Earth and Atmospheric Sciences, Department of


Date of this Version



National Weather Digest (December 2007) 31(2): 137-152


United States government work


During the early morning hours of 6 July 2002, a mesoscale convective system (MCS) traversed southwestern Nebraska and produced more than 40 cm of precipitation, resulting in a flash flood that closed Interstate 80 and caused one fatality near Ogallala, Nebraska. Regional climatology yields that this flash flood ranked first in precipitation amount for a 24 hour period over the past one hundred years. Synoptic and mesoscale features similar to other flash flooding events and conducive to extremely heavy precipitation were in place over the Central Plains, including a weak upper level ridge, high precipitable water values (180% of normal), significant moisture advection and weak mid level flow permitting slow storm motion. Somewhat unique to this flash flood case was the lack of a “traditional” initiation mechanism, such as a synoptic front or distinct boundary often observed in other cases. Convective initiation was aided by an upslope flow component along terrain in northwestern Kansas as well as an outflow boundary from pre existing thunderstorms that developed earlier that day. The existence of a low-to mid level moisture and instability axis and low-level jet were noted as well, which aided to focus and maintain convection over Ogallala. An analysis of the synoptic and mesoscale environments responsible for producing the flash flooding convection is presented.