In this age of instant communication via cellphones and round-the-clock cable TV, when severe weather hits, the lives of many Americans are still partly dependent on the ear-splitting blasts from an old technology: tornado sirens. Six people were killed when a tornado rolled through Woodward, Okla., this month. Some of the town’s tornado sirens failed after a lightning-caused power outage from storms earlier that day. But many lives were saved in other parts of the Plains in part by tornado sirens.