically a few hundred kilometers wide and can be seen on satellite images.

It will bring an abundance of moisture from the tropics near Hawaii, and as it makes landfall the mountains will wring the moisture from the air like a soaking wet towel, causing record amounts of precipitation.

The angle and orientation of the mountains can have a major effect on the amount of precipitation that can be created from an atmospheric river, and this latest system will have a southwesterly to southerly orientation, affecting the Transverse Ranges from Santa Barbara to Los Angeles. perfect angle to create a worst possible outcome.

Making matters worse are higher ocean temperatures just off the coast.

“Any time these amounts of rainfall cross the terrain and the built environment, you're going to have problems,” said Greg Carbin, division chief of forecast operations for the National Weather Service.

Some of the heaviest rainfall could reach daily records of one or two days, Mr. Carbin said.

Up to a meter of rain can fall.

This storm could bring “unprecedented” amounts of rain to a widespread area — up to a foot in less than two days in some locations, the weather service said.

Forecasters say it has the potential to match or surpass the atmospheric river event that led to people being saved from flash flooding in Los Angeles last winter.

This storm is expected to be even stronger than the atmospheric river that hit the same region earlier this week, and significantly stronger than the very weak storm that sent a flood through San Diego last week. That storm was strengthened by winds whipping over higher-than-average ocean temperatures.

In its stark warning about the Los Angeles River, the Weather Service said flooding was likely in urban areas and other places with poor drainage.

“Storm drains and ditches can become clogged with debris,” the report said. “Major flooding of streets and flooding of creeks and rivers is possible.”

Warmer than usual temperatures in North Pacific waters will continue to be a factor in this atmospheric river.

The water isn't necessarily warm in an absolute sense (you might still want to wear a wet suit), but it is warmer than average in January.

The warmer ocean means more evaporation can occur in the lower atmosphere, which could translate into more rainfall on land, said Daniel Swain, a climate scientist at the University of California, Los Angeles. on social media.

Martin Ralph, the director of the Center for Western Weather and Water Extremes, said that “this atmospheric river will hit the area with winds almost straight into the mountains from Santa Barbara to LA, and will cross exceptionally warm coastal waters, bringing moisture and heat that could increase rainfall on land.”

The atmospheric river alone would be problematic, but when you add the higher ocean temperatures, the right angle to the mountains and the timing, following in the footsteps of another storm system, it has meteorologists worried and stronger than normal use language such as “a very dangerous situation.”

Mr Carbin said he believed that before the weekend was over there would be reports of mudslides, landslides and road closures.

As confident as forecasters are that this could be a major storm, there is still some hesitation in determining exactly where the heaviest rain will fall.

“It is still possible that there will be some shifts as forecasts become clearer,” said Alex Lamers, a warning coordination meteorologist with the Weather Service.

There is one situation where the main plume of heavy rain could linger over Santa Barbara and Ventura counties, increasing the risk of severe flooding in those areas and limiting the amount of rain that falls in Los Angeles County.

Precipitation totals will likely average three to six inches in most coastal and valley areas, and six to twelve inches in the foothills and mountains.

“Even if urban areas don't necessarily see higher numbers,” Mr. Lamers said, “intense bursts of rain could cause significant flooding given the amount of hardened ground.”

Raymond Zhong reporting contributed.