After a very nice Friday and Saturday, there will be a slow descent in the weather tomorrow as a frontal boundary approaches us from the west. The city and southeast should stay dry tomorrow but the farther west one goes the better the chances of rain and thunder. This front will be slow to progress through the region -- it is May and fronts tend to get hung up over our region as they lose punch thanks to the Bermuda High establishing itself and the last gasps of the northern jet -- and the result is another shot at steady rains for the Northeast for Sunday (to our west) and Monday and Tuesday (overhead).
Sunday's main threat is afternoon and evening shower activity for those in Central and Western Pennsylvania as the front approaches the region and then starts to slow down. Moisture that is currently in the Southeast will push northeast along this boundary and pass through Pennsylvania tomorrow afternoon, perhaps getting into the far western suburbs by evening (if the NAM is right). Thankfully, city and points east should stay dry and have a pretty decent Sunday to trudge through although the weather may not be as nice as it is today.
The front slows further as we move into Monday and low pressure may organize along the front. This area of low pressure has help trigger steadier and heavier rain and thunder chances for Monday night and Tuesday around here. Both the GFS (below) and Euro show this scenario setting up, with rainfall potentially exceeding 1.50" between Monday/Tuesday in parts of the region. This *could* extend into Wednesday if the GFS is right...it is slow in pushing this front off the coast and fires another wave of energy along the front that brings more rains on Wednesday around here.
Since we're still working a rainfall deficit (a diminishing one, mind you) for the year the rains are still appreciated...but heavy rainfall that pushes past the 1.50" mark might lead to some ponding of water and localized stream floods in spots. The good news in all of this is severe weather potential is limited...this is just an old-fashioned rain and thunder event moving on in to start the new week on a soggy foot.