Reuters) - Relentless heat was forecast for much of the eastern United States for a fourth straight day on Monday, after violent storms killed at least 15 people and knocked out power to more than 3 million customers. "Hot and hotter will continue to be the story from the plains to the Atlantic Coast for the next few days," the National Weather Service said. Emergencies were declared in Maryland, Ohio, Virginia, West Virginia and Washington, D.C., during the weekend because of damage from storms that unleashed hurricane-force winds across a 500-mile (800-kilometre) stretch of the mid-Atlantic region. Almost 2.4 million people from Illinois to Virginia were still without power Monday morning, with the biggest concentration of outages in the Washington, D.C. area.