The frontal boundary that's to our south will gradually fizzle as we work into the latter portions of the week. While more humid air will nudge northward as the front dies away, a disturbance will push through the Ohio Valley and into the Great Lakes on Saturday and Sunday. This disturbance will help pull some moisture-rich air northward into Pennsylvania and agitate the atmosphere to allow for thunderstorm development.
One of the indicators that forecasters look at for precipitation potential is "PWAT" or "Precipitable Water"...it's a basic indicator that shows how much rain per hour the atmosphere can generate. PWAT's over two inches suggest the potential for heavy rain in a short period of time and can result in localized flooding, especially in heavier thunderstorms. The GFS (below) show PWAT's for Saturday afternoon across Maryland at and over two inches, with those two inch plus PWAT's moving overhead Saturday night and into Sunday as the disturbance pushes by to our west.
You can see the thunderstorms and rainfall on the GFS below for Saturday night and early Sunday. This would continue into Sunday across our region if the model is right. The Euro also is hinting at this in today's runs although the best support and dynamics would be to our west and north....not necessarily overhead. That said, the weekend does promise to be more humid as this moisture-rich air moves in place.
Predicting how much rain falls and where thunderstorms pop this weekend...at least from a Tuesday timeframe, is a tough task since we're dealing with a system that's five days out. That said, the potential exists for some areas to get some needed rainfall...perhaps a case of too much, too quick in some locations if this trend of very moisture-rich atmosphere continues to hold serve.