The matter is a minefield.
Failure to insist residents leave could spell their death -- or, less dramatically, leave them facing days or weeks without electricity, food or medical care.
Meantime, pulling the trigger risks subjecting people to long, expensive trips out of town or long stays in shelters, with government footing the bill.
If the predicted disaster doesn't strike, it could contribute to what experts dub "cry wolf syndrome," a mindset that leaves people skeptical of official warnings -- and more likely to ignore them next time.
And, of course, either decision is likely to earn criticism from a public whose lives, homes and incomes depend on what call gets made.
Emergency managers in Santa Barbara County, California, pushed back aggressively against reports that certain public alerts weren't blasted out until after mudslides began flowing in Montecito on January 9. Officials had warned people days in advance via televised news conferences, then sent email and text alerts when conditions worsened, they said, adding that local sheriff's office employees knocked on 1,400 doors to deliver the message in person.
"We warned them before, we warned them during, and we warned them after about the possibilities of more flooding," county emergency manager Robert Lewin, a former fire chief, told CNN.
Houston Mayor Sylvester Turner in August defended his decision not to order evacuations before his city was hit by torrential rain from Hurricane Harvey.
"You literally cannot put 6.5 million people on the road," he said. "If you think the situation right now is bad, you give an order to evacuate, you are creating a nightmare."
And though then-Mayor Ray Nagin earned praise for ordering a mandatory evacuation
ahead of Hurricane Katrina in 2005 -- with analysts saying the death toll would have been much higher if he'd not -- Nagin admitted months later that if he'd gotten a dire warning sooner
from the National Hurricane Center director, "I would have issued it earlier."
Deciding to order an evacuation -- voluntary or mandatory -- hinges on multiple factors, disaster management experts told CNN, from weather forecasts to residents' trust in their leaders to the communications networks used to spread the word. And human nature affects every turn, making the decision to tell residents to hunker down or flee all the more challenging.
Cost-benefit analysis in play
Rather than rely on federal or state actors, evacuation decisions fall to local officials -- municipal disaster experts, elected officeholders, hospital managers, nonprofit directors, business leaders -- said Richard Serino, a FEMA deputy administrator from 2009 to 2014 who is now a fellow at Harvard University's National Preparedness Leadership Initiative.
"They have the best information on the ground at the time to make that decision," he said. "And they're not just making the decision for themselves, they're making it for their family and for everyone else in the community where they live."
These leaders rely on climate science to know when a storm or a wildfire will reach an area and for understanding the risk that place faces, often based on topography and disaster modeling, the experts said.
Other factors also are key, including the capacity of roads to carry heavy traffic, the availability of shelter space and public resources to move elderly or ill patients, and the potential economic impact on a community if it effectively shuts down for days, the experts said.
"There's all this cost-benefit analysis in everyday life that comes into play," said Tim Frazier, faculty director of the Emergency and Disaster Management program at Georgetown University. "The decision to evacuate or not evacuate is complex for lots of different reasons."
People's perception of risk also plays a huge role, especially for those with a disaster experience fresh in their minds.
"Everybody remembered Andrew, then here comes Floyd," Frazier said, referring to the 1992 and 1999 Atlantic hurricanes. "Not a lot of coercing needed to be done by the governor. Risk perception was high, and nobody was having to beg anyone to go."
By contrast, he said, "people don't want to go (if) last time they rode it out and it was fine."
"Disaster fatigue is very real," said retired Air Force Lt. Col. Chris Reynolds, who is now a dean at the American Public University System. "In their mind, it's the sense (that) nothing happened last time, it's likely be the same this time. Now, that's dangerous."
Bolstering public trust
For decades, people simply heeded warnings from emergency managers, no questions asked.
"What's happened now with the internet and Twitter and Facebook and all the other media we have out there, it allows us to have this high volume of information from all these sources. Everyone individually becomes their own emergency manager," Frazier said. That "could be unfortunate ... if you think you're the expert because you have (a) few Twitter posts or some buddy of yours on Facebook says you should ride it out."
To cut through the clutter -- and convince residents they're not "crying wolf" -- local leaders can bolster evacuation orders with detailed explanations of how they came to their decision, including storm surge models, wind forecasts and historical disaster data, he said.
It all comes down to the public's trust.
"The 'cry wolf syndrome' is real, and it all comes down to: do you perceive your risk is great," Frazier said.
Indeed, local governments can build trust with the communities they serve by making disaster management an ongoing conversation, Reynolds said. That could mean frequently reevaluating evacuation zones, updating floodplain maps and reassessing wildfire risk -- and always sharing that information with residents.
Communicate early and often
Local leaders should begin warning the public of an impending disaster days in advance, if possible -- and make evacuation decisions early enough so people can get out.
"It all comes down to pulling the trigger: the warning point, the notification point and the plan to implement it," Reynolds said.
All lines of communication should be in play: TV, radio, social media, email, text messages, public sirens, door-to-door canvassing, reverse 911 calls and cell-phone push-alerts, with opt-out services, like those used for Amber Alerts and severe weather warnings, the best choice in the worst situations, Serino said. Local leaders can time the use of different methods to maximize their reach, he added.
As technology improves, micro-targeting is sure to factor in, too, with alerts sent only to residents of a certain block or neighborhood where conditions are most likely to deteriorate, he said.
Being clear about the consequences for residents who ignore evacuation orders also is important, Frazier said.
Katrina "was one of the turning points for this realization that ... if you don't go before the storm hits, (emergency responders) will come as soon as we think it's safe," he said. "We're not putting our people in harm's way because of your stupidity."
And ultimately, "people also have to accept the responsibly themselves," Serino said.
In the end, ordering an evacuation is a team effort, as much art as science, and never a small matter for those making the call, the experts said.
"It's very easy to Monday-morning quarterback," Serino said. "Everybody's dealing with the best information they have at the time."