Numerical weather modeling is a quickly and continuously growing area of meteorology, and with this growth comes a rising dependence on the models as a forecasting tool. This reliance places great value on both the quality of the model forecasts, as well as model forecast improvement. Precipitation, especially of the convective type, affects the public considerably and it must therefore be predicted as accurately as possible. Unfortunately, quantitative precipitation forecasting (QPF) has been shown to the weakest parameter of numerical weather prediction. Therefore, it is important for forecasters to learn to interpret observations and model output so that they can anticipate flooding events even when the model forecasts fail.

An example of a high-impact precipitation event occurred over Wake County on the 4th of July holiday in 2001, producing flash flooding, frequent lightning and up to 6 inches of rain. This event was not accurately predicted by numerical models and it had major impacts on the public, as people were caught outdoors and unprepared at holiday events. The objectives of studying such a significant, high-impact event were to understand the basic factors that resulted in such focused, heavy convection. It was determined that the focus for heavy thunderstorm activity was related to a cold front, a sea breeze, environmental instability, outflow boundaries that propagated convection, and the presence of other surface-driven/mesoscale boundaries. Additionally, observations, model output, and sensitivity test results were examined in order to identify signatures that can alert forecasters to flash flood potential in the future, even when model QPF guidance is lacking.

Results reveal that because it is highly unlikely that a numerical model can represent such a focused and rare event, forecasters must instead look to observations and environmental factors that possibly signal flash flooding, even in the absence of trustworthy model QPF guidance.