The heat wave that has been broiling the Midwest began intensifying in the Northeast, with temperatures expected to approach or climb past 100 degrees Fahrenheit (38 degrees Celsius) in cities including New York, Philadelphia, Boston and Washington.The heat index -- which measures how hot it feels based on temperature and humidity combined -- was even more merciless. The National Weather Service warned the index could peak as high as 115F (46C) in the mid-Atlantic region.The intense heat was of particular concern given the Fourth of July celebrations that include many outdoor activities like barbeques and fireworks displays. The heat wave also coincides with a string of weekend World Cup matches."This level of rare and long-duration heat, with little or no overnight relief, affects anyone without effective cooling and/or adequate hydration," warned the National Weather Service. Nighttime temperatures across the Northeast are not anticipated to fall much below 80F (27C).Authorities across the country urged people to stay indoors especially in the hottest parts of the afternoon, to check on their neighbors, drink more water than usual, and to find air conditioning if they don't have access at home.