A strong statistically significant spectral peak with a frequency of 7--8 years is computed for tree-ring reconstructed summer PDSI averaged at continental and regional scales over North America for the period 1645--1990. A similar, though non-significant spectral peak is computed for instrumental summer PDSI averaged over North America for the shorter period from 1900--1990. The winter NAO index (1781--2002) has a very strong spectral signature at this same 7--8 year sub-decadal frequency and is coherent with summer PDSI across a broad sector of the central U.S. Composite analyses confirm a PDSI response to extremes of the NAO over the central U.S., with drought prevalent during negative extremes, and wetness during positive extremes. The winter NAO index leads summer PDSI and a contingency table analysis indicates that extrema in the winter NAO index may have modest forecast value for the following spring-summer moisture regimes over the central U.S.