PHOENIX — Wildfire season started early here this year.

In two months, more than 20 fires have broken out across the bottom half of the state, from its southeastern tip, near New Mexico, to the outskirts of Tucson and west of Phoenix. Flames have consumed so many acres that Gov. Doug Ducey declared a state of emergency last week, freeing up more money to cover the growing costs of fighting fire on abnormally dry land.

More land has burned so far this year in the Southwest than anywhere else in the country as a new season of fast-moving fires has unfolded across the western United States. Firefighters, guided by lessons learned in past deadly fires, are struggling to stay ahead of the flames.

Arizona is one of four states in the region using a system to identify communities made more susceptible to burning by overgrown vegetation. The goal is to enlist residents to do their part: “Clean up around your yard, trim the trees around your cabin, get those fuels removed,” said Arizona’s state forester, Jeff Whitney.

“Prevention is our most important opportunity,” he said. “If we can treat the fuels before the fire starts — in other words, if we can minimize the vegetation that feeds a fire — we’ll be more successful at extinguishing that fire.”

There is also a heightened emphasis on firefighter safety. The deadly fire of June 30, 2013, when 19 firefighters died battling a blaze in the tiny town of Yarnell, has led firefighting teams to decide much earlier to move out of the path of flames, even if it means that communities may burn.

Earlier this year, Mr. Whitney had some good and bad news for Governor Ducey about the current fire season: After a snowy winter, the mountaintop forests were moist and green, likely safe from burning. But not the vegetation that lines the lower elevations.

“Tinder-dry” is how firefighters on the ground have described the parched land in the area, seared by a lack of rain and increasing heat. From the southern edge of the state to the high desert of Central Arizona, brush and grass were brittle, Mr. Whitney told the governor, and “ready to combust.”

His prediction is coming true.

Whipped by strong winds, the Goodwin Fire, which began on Saturday near the city of Prescott, had quintupled in size by midweek, a reminder of the risks that residents know far too well. The lethal blaze four years ago took place 60 miles west, under similar conditions and on similar terrain.

Fire historians say the need for preventive measures like clearing brush is an important realization and a hard lesson learned from the fatalities in 2013. When Yarnell burned again last year, the flames were stopped by large fuel breaks carved between the town and the wilderness that surrounds it. No one was hurt, and only a handful of structures were damaged. The work was paid for by grants funded by the Arizona Legislature for the first time, in direct response to the need to thin out vegetation.

Still, the historians say, the changes seem almost meek when compared with the shock and magnitude of the tragedy of four years ago.

In the past, each cluster of fatal wildfires led to a round of new rules for firefighters on the ground. The 10 standard firefighting orders of the 1950s warned, “Know what your fire is doing at all times.” The 18 “watch out” situations of the 1980s laid out the risks of fighting fire when instructions and assignments are not clear. A system known as L.C.E.S., for the elements of fire safety it emphasizes — lookouts, communication, escape routes and safety zones — became the focus in the 1990s, after six firefighters died in the Dude Fire in Arizona.

But the deadly fire in Yarnell has not translated into practical reforms, in part because of where it took place. While most wildfires happen on federal land and are fought by federal crews, the one in Yarnell burned on state land, and the 19 firefighters who died were members of the only city-based wildfire-fighting crew in the country, the Granite Mountain Hotshots of Prescott.

“Without a stronger federal presence, there was little way to nationalize the experience,” said Stephen J. Pyne, a professor in the School of Life Sciences at Arizona State University who specializes in the history of fire.

Nonetheless, the Yarnell Hill Fire, as it is known, has made it easier to use danger as a reason to move firefighters out of the path of flames, even if it means that communities may burn.

In public meetings and on social media, incident commanders — the managers who oversee the work of crews assigned to specific wildfires — are no longer reluctant to tell residents that no home is worth risking a firefighter’s life.

“It is important for people to understand the limitations we have at protecting values at risk when firefighter safety is on the line,” said Alan Sinclair, an incident commander stationed in the Southwest.

Earlier this month, Mr. Sinclair kept crews away from a rugged corner of the Frye Fire in southeastern Arizona, even as it threatened the Mount Graham International Observatory, home to several of the world’s largest and most powerful telescopes.

“Crews assessed this spot and made a determination that it would not be safe to hike in there and engage it,” he said in a video posted on Facebook. “Safety is the No. 1 priority that we have for our firefighters.”

On Tuesday, as he met with residents displaced by the Goodwin Fire near Prescott, Pete Gordon, fire chief for the Prescott National Forest, invoked the Granite Mountain Hotshots. “Please understand that while we send firefighters into difficult places, there are places we will not go,” he said.

Four years ago, the 19 members of the Hotshots were trapped by flames in a canyon choked with dried brush west of Yarnell, presumably while trying to protect nearby homes.

Mr. Gordon and Mr. Sinclair were among a small group of firefighters who brought their bodies out of the canyon. They knew the fallen firefighters well. At Tuesday’s meeting, Mr. Gordon said the experience had caused him to question safety decisions constantly.

Mr. Sinclair agreed. “I’ve always known that the job was dangerous,” he said, “but when you’re closely connected to a tragedy as I was to Yarnell, it really woke me up.”

The issue, he said, is not simply what decisions to make, but whether he is prepared to make them.

“After Yarnell,” he said, “I had to ensure that I was willing to face the uncertainties of an inherently dangerous job.”