Boulder , Colorado ( CNN ) -- Authorities in Boulder , Colorado , warned some residents to prepare to evacuate Thursday as winds gusting as high as 50 mph threatened to drive a raging wildfire out of the hills and into city limits . The Fourmile Canyon fire already had charred nearly 6,400 acres of countryside and destroyed 169 homes west of Boulder by mid-morning Thursday . High winds that could whip the fire toward the city were expected to kick up Thursday evening and last throughout the night , forecasters predicted . About 8,000 people in the hills west of Boulder have already been forced to evacuate . No evacuations have been ordered for city residents yet , but Boulder municipal spokeswoman Sarah Huntley said they should `` think about the possibility of evacuating '' and have clothing , important documents and needed medication ready to go if necessary . `` We absolutely hope this wo n't be necessary , but this is a really good reminder that we live in a community that does potentially have a fire danger , '' Huntley told reporters . `` Obviously , some of our neighbors have found that out , and our hearts definitely go out to them . We 're asking city residents to be on notice . '' Huntley said people living on the northwestern edge of the picturesque university city should `` do what they can to help us protect their home '' by cutting grass short , removing combustible material from yards and moving propane tanks to the east side of their homes and making sure they 're `` clearly visible '' to firefighters . Boulder is about 35 miles northwest of Denver , at the eastern edge of the Rocky Mountains . The Fourmile Canyon fire was about 30 percent contained Thursay morning , and could be as much as 45 percent contained by the end of the day , said Winslow Robertson , operations chief for the Rocky Mountain Incident Management Team . There have been no deaths or injuries , and four people who had been listed as unaccounted for have been contacted and are unhurt , Boulder County Sheriff 's Cmdr . Rick Brough said . But while overcast skies and a bit of rain helped firefighters on Wednesday , the projected high winds raised new concerns Thursday . `` It takes one spark outside the line with the conditions tonight , and it 's a new race , '' firefighter Rob Bozeman told reporters earlier Thursday . He said some portions of the blaze were `` as extreme a fire behavior as I 've seen -- not a front a mile wide coming at me , but fingers coming up . '' Bozeman said `` entire chains of trees '' were ablaze , running up the sides of the hills into subdivisions and setting houses on fire . `` The fire behavior was so intense that when that heat hits , the firefighters have to pull back , '' he said . If firefighters can get back into the area and start spraying water onto a home , `` You can save it . But if it 's happening all over the place , you ca n't get to all of them , '' he said . The fire started Monday morning . Its cause was still under investigation , Brough said . Three subdivisions reopened briefly to residents Thursday morning , but authorities shut them off again after conditions worsened . About 550 firefighters were battling the blaze Thursday , with 160 others arriving for duty , said Laura McConnell , a spokeswoman for the Boulder County Incident Management Team . Aircraft were also being used , with more than 100,000 gallons of fire retardant dropped from the air Wednesday , authorities said . Colorado Gov . Bill Ritter declared a state of emergency this week , throwing $ 5 million into the battle to save homes and forests in the canyon . CNN 's Jim Spellman contributed to this report .
NEW : Some Boulder residents should `` think about '' evacuating , city says . `` Entire chains of trees '' ablaze in Rocky Mountain foothills . Wind-whipped fire claims nearly 170 homes . No deaths or injuries reported since blaze started Monday .