Dry winds gusting as fast as 40 miles per hour are keeping “red flag” wildfire warnings locked in place Thursday in California, where PG&E Corp. has cut off power to about 750,000 customers.

The utility has blacked out about 14% of its 5.4 million electric customers to prevent strong winds from knocking down electrical lines and igniting fires. A series of wind-driven blazes sparked by the utility’s equipment forced PG&E to seek bankruptcy protection in January after they destroyed tens of thousands of structures and killed more than 100 people.

Wind gusts are ranging from 20 mph to 40 mph in some areas, according to Marc Chenard, a senior branch forecaster at the U.S. Weather Prediction Center in College Park, Maryland. Adding to the threat is desert-like humidity across much of the area, he said.

“When the relative humidity is between 10 and 20%, that is quite dry,” Chenard said. “For this area it happens but it is not an everyday thing.”

Red flag warnings are issued by the National Weather Service when high and gusty winds combine with very low humidity in the air, and a lack of moisture in local vegetation. The very dry air now sweeping across California has turned trees and shrubs into “cured fine fuels,” the analysis noted.

Fire concerns will linger across Northern California Thursday and become critical in the south, through parts of Los Angeles, Ventura and San Bernardino counties. The biggest fire concerns will occur in the late morning, the center said in a forecast analysis.

Southern California from Santa Barbara to San Diego will still be at risk on Friday.

In addition to the threat of wildfires, the winds have also triggered an air quality alert in the agriculture-rich San Joaquin Valley, including Fresno and Merced through Thursday, the National Weather Service said. Gusty conditions are expected to kick up dust and dirt that could cause asthma attacks, irritate lungs and raise the risk of respiratory infections.