HARRIS COUNTY, Texas (KTRK) -- Drivers who take the Tomball Parkway could expect delays after an unknown vehicle hit an overpass in northwest Harris County on Thursday morning, authorities said.

According to Harris County Precinct 4 Constable Mark Herman's Office, all southbound lanes in the 20500 block of Tomball Parkway near Grant Road are closed.

Precinct 4 said the unknown vehicle hit the overpass, causing concrete debris to fall onto the roadway.