"They're anticipating quite a bit of activity and potential growth," Crapser said. "We are going to have at least a couple days of pretty active fire behavior."
More than 150 homes were evacuated - about 400 people Sunday and another 100 Monday. About three-quarters of the homes were year-round residences and the rest were seasonal cabins, Crapser estimated.
The American Red Cross lodged nine evacuated families at the Parkway Plaza Hotel & Convention Center in Casper.
"We're assessing the situation today and will possibly set up a shelter," said Traci Aguilar, Red Cross branch manager for Natrona and Converse counties.
Firefighters requested more help from heavy air tankers and helicopters. The fire's cause was unknown.
Less than two miles south of Jackson, about 1,000 residents in the ski resort town in northwest Wyoming were being told to prepare to leave Monday because of the Little Horsethief Fire. The blaze had burned about 1,500 acres in a forested, mountainous area.
"Last night you could see a glow and trees torching here and there," fire spokeswoman Jesse Bender said Monday.
Snow King Resort, which borders the south side of Jackson, closed its summer lift line and trails.
More than 100 firefighters aided by three helicopters were battling the blaze, and more firefighting resources were ordered. The fire was 15 percent contained.
The online InciWeb incident information network described the Little Horsethief Fire, which began Saturday, as human-caused.
The forecast in much of Wyoming called for cooler weather and a chance of rain later in the week. But firefighters were concerned that winds from the south could push the Horsethief Fire to Snow King, and downhill to Jackson, before then.
Wyoming is ripe for more wildfires. This summer has not only been the state's driest on record but its warmest, according to the National Oceanic and Atmospheric Administration.