August was the wettest month on record for the area and the records go back about 100 years. However, the phenomenon of a sudden influx of rain leading to flooding happens not infrequently: Hurricane Agnes
in '72, the blizzard of 1/25/79, Hurricane Jeanne
in '04, and T.S. Tammy
in '05 all caused flooding as bad or worse than the recent floods, depending on which waterway one looks at.
Many folks don't know that the USGS maintains near-real-time feeds
from hydrographs on many area rivers and tributaries. If you live near a waterway, consider bookmarking a nearby hydrograph to give you advance notice of rising water levels.
(We sent our landlord a link to a hydrograph so they'd stop calling us to ask whether the house was flooded.)