(CNN) -- A wind-fed fire in southern Arizona grew dramatically Friday, racing down from canyons and gobbling parched grasslands and brush.
Arizona and New Mexico were under red-flag alerts through the weekend as firefighters battled several blazes in Arizona.
Conditions will be especially difficult Sunday at the Monument fire just south of Sierra Vista, Arizona, according to Glenn Lader of the National Weather Service office in Tucson.
Relentless winds have spread the 18,580-acre fire that has torched at least 40 homes and forced the evacuation of hundreds Thursday. Evacuations of several areas continued Friday.
The Monument fire has spread across the Huachuca Mountains near the Mexican border.
The area is known as the "Sky Islands" region.
"These mountain 'islands,' forested ranges separated by vast expanses of desert and grassland plains, are among the most diverse ecosystems in the world," according to the Sky Island Alliance, a conservation group.
Garry Foss, who operates Oaks of the Wild West nursery in Hereford, a community just east of the fire, said grasses 2 to 3 feet tall have not seen rain since December, making for a ready fire fuel source.
Sierra Vista, home to the Army's Fort Huachuca, has had 0.13 inches of rain this year, according to Lader, compared with 2.5 to 3 inches in a normal year. There's no rain in the forecast.
"I'm trying to sit it out," Foss said, adding that he now has to drive east to Bisbee and northwest to reach Sierra Vista. "This grass is so dry."
Much of Highway 92 south of Sierra Vista was ordered closed. A brush fire was reported on Fort Huachuca
Arizona Gov. Jan Brewer, meanwhile, issued emergency declarations for the Monument fire and another blaze, Horseshoe II, making Cochise County eligible for $100,000 toward response and recovery expenses.
High winds also could undo some of the progress made by crews battling the Wallow blaze, which has burned nearly a half-million acres in the Apache-Sitgreaves National Forest.
"Yes, it is a threat. Anything could happen," said Terry Stemmler, a spokesman for the Southwest Incident Management Team.
Crews are facing four straight days of red flag warnings that started Thursday with the return of strong winds to a dry region, Stemmler said. A red flag warning means weather conditions -- high winds and low humidity -- pose an extreme risk for fire.
"The hot, dry and windy weather coupled with the drought-stressed vegetation and heavy fuel loading in mixed conifer forests has the potential to create extreme fire behavior," officials with the Wallow fire team said.
As of Friday morning, the Wallow fire had scorched 495,016 acres -- more than 770 square miles -- in eastern Arizona and western New Mexico, the vast majority of it in the Apache-Sitgreaves National Forest in Arizona, authorities said.
The blaze, which is 33% contained, became the largest wildfire in Arizona history on Tuesday.
CNN's Phil Gast, Samuel Gardner III and Dave Alsup contributed to this report.