Composites of Heavy Rain Producing Elevated Thunderstorms in the Central United States

Abstract

Composite analyses of the atmosphere over the central United States during elevated thunderstorms producing heavy rainfall are presented. Composites were created for five National Weather Service County Warning Areas (CWAs) in the region. Events studied occurred during the warm season (April–September) during 1979–2012. These CWAs encompass the region determined previously to experience the greatest frequency of elevated thunderstorms in the United States. Composited events produced rainfall of >50 mm 24 hr−1 within the selected CWA. Composites were generated for the 0–3 hr period prior to the heaviest rainfall, 6–9 hours prior to it, and 12–15 hours prior to it. This paper focuses on the Pleasant Hill, Missouri (EAX) composites, as all CWA results were similar; also these analyses focus on the period 0–3 hours prior to event occurrence. These findings corroborate the findings of previous authors. What is offered here that is unique is (1) a measure of the interquartile range within the composite mean fields, allowing for discrimination between variable fields that provided a strong reliable signal, from those that may appear strong but possess large variability, and (2) composite soundings of two subclasses of elevated thunderstorms. Also, a null case (one that fits the composite but failed to produce significant rainfall) is also examined for comparison.

Lightning and Rainfall Characteristics in Elevated vs. Surface Based Convection in the Midwest that Produce Heavy Rainfall

“…Elevated thunderstorm cases were selected for dates between May and September during the years 2007 to 2010. Cases identified in [5] were used. Surface based thunderstorms were only selected if they occurred during the same months and year as the elevated thunderstorm case.…”

“…The studies of [1,2] defined an elevated thunderstorm via the following selection criteria based on observations from stations reporting a thunderstorm: (1) the observation must lie on the cold side of an analyzed front that shows a clear contrast in temperature, dew point, and wind; (2) the station's wind, temperature, and dew point must be qualitatively similar to the immediately surrounding values; and (3) the surface air on the warm side of the analyzed front must have a higher equivalent potential temperature (θ e ) than the air on the cold side of the front. Similar criteria were also used by [3][4][5]19,20] for studies involving elevated thunderstorms and will be used for this study as well.…”

“…Additionally, McCoy [5] investigated over 250 cases of elevated thunderstorms over the central CONUS, which produced composites of conditions that were found just prior to elevated convection. This work proved useful as they provided a list of elevated cases in the time frame of this study.…”

“…A climatology of elevated thunderstorms [1,2] showed that the central United States is the focus for occurrences of elevated convection. Elevated thunderstorm complexes are often associated with heavy rain and can lead to flash flooding [3][4][5]. However, surface based thunderstorms also produce heavy rain and flash flooding.…”

“…The heavy rainfall area is defined as the continuous area in which >50.8 mm·day −1 (two inches) of rain fell in association with elevated thunderstorms or surface based thunderstorms. This threshold was selected due to previous work with elevated convection [5]. All of the cases The results of this study are important as they show that there is a clear difference in mesoscale environmental conditions between elevated and surface based cases.…”