Although often hard to correctly forecast, mesoscale convective systems (MCSs) are responsible for a majority of warm-season, localized extreme rain events. This study investigates displacement errors often observed by forecasters and researchers in the Global Forecast System (GFS) and the North American Mesoscale (NAM) models. Using archived radar data and Stage IV precipitation data from April to August of 2009 and 2010, MCSs were recorded and sorted into unique six-hour intervals. The locations of these MCSs were compared to the associated predicted precipitation field in both models using a basic “eye-ball” test and the Method for Object-Based Diagnostic Evaluation (MODE) tool, produced by the Developmental Testbed Center.

Both tests discovered a northward bias in the location of the forecasts in all lead teams of both models. The MODE tool found that 72% and 84% of the forecasts were too far to the north of the observed rainfall in the GFS and NAM models respectively. The GFS model consistently moved the MCS too quickly with 65% of the cases located to the east of the observed MCS. While no clear relationship exists between forecast error (distance from observed) and forecast area, the forecast error from the GFS and NAM were on average 266 km and 249 km, respectively.