Several highways along the coast were submerged in floodwater on Thursday morning following a heavy downpour over San Mateo County, SFGATE reported.

A video showing the flood water reaching waist-high on Coral Reef Avenue in El Granada was shared by Cal Fire’s unit in San Mateo-Santa Cruz.

Flood alerts were also released on other locations including Highway 1 near Half Moon Bay Airport, on Highway 1 in Moss Beach, and on Highway 1 at California Avenue.

Some roadways in San Mateo County’s inland places also suffered from flooding.

On Twitter, ABC7 reporter Amy Hollyfield posted a photo of the standing water at Hemlock and Hillcrest in Millbrae.

“Someone told authorities cars are trapped and that they were able to help some to safety, but not all,” Hollyfield said. “First responders are trying to figure out if people did not make it out- say at this point it would be a recovery effort, not rescue.”

Also among the images Hollyfield shared was the one showing a Millbrae residential area covered in standing water.

Motorists were cautioned by the National Weather Service against driving through the flooded roads.

At 8:00 a.m., the part of the coast including Half Moon Bay had recorded an inch in the past hour, said weather service forecaster Sean Miller.

About 3 inches of rain was logged in the past 24 hours using a weather gauge located at the Half Moon Bay Airport. More downpours can fall throughout the day but in the late afternoon to evening, precipitation is said to reduce.

“It’s going to be a slow progression through the day,” Miller said. “At least through early afternoon-ish. The system will slowly sink toward the South Bay. The rain is going to continue through the afternoon.”

Rainfall rates over the CZU Lightning Complex Fire burn scar located in San Mateo County as well as in the Santa Cruz Mountains are placed under close monitoring by the weather agency.

“No issues down there at the moment,” Miller said. “That’s something we’ll be watching through the day. In general, the rain rates haven’t been high enough to trigger debris flows.”