Saturday was the perfect summer day with the high at Philly International Airport being 83 shortly before 4pm(albeit the average high should be 87); there was plenty of sunshine and during the morning there was a northwesterly wind between 10-15mph, but then subsided by the afternoon.
Overnight will be clear and cool with lows in the lower 60s(the average low is 69) as the wind shifts to the west-southwest. Sunday will be continued sunny with some fair weather cumulus clouds in the afternoon as we reach our convective temperature(the estimated temperature that the surface must warm to for clouds to develop). Highs will be in the middle 80s.
It's not until Monday we'll get more a southerly flow and temperatures will respond as highs will be in the lower 90s; the humidity will begin to return as well and there will also be a short wave trying to push through. This will result in some isolated showers and/or thunderstorms to pop up in the afternoon(primarily to the north and west).
Come Tuesday we'll have a cold front approach from the west; the day will start off hot and humid with highs once again in the lower 90s, but then have showers and thunderstorms by the afternoon. It will linger into the overnight with lows in the middle 70s as the front stalls on top of the region. This means that Wednesday will feature on-and-off showers and thunderstorms with highs still in the lower 90s(making it a heat wave as we'd have three consecutive days of temperatures 90 degrees or higher); the front will eventually push eastward in the early morning hours on Thursday so the afternoon shall be drier with high pressure moving in; sunshine and clouds will be on tap with highs in the upper 80s
Now be sure to check out the rest of the 7-day forecast(you can any time from anywhere ) at www.scottderekwx.com/forecast.html.