Our comfortable weekend and our very comfortable start is evident in the fact that there's very little moisture in the atmosphere. Surface dewpoints are in the 40's today and were in the 40's and 50's for a large chunk of the weekend (Sunday's brief jump into the 60's notwithstanding). Looking at my "friendly" PWAT graphic, you can see how little moisture there was in the atmosphere this morning -- around a half inch throughout the entire column of air. By September standards, that's pretty dry air.
That will change -- look out to the west over the Great Lakes and Minnesota where PWAT's quickly climb to over 1.5 by the time you reach Michigan and Ohio. That pooling of moisture is in front of a low pressure center in the Dakotas that will push east over the coming days, dragging a cold front eastward with it.
That moisture, plus additional Gulf moisture, will lead to a more humid atmosphere in the region starting tomorrow as dewpoints jump into the upper half of the 60's...and then into the 70's in a few spots by Wednesday. Wednesday also serves as a pretty good shot at getting to 90 degrees as modeling continues to show the potential of heat, west-southwest winds, and enough sunshine for many of us to sweat our way to that threshold. There will be a scattered risk for thunder in the region on Wednesday but a better risk for storms around here on Thursday as a frontal boundary approaches.
With the increased moisture in the atmosphere and the boundary nearby, the potential for heavy rainfall will be on the increase. Some of the scattered activity on Wednesday could produce heavy rainfall as well but the best chance for the majority of the region to get thundered upon is on Thursday with the front's nearby positioning. With PWAT's over 1.75 and pushing 2, that potential could yield some real soakers on the order of what occurred this past Labor Day around parts of the region.
As always, we will update this in the days to come but be aware that the comfort of today is going to quickly go out the window in short order as humidity and heat push on in.