A SYNOPTIC CLIMATOLOGY OF EXTREME UNSEASONABLE FLOODS IN THE SOUTHEASTERN UNITED-STATES, 1950-1990

Unseasonable floods are floods that occur in the season of lowest floo d frequency, or dry season. Such floods pose a unique problem to flood planners and forecasters, yet little research has investigated the ph ysical processes associated with unseasonable floods. The purpose of t his study is to construct a synoptic climatology of extreme unseasonab le floods for the southeastern United States. Results indicate that th e types of storms creating unseasonable floods are location specific, with four unique regions across the study area: Carolina (tropical sto rms/hurricanes), Georgia Coastal Plain (Gulf depressions), Gulf-Atlant ic (frontal), and Tennessee (frontal with upper-air enhancement). The precipitation created by these storms is low to moderate, rarely excee ding the 10-year 24-hour storm total. The precipitation levels suggest that a combination of meteorological conditions and land-surface cond itions create the extreme events. A statistical analysis indicates tha t high soil-moisture levels combine with the moderate rains to produce extreme unseasonable floods.