Before Hurricane Harvey barreled into Texas and then hung around for days like an unwanted house guest, dropping more than 4 feet of rain in some areas, Houston's leaders had a tough call to make.

Should they try to evacuate a metropolitan area of 6.4 million people for a hurricane whose greatest potential threat was torrential rainfall, not high storm surge or destructive winds — especially given the perils that could befall that many residents trying to flee?

Their eventual choice to forgo a mass exodus and avoid the chaos that accompanied evacuations ahead of Hurricane Rita in 2005 is one likely to be debated long after the death toll from Harvey's catastrophic floods is made final.

Roughly 60 deaths, mostly of people stuck in endless traffic jams, were attributed to the abortive Rita evacuation. That's more deaths than officials have attributed to Harvey to date, although the toll from the storm continues to climb. 

But in New Orleans, a city that has learned hard lessons of its own after weathering disasters, there would likely have been less debate. For one thing, New Orleans — whose metropolitan area has roughly one-fifth as many people as Houston's — showed in 2008, as Hurricane Gustav approached, that it could pull off a successful evacuation.

And with the city's seemingly diminished drainage capacity, an evacuation would likely be called more hastily than it was in 2008, at least in Orleans Parish.

The typical trigger for a local evacuation has long been the approach of a storm with Category 3 winds or greater. But that threshold has been lowered to a Category 1 with heavy rain-making potential, in light of the city’s diminished drainage capacity, the Governor’s Office of Homeland Security and Emergency Preparedness said.

“We are prepared to execute what we would normally execute for a Category 3 or higher. We are prepared to pull those triggers on perhaps a Category 1 with a lot of rainfall,” said Christopher Guilbeaux, an assistant deputy director with GOHSEP.

New Orleans would likely also clear out if any rain event — with or without strong winds — is forecast to bring 20 inches of rain to the area, according to GOHSEP.

GOHSEP’s revelation of the triggers being considered appeared to peeve Mayor Mitch Landrieu’s administration, which has been hesitant to publicly cite new thresholds for evacuations as repairs are being made to the city's pumps. Instead, administration officials have said they would consider storms on a case-by-case basis.

And city officials on Friday said any previously announced threshold for evacuation could change, depending on a specific storm’s track, strength and character. They also said that they, not the governor's office, should be considered the final authority on when and whether citizens should leave.

An evacuation for a strong storm that threatens to drop lots of rain, but not the devastating storm surge that an oncoming hurricane can bring, would be unprecedented locally.

The reason Category 3 storms, which pack winds of 111 to 129 mph, have long been the city’s threshold is that a storm of that force can push water from the Gulf of Mexico into Lake Pontchartrain and other bodies of water, which can then rise and overtop the city’s levees.

Such potentially devastating storm surges have historically been seen as the key threat to New Orleans during hurricanes. It was the storm surge from Hurricane Katrina, combined with engineering failures, that led to catastrophic levee breaches in New Orleans in 2005.

But extreme rain poses its own threats, as the flooding caused by Harvey underscored. Because Harvey was not forecast to have a storm surge high enough to inundate the Houston area’s low-lying counties or winds powerful enough to destroy its buildings, local officials did not issue an evacuation order, the Los Angeles Times reported.

Houston, unlike New Orleans, drains naturally, if not rapidly; it does not have levees surrounding it.

The city’s leaders were clearly mindful of the chaos that ensued after 3.7 million Houstonians followed an order to leave as Rita approached a dozen years ago. The exodus led to traffic jams that stretched for miles, stalled cars, and the explosion of a bus carrying elderly residents that stalled on the long drive, catching fire and killing 23 people.

But the decision to stay put this time was made with the benefit of repeated warnings from the National Weather Service that parts of the Houston/Galveston area could receive as much as 50 inches by the time the storm ended — predictions that unhappily came true.

It was an ungodly amount of rain — equivalent to the past eight months of rainfall in New Orleans, according to data provided by the National Oceanic and Atmospheric Administration and compiled by the New York Times.

Whether sheltering in place was the right call is something experts have continued to debate since the storm's arrival last weekend. Both GOHSEP and Landrieu shied away recently from that debate, with Landrieu telling National Public Radio that he wouldn’t “second-guess a local official or the governor in that area” and GOHSEP’s Guilbeaux saying Louisiana wasn’t “trying to armchair quarterback why Houston did or didn’t do anything.”

But New Orleans’ established practice of telling people to leave when strong hurricanes threaten would likely have made that issue a moot point here. And with continued uncertainty about how much water the city’s system can drain, weaker storms could spark evacuations in the near future.

Even the prospect of a few rain bands from Harvey was a big enough threat to the diminished system for Landrieu to shut the city down on Tuesday, even though the storm's center was hundreds of miles away.

In a scenario where the trigger is pulled — whether because of prospective heavy rain or high storm surge — the city could order residents to leave as early as 72 hours ahead of a storm’s landfall, according to one sample timeline provided by City Hall.

The city then could shuttle tourists from hotels to Louis Armstrong International Airport and begin picking up residents from designated evacuation points less than 24 hours later. During Harvey, for example, officials had four shelters on the east and west banks at the ready in the event heavier rains began to fall here.

Given the precarious state of New Orleans’ drainage system, it’s possible that some steps often taken during evacuations, such as the enactment of contraflow rules on the highways leading from the city, would be modified or abandoned if only the city proper is told to evacuate, Guilbeaux said.

Landrieu spokesman Tyronne Walker and the city's emergency preparedness director, Aaron Miller, said residents should look to the mayor’s office for guidance on how they should respond to a storm.

Once an evacuation is called, the timeline for how it will play out is also subject to change.

“The only source that will matter when we are talking about what New Orleans will do is the city of New Orleans,” Walker said.

But both he and Miller acknowledged that, as Guilbeaux suggested, a Category 1 storm that packs heavy rainfall could prompt an evacuation.

“If it was a scenario where there was an excessive amount of rainfall and we saw a rapid increase in intensification of the system, all of those things are going to determine what we do,” Miller said.

Follow Jessica Williams on Twitter, @jwilliamsNOLA​.