(Reuters) – A unrelenting heat wave on Thursday was expected to scorch the U.S. East Coast and South for the third day as dangerously high temperatures had forecasters warning about the dangers of dehydration and exposure. Heat warnings and advisories were in effect from South Carolina up into parts of New England as heat index values were expected to reach 110 degrees Fahrenheit (41 C) in Washington D.C. and near 100 degrees in New York City on Thursday, the National Weather Service (NWS) said. The U.S. heat wave follows conditions in Europe this week that have touched off wildfires and set r…