Heat advisories are in effect for much of the Acela Corridor in the Northeast, including Philadelphia, Newark, parts of the New York City metro, Hartford, Conn., Providence, R.I., and Boston, where heat indexes — reflecting how hot it feels factoring in the air temperature and humidity — could range between 100 and 105 degrees. It’s the first heat advisory issued during the month of May by the National Weather Service in New York dating back to at least 2006; the same is true for the Boston office.