Late summer Iowa rainfall events in weakly forced environments

This study analyzes convective events that propagate through Iowa, but do not initiate near a major frontal boundary. Storm reports taken from the National Climatic Data Center (NCDC) for the state of Iowa and surface analysis maps from the Weather Prediction Center (WPC) are used to determine if the convective event occurred near a major front. Fifteen cases characterized by the vertical phase reversal of the monsoon circulation were selected for analysis. A synoptic and mesoscale diagnostic analysis is done to gain an understanding of the mechanisms responsible for convection. An analysis of the North American Mesoscale Model's (NAM) forecasting capability for these non-frontal convective rainfall events was also pursued.;A detailed study for one of the 15 cases was conducted to illustrate the characteristics of this type of summer rain-producing disturbance. The rainfall propagates through Iowa in the warm sector southeast of the low-pressure system and south of a warm front. Low-level warm and moist air advection along with convergence at the terminus of the Great Plains low-level jet (LLJ) develop an unstable environment for convection. The Great Plains LLJ is defined as a wind magnitude maximum in the lower troposphere. Systematic and vertical shear below 700 hPa is conducive to create the instability needed for convection. The anticyclone prohibits the perturbation from growing at initiation because of heavy, dry air just above 700 hPa. Strong positive vortex stretching generates convective vorticity to initiate the convection. The semidiurnal wave also proves to create a perturbation in an unstable environment, helping to strengthen the convection or even initiate it. A composite analysis of all 15 cases shows similar results.;The NAM 12 hour forecast is analyzed for its capability to simulate the non-frontal convection accurately. The divergent circulation associated with convection is not forecasted correctly, suppressing (enhancing) the rainfall where it should be stronger (weaker). Analysis of the precipitable water error in the forecast shows that the forecasted moisture in the atmosphere is not correct either. It is either too strong when the rainfall is over-predicted or not strong enough when the rainfall is under-predicted.