Notice how the dewpoints are in the upper 60's/lower 70's across the Midwest and Ohio Valley, but only in the 50's and low 60's further south. Return flow has not yet developed fully afer the last cold front came through, hence the low dewpoints in the south. The corn belt contributes substantially to the surface moisture in the Midwest during the summer, especially during years of heavy rain and high vegetation coverage (like this year's record rains, cool summer and record corn crop). A few weeks ago, parts of IL, IN, MO and IA had dewpoints in the lower to mid 80's, which is almost unheard of, even in the tropics. The models often have a really difficult time getting the dewpoints up there because land surface models need a lot of improvement. This can ultimately affect their forecasts of precipitation, as just a little extra surface moisture can provide the instability to initiate convection. Another reason for the higher dewpoints could be due to the fact that moisture is pooling along a cold front. Still, the largest contributor at the moment is evapotranspiration from corn, soil and soybeans.
