A stubborn thunderstorm last week dumped a downpour on Sarasota, which brought fast-rising floodwaters to St. Armands Circle.Sarasota County officials, who are responsible for stormwater in the shopping and dining district and the rest of the city, attributed the slow drainage to a failed sensor that prevented one of the five pumps in the circle from starting. It has since been repaired.Data from University of South Florida’s real-time rainfall …