A wildfire burning in Riverside was threatening homes in the La Sierra Acres neighborhood today. The blaze was reported at around noon near Jurupa Avenue and Tyler Street, according to the Riverside Fire Department. Authorities said residents on the north side of Valley Drive between Mitchell and Chadbourne avenues were evacuated. Fire officials said they believe an off-road vehicle crash may have sparked the blaze. Firefighters were on high alert Sunday as strong gusts and low humidity triggered a red flag warning for much of the Southland.