DALLAS—A winter storm dropping snow and ice also sent temperatures plunging across the southern Plains, prompting a power emergency in Texas a day after conditions canceled flights and impacted traffic across large swaths of the United States. Rotating power outages were initiated by the Electric Reliability Council of Texas, or ERCOT, early Monday morning, meaning thousands went without electricity for short periods as temperatures fell into the teens near Dallas and 20s (about minus 5 degrees Celsius) around Houston. “We urge Texans to put safety first,” ERCOT tweeted as it urged residents to reduce electricity use. The council manages the flow of electric power in the state. “Traffic lights and other infrastructure may be temporarily without power,” the council said. The council described the rotating outages as a “last resort to preserve the reliability of the electric system as a whole,” adding that utility transmission companies are tasked with determining …