Use of nwp for nowcasting convective precipitation: Recent progress and challenges

The demand for accurate nowcasts of convective precipitation has driven the development of high-resolution data assimilation and rapid cycling numerical weather prediction. The least complex form of nowcasting involves predicting storm evolution by extrapolating radar reflectivity echoes with or without the use of trends in echo size and intensity. Adding a bit more complexity is the so-called expert systems3 that attempt to nowcast storm initiation and dissipation in addition to echo extrapolation. Since it has been shown repeatedly that NWP models generally produce superior quantitative precipitation forecasts (QPF) than nowcasting systems beyond a few forecast hours, it is logical to blend radar echo extrapolation with a numerical model to generate a seamless 0-6-h forecast. The corrected model forecast skill exceeds that of extrapolation at a forecast lead of four hours. Key to this level of model skill is the fact that latent heat estimated from radar reflectivity data is used to provide the model improved initial conditions wherever storms are present.