Jeff Masters Weather Blog

The rules … and the go or no-go responses » Yale Climate Connections

Some years back, a big but slow-moving hurricane threatened a place I knew well. Close family members were not only in the storm’s path but living in an area thrust into a mandatory evacuation. While many of their neighbors shuttered up and hit the road, they, like others, saw only a road fraught with complication from unnecessary panic. When decisions feel paralyzing, it’s only human to delay.

Their adult children weren’t leaving, and they certainly weren’t about to leave their kids – even if their kids were healthy twentysomethings and even for a hurricane. “It’s only a Category 1. They’re saying it’s not going to be that bad.” The elusive “they.” It taunts every meteorologist that’s ever made a forecast, but without a face or a name to spurn. I stood outside the concrete walls of my office at the National Hurricane Center in Miami that day in 2012 searching for something between empathy and indignation. “I am the ‘they,’ and I’m telling you it’s going to be bad!” I erred on the side of indignation.

What motivates us to evacuate?

Evacuations are a messy business. Compliance on evacuation orders isn’t great – between 30% and 70% of people under mandatory evacuation orders in the U.S. typically leave, with higher compliance for stronger hurricanes. To complicate matters, we throw in voluntary evacuations for good measure. Ordering a voluntary evacuation is paradoxical at best and counterproductive at worst. How is an order voluntary? Do voluntary evacuations imply a less credible threat? Studies show voluntary evacuation calls are about as effective as having no evacuation orders at all.

Evacuation orders are only one row of the Rubik’s cube. We can’t discuss evacuations without first recognizing that those most disproportionately impacted by hurricane disasters – older adults, the economically disadvantaged, communities of color, people with disabilities, etc. – are often the ones left behind. Coordinating comprehensive plans to support individuals and households with physical or financial limitations to evacuate is a critical first step.

But what about the ones able to go who choose to stay? Social scientists have identified a cacophony of possible barriers, from pets to hotel costs to social cues and community ties, that impede personal evacuation decisions. Among the decades of evacuation research, however, three factors in particular convince people to go: the issuance of mandatory evacuation orders, a perceived threat of flooding, and early, official notices to leave.

All disasters are local

There’s an emergency management adage that disaster responses are locally executed, state managed, and federally supported. This means local officials are the ones driving the decisions affecting their threatened communities. This is especially true with evacuations. With some exception, as in South Carolina where the governor directs evacuations, the feds and state are largely hands-off with evacuation orders. While federal and state agencies support evacuation and sheltering operations, in general, city, county, parish, or regional governments set their evacuation zones and decide if and when to evacuate an area. This is how it should be. Washington can’t know if the community surrounded by low-walled earthen levees at the head of the Five-Forks River needs to pack it up.

The rules of evacuation

Hurricane evacuations are a bespoke service, but certain universal rules apply. Although some higher risk populations like those living in nursing homes or in manufactured homes may need to evacuate for high winds, the vast majority of hurricane evacuations are predicated on life-threatening flooding from storm surge. Storm surge atlases developed by the National Hurricane Center outside hurricane season allow officials to update and refine their evacuation zones before the first storm forms.

While the storm surge threat helps officials decide if to evacuate, the timing of tropical storm winds (winds above 38 mph) helps decisionmakers determine when to evacuate. Extensive studies of regional and local population and behavior, nearby sheltering capacity, and road networks help in the determination of appropriate clearance times, the time needed to move at-risk populations to safety. The goal of local officials is to begin evacuations with enough time to get all evacuees to a safe place before dangerous tropical storm winds arrive. In some cases, those evacuations may require only a few hours, but in other high-density and storm-surge-prone areas, clearance times can be days.

Politics at play

The decision to evacuate an area is part science, part art, and inescapably political. Officials making the evacuation calls are usually elected or appointed, and a go or no-go decision can have significant consequences on their municipality and electorate, not to mention on their own futures.

Evacuations are a significant indirect contributor of hurricane deaths (about 15% of all indirect hurricane fatalities are evacuation-related), so putting people on the road unnecessarily can be costly and deadly. In 2005, as Category 5 Hurricane Rita aimed toward the Texas coast, nearly 4 million people jammed the roadways, many unnecessarily in a post-Katrina panic as “shadow evacuees” (people who evacuate even though they’re not directly ordered to do so). The result was 100 deaths from evacuations alone, accounting for over 80% of the storm-related deaths. Officials may also worry about jading constituents into not evacuating next time when a hurricane threat becomes a false alarm (numerous studies have proven the “crying wolf” worry to be unfounded).

But delaying evacuations has consequences too. Those ordered to evacuate are significantly more likely to do so when they’re given at least two to four days’ notice. Short-fuse evacuations notices aren’t always avoidable – especially with rapidly forming storms – but evacuation compliance is markedly lower when a day or less notice is given. When people stay that shouldn’t, it not only endangers their lives, but the lives of first responders and rescue teams that follow.

Officials aren’t always uniform or consistent in seeking information on which to base their decisions, either. Public officials and emergency managers may not have the technical background or appropriate skillset to interpret complicated scientific products from forecasters. Even common visuals like the much-maligned forecast cone can be a point of confusion for decisionmakers. They’re often left to make binary decisions out of unfamiliar probabilities. The typical weather rules of thumb tend to fail when the weather turns extreme. A 40% chance of rain and a 40% chance of hurricane force winds should elicit very different behavior, but as scientists have discovered, they often don’t.

Next time we’ll leave

The personal plea I made between forecasts that late summer afternoon at the National Hurricane Center didn’t work. My family members stayed despite the mandatory evacuations. They were terrified, powerless against the dark whims and piercing whistles of the oncoming storm. Their neighbors only a few miles away flooded, but they didn’t.

They said they’ll evacuate next time, but the science suggests otherwise. We tend to not change our future behavior based on near misses. Only once we perceive a threat to be bad enough – whether it is or isn’t – do we seek safety, but as we learned in Ian, with hurricanes and evacuations, for too many, that next time may never come.

Michael Lowry is Hurricane Specialist and Storm Surge Expert at WPLG, the ABC affiliate in Miami, Florida. He is a former emergency management official with FEMA and senior scientist at the National Hurricane Center.

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