Heat and humidity continues to build from the Northeast into the Plains with more than a dozen states under heat alerts. Excessive Heat Warnings include St. Louis and Boston as the heat index climbs from 100 to 110. The mercury will reach well into the 90s from New England to the Mississippi Valley. High temperatures will hit 10 to 20 degrees above average today (Tuesday). A cold front will move only slightly further south, through the Ohio Valley on Wednesday. Highs may approach the century mark near Boston and New York, as the heat index climbs into the triple digits. High temperatures will be 20 to 25 degrees above average in parts of the Northeast. As the cold front pushes further south on Thursday, highs will drop out of the oppressive range from the Northeast to the Tennessee Valley. Temperatures will still reach into the 90s from the Middle Atlantic to the Gulf of Mexico. For WeatherNation: Meteorologist Mace Michaels