More than 100 TV stations have signed up for “Flood Aid,” an hourlong primetime telethon tonight that will try to coax viewers to send money to the Red Cross for victims of the Midwest floods.Willard Scott, the “Today” show weatherman, will host the live telecast on location from the banks of the Des Moines River, with cameras, satellite uplinks and other technical support provided by the Des Moines, Iowa, network affiliates KCCI (CBS), WHO (NBC) and WOI (ABC). The executive producer is Bill Anderson, station manager of KGAN, the CBS affiliate in Cedar Rapids, Iowa. Scott will serve as the bridge for a series of five-minute pieces supplied by TV stations throughout the region most affected by the flood damage. A toll-free number tied in to the Red Cross will be on screen for the entire hour. Event coordinator John Sloan, president and general manager of KWQC, the NBC affiliate in Davenport, Iowa, said the taped pieces will graphically show the devastation and place the tragedy in historical context. The biggest TV station to sign up so far, Sloan said, is KSDK, the NBC affiliate in St. Louis. Other network affiliates on board include WHTM Harrisburg, Pa.; WEAR Pensacola, Fla.; and WHEC Rochester, N.Y. He acknowledges that the network that can least afford heavy preemptions on July 29 is NBC, which is shifting its sitcom “Mad About You” to 8 p.m. to get viewers used to its new home for the 1993-94 season. The head of NBC-TV, Pier Mapes, “was agonized when we told him about the time period we were preempting for the telethon,” Sloan said. “But he understands the severity of the situation and told us he knew we had to do something like this.” The closest television station to Los Angeles scheduled to air the telethon is KMIR in Palm Springs.