Handout (373.4 kB)
Mid-level relative vorticity and surface precipitation were analyzed using the NCEP North American Regional Reanalysis from 1979-2011. Hovmӧller diagrams were examined to identify time periods with long-lasting, slow-moving vorticity maxima. Six cases were selected for further analysis composite analyses for heavy precipitation cases and dry cases were created for multiple variables of the atmosphere including 500-mb geopotential height anomalies, 925-mb meridional wind anomalies, precipitable water anomalies, and mean integrated vapor flux.
A ridge was found to have been displaced further south for heavy precipitation cases in the 500-mb geopotential height composite anomalies along with uniform distribution of higher heights over the United States. Dry cases displayed a tighter ridge further north and larger gradients between lower and higher heights. Strong southerly flow was evident in the 925-mb meridional wind composite anomalies for the heavy precipitation cases. This flow helped transport moisture from the Gulf of Mexico for three MCV series' that went on to produce heavy floods in the Southern Plains. Sounding data confirms southerly flow and a moist atmosphere for heavy rain events.