A layer of smoke and haze blanketed the central San Joaquin Valley on Sunday as the Rim fire in Yosemite National Park spread southward.
The National Weather Service in Hanford attributed shifting winds and low humidity to the fire's increased activity.
Meteorologist David Spector said the haze may remain trapped in the Valley for a few days until a stronger pressure system arrives to push the smoke east.
An inversion layer will keep the smoke near to the ground and temperatures hot.
Monday's high for Fresno is expected to reach 103 degrees with an overnight low of 71, Spector said.
Air quality in Fresno is forecast to be unhealthy for sensitive groups, according to the San Joaquin Valley Air Pollution Control District.
Daytime high temperatures in Fresno are expected to hit triple digits again on Tuesday before slipping back into the upper 90s the rest of the week.
A slight chance of thunderstorms is forecast for the southern Sierra Nevada Tuesday and Wednesday, the weather service said.
To date, the Rim fire has burned 253,332 acres and is 80% contained.
The estimated cost of the fire is $96.2 million so far with more than 3,104 firefighters working the fire.
The fire has destroyed 11 buildings and nine people have been injured since it started burning four weeks ago.
Highway 120 (Tioga Road) remains closed from Crane Flat to 1 1/2 miles east of White Wolf.
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