When a fast-moving, destructive fire strikes in the middle of the night, how do you get people out of their homes to safety?

For Paul Lowenthal, assistant fire marshal in Santa Rosa, Calif., there was only one answer. When he saw smoke and flames dart across the land toward the heart of the city with astonishing speed Sunday night, there was no time for phone alerts or Facebook postings. He called the local police department to order an evacuation, and then he drove through the nearest neighborhood with his sirens blaring, yelling at people to get up and get out.

"The smoke and ash and embers were raining down, sparking spot fires," he recalled in an interview Wednesday. "It didn't take but moments for people to look out their front doors and see what was happening."

The deadly wildfires racing unpredictably across Northern California this week have exposed the difficulties that persist for authorities trying to communicate emergencies to vulnerable residents, even in the hyper-connected 21st century.

It is now standard for fire departments, county emergency management offices, and police and sheriff's departments to have multiple Twitter accounts, Facebook pages, programs that allow residents to sign up for real-time text alerts and systems that can robo-dial home phones.

But all that technology wasn't enough, as the fires caught authorities and residents off guard, claiming at least 21 lives and untold homes from Sonoma County to Mendocino County.

The fires incinerated or damaged more than 70 cell towers, including a major communications hub, leaving many people without the ability to communicate or receive warning mes­sages. Some people reported getting text evacuation warnings long after they had left their homes, while others who said they had signed up for the critical alerts didn't get them at all.

Many residents said they found the lack of information from local authorities frustrating, particularly the silence regarding which communities might be vulnerable to the flames, although the high winds made predicting the fire's path difficult.

Janet Balatti saw flames Sunday night in Santa Rosa probably before authorities even knew of the unfolding catastrophe. She watched the sky and packed belongings before fleeing with her husband from the now-devastated Fountaingrove section of the city, but they did that on their own.

"No official alert was put out as far as I know," she said.

Other evacuees were awakened by a neighbor or law enforcement officer pounding on the door. William DeLeon, a machinist who was home Monday morning with his wife, his daughter and her guinea pig, isn't sure whether he ever got an alert; a police officer woke his family up telling them to flee.

An announcement came over the public address system at Spring Lake Village in Santa Rosa, "but you couldn't really understand it," said Jean Kalsted, 79. Residents at the senior community had run emergency drills, she said, so they went into action, leaving for the Sonoma County Fairgrounds, which is an emergency shelter.

Imad Mouline, chief technology officer of Everbridge, a company that provides text and other emergency notifications for local jurisdictions around the country, said the company looked into reports in Northern California that some of the messages through their Nixle program were delayed during the fire.

They concluded it was because of a bottleneck in communications caused by the damaged towers and the large number of people trying to use cellular services, Mouline said.

One danger is that people sometimes receive emergency alerts but turn them off or simply tune them out, said J. Keith Gilless, dean of the College of Natural Resources at the University of California at Berkeley.

"My biggest worry is that people find warnings annoying and either don't want to regard them anymore or fiddle around with settings on their phones," he said. "You don't sign up for systems, or you try to mute them on their phone."

Some people can become overloaded with information and others might be communicating — or searching for communications — when they should be evacuating, said Meg Krawchuk, an assistant professor at the Oregon State University's College of Forestry. Other times, the information just can't get out fast enough, particularly with an unpredictable situation — leaving residents with little to go on.

"We don't always have information as quickly as we think we might, given what we think of as our sophisticated culture when it comes to tech," she said. "There are weak links in that culture that don't do what we think they should be able to do."

At a news conference Wednesday, Sonoma County Sheriff Rob Giordano said one challenge is that some notification systems rely on landlines. One such system, Reverse 9-1-1, allows public-safety organizations to use public databases to send robo-calls to residential phone lines.

But Giordano said it was critical for residents who primarily use their cellphones to sign up for a system called SoCoAlert, which can deliver emergency information via text, phone call and social media.

"The world has changed," he said. "People don't have landlines anymore."

However, getting people to opt in to such systems might not be so easy. As of June, about 10,500 people were signed up for the alerts, according to the SoCoAlert website. There are 500,000 people living in Sonoma County.

The Federal Emergency Management Agency also manages a cellphone alert system, which uses different technology than regular text messages that are meant to overcome jammed lines — they turn up automatically without requiring a person to opt in. Many people might recognize them in the form of flash-flood warnings delivered by the National Weather Service, alerts that are accompanied by piercingly loud beeps.

It's unclear how many Northern California agencies might have used the system to try to communicate with residents.

For Melissa and Ken Moholt-Siebert, the first warning that something was amiss came in the form of the acrid smell of smoke about 9 p.m. Sunday. At about midnight, a neighbor came over to point out the fire in the distance. A half-hour later, the two were fleeing.

"It was just explosive," Ken Moholt-Siebert said.

Lowenthal, the Santa Rosa assistant fire marshal, had been headed out for what he thought was a quick work assignment Sunday night. He was to serve as a spokesman for a couple of hours for emergency officials in neighboring Napa County, which was grappling with its own fire.

But he stopped in his tracks when he saw the line of flames across his path.

"I was just watching it barrel toward the city," he said. "No one would have realized it would have gotten there that quickly in the middle of the night."

He grabbed his cellphone and called the police department, telling them to order an immediate evacuation of a broad swath of the city. He guesses that cellphones in the area stopped working a short time later. Police and fire personnel then swarmed neighborhoods, blasting their sirens and banging on doors, he said.

"There was no, 'Let's get on social media and Twitter and Facebook,' " he said. "This was a situation of, 'Oh, my God, it's here. We just need to start getting people out any way we can.' "

Donosky reported from Sonoma County; Somashekhar and Zezima reported from Washington.