Moisture associated with Tropical Depression Barry has already worked into the middle of the nation. In this image (left) of precipitable water, areas in green indicate the very high levels of atmospheric moisture associated with Barry.
Actively-growing crops have likely tapped sub-soil moisture resulting in enhanced evapotranspiration. Weather models are forecasting dew points (a measurement of low-level atmospheric moisture) well into the 70's over the Midwest later this week.
Note that it is historically the warmest time of the year for much of the Midwest, Ohio Valley, and Mid Atlantic regions.