The combination of heat and humidity on Tuesday and Wednesday is such that the National Weather Service has bumped the region into an excessive heat warning, with heat advisories out (for now) in the Lehigh Valley and Central Pennsylvania.
Tuesday and Wednesday bring the potential for temperatures in the middle and upper 90's, with heat index values that could hang around 100 tomorrow afternoon at peak heat in the region as dew point values will be in the lower half of the 60's. Nothing terribly humid but it will be a hot day. Wednesday will feature more humidity as a cool front approaches from the north. As increased moisture pulls northward ahead of the frontal boundary, the combination of heat and humidity could result in heat index values of 100 to over 105 for the Delaware Valley and points south.
The front's approach on Wednesday will increase the chances of thunderstorms for the city and points north during the afternoon and evening hours. Since this front will be pushing against heat and humidity and because the front will be a bit more robust than your average July front, the potential for strong and severe storms is pretty high for some locations in the region. Given timing and the front's slower movement in recent modeling, odds favor north of the city on Wednesday afternoon, with the city getting a shot of thunderstorms Wednesday night, and south of the city on Thursday. Heavy rainfall is again a threat in some of the thunderstorms. Those storms will signal the end of the heat as lower humidity and less oppressive temperatures return for the end of the week.