The National Weather Service said in a tweet that US west will see "elevated and critical fire weather conditions and widespread chances for dry thunderstorms."
Gusty winds and dry, hot weather have been increasing the size of dozens of wildfires scorching vast stretches of Oregon, California and other western states.
The Bootleg Fire in Oregon - the largest wildfire raging in the United States – is pushing to the north and east. It grew in size to nearly 212 square kilometers (468 square miles ) – an area larger than Hong Kong.
Some 2,000 people have had to evacuate, with more following on Sunday.
"The weather is really against us," said John Flannigan, an operations section chief with the Oregon Department of Forestry (ODF), in charge of battling the blaze.
He noted that all weather factors were "negative for firefighters and positive for fire."
No relief is in sight. The National Weather Service said on Twitter that the coming days in the US west will see "elevated and critical fire weather conditions and widespread chances for dry thunderstorms."
One of the big weather stories this week in the #West and U.S. will be the elevated and critical #fire weather conditions and widespread chances for isolated dry thunderstorms. Here are the current red flag warnings and fire weather watches in effect across the region. pic.twitter.com/B9IwENzMcL— National Weather Service (@NWS) July 18, 2021
The Bootleg fire, which started just north of the California state line, remains 22 percent contained, the same as the day before. Over 2,000 fire personnel worked the perimeters in an effort to keep the blaze from growing too fast.
The ODF said Bootleg is threatening over 3,100 homes, and that it had destroyed 67 so far.
Authorities are investigating the cause of the fire.
The National Interagency Fire Center said the outlook was for "very hot, dry and unstable conditions across the inland Pacific Northwest, Northern Rockies and Plains into northern Minnesota."
It said nearly 20,000 firefighters and support personnel are struggling to contain fires raging across the Western states, with more than 2.5 million acres already having burned this year.
Firefighters in Canada, meanwhile, continued to battle dozens of blazes, including some 20 new ones in British Columbia province and about 15 new ones in northwest Ontario province.
In California, the Dixie fire, the state's largest, continues burning toward the northeast in a remote area that is difficult to reach, officials said.
That wildfire – which has burned over 23 square miles (59.5 square kilometers) and is 15 percent contained – has prompted an evacuation order for Plumas County.
The Tamarack fire that was ignited by lightning on July 4 has grown to 74 square kilometers (28.5 square miles) and remains 0 percent contained, authorities said.
The blaze is located about 18 miles southeast of Lake Tahoe.
Mandatory evacuations of several small towns and other areas in the path of the fire has forced hundreds of people to leave their homes.