Extreme Cases of Vertically Integrated Moisture Flux Convergence and their Relationship to Precipitation in the US Midwest

The Midwest US is prone to receiving heavy precipitation events, including Mesoscale Convective Systems (MCSs), during the warm season. These MCSs may produce rainfalls, in a 1-2 day period, with amounts well above the monthly climatology. The ability to recognize the patterns and mechanisms that accompany these convective systems is crucial for forecasting their timing and location. Moisture Flux Convergence (MFC) is a useful tool in short-term prediction of convection since it contains information on the low-level forcing and the amount of moisture available. However, if there is no support in the upper levels then the likelihood of thunderstorm development is significantly diminished. Using the NARR (North American Regional Reanalysis) data set, the vertically-integrated MFC was computed every 3 hours during a 36-year period (1979-2014) through the months of May to August. A 24-hour running mean was applied to the 3-hourly vertically-integrated MFC, and the 99th percentile was selected. An additional criterion ensuring synoptic independence (five days before/after the largest event) was applied, yielding 72 cases. The low-level moisture transport (1000-700hPa) was analyzed for each case, facilitating the classification of cases. Classifications include 1) southwesterly, 2) high amplitude, 3) low amplitude, 4) cold surge, and 5) southeasterly flow. The overall atmospheric circulation structures differ substantially among the categories. Perhaps surprisingly, there were several cases that actually showed very little precipitation. Further analyses of these light precipitation cases are being studied.