By Brendan O’Brien CHICAGO (Reuters) – A heatwave will bake the southwestern and central United States starting on Monday when record-breaking temperatures are expected to soar above 100 degrees (38C), with forecasters warning people to stay indoors and drink plenty of fluids. A large swath of the nation, stretching from central Nebraska to West Virginia, north into Wisconsin and south into Mississippi will see the heat index rise from Monday morning until midweek, the National Weather Service said. Midwesterners who woke up on Monday to mild early summer conditions could see showers and thund…