Federal meteorologists say a “catastrophic” tornado cut through Little Rock, Arkansas, on Friday afternoon—toppling 18-wheelers and injuring so many people that police were forced to transport victims to the hospital because there were no more ambulances, THV 11 reported. The terrifying twister, whose official strength has not yet been classified, was captured on video around lunchtime in Arkansas’ capital city. Damage reports are still developing, but the University of Arkansas for Medical Sciences deemed the carnage a “mass casualty event.” The Metropolitan Emergency Medical Services estimated that as many as 600 people were injured. The tornado, which also tore through nearby Central Arkansas towns, formed on a day forecasted to be particularly dangerous in the great plains and Midwest—with a tornado watch issued as far north as Milwaukee, and separate twisters touching down in Iowa.