Isaac lashed Louisiana last week with heavy rain, gusty winds, and painstakingly slow movement as it made landfall. It's still in the Midwest, having moved little over the past few days and spinning up rounds of rain and thunderstorms across the Midwest and the Ohio Valley. As much as 20 inches of rain fell in the New Orleans area, with several inches common into Missouri and Illinois.
Some of that rainfall will work its way east in the coming days. Not the 20 inch variety, thankfully, but we have a shot at rounds of heavy rain over the next few days as we get pieces of energy from Isaac's leftovers. These will move east ahead of a frontal boundary that will take until perhaps Thursday morning to clear our region. Thunderstorms are in the forecast daily through Wednesday and this may also need to include Thursday as my optimistic thinking is that the front will get out of here by then. However, trends have been fickle about when the front finally clears. It will not rain at all times through the next few days — scattered showers and thunderstorms should be expected, though, with the greatest coverage of thunderstorms in the afternoon and early evening hours.
Once the front finally clears, humidity will lower and temperatures will be nice for the latter portions of the week. However, another low will develop in the Ohio Valley over next weekend and bring rain back to our forecast, perhaps for next Sunday.
The good news, despite the rain chances over the next several days, is that we may be 90s-free this week after hitting it last Friday and Saturday around the region. Philadelphia is up to 39 days of 90 degrees or warmer, which is tied for the 9th most on record in Philadelphia. September typically provides two 90 degree days in the course of the month, with this past Saturday already knocking one of those two out. It's possible we'll end up with our second year in three with at least forty 90-degree days before all is said and done.