Dew point depression (DD) is useful indicator of how moist the air is and is calculated by taking the difference between the temperature of the air and the dew point temperature. Lower dew point depression values mean that the air is very moist, and an increased likelihood that clouds will develop. In the 700mb map below, the lowest DD values are located over the Southeast (shaded in yellow).
Examination of the visible satellite image from the same time reveals an extensive cloud deck present over the Southeast, corresponding very well with the low DD values identified above.
Also observe the lack of clouds over Texas and Louisiana, areas of much greater DD, indicating drier air.