Warm-season precipitation plays an important role in the global hydrological cycle. Unfortunately, the progress in warm-season quantitative precipitation forecasts (QPFs) has been particularly slow due to the dominant weak dynamical forcing. Nevertheless, it is encouraging from recent real-data modeling studies that the warm-season QPFs could be significantly improved by simply incorporating high-grid resolution and realistic model (cloud and boundary-layer) physics. Similarly, recent regional climate modeling studies showed that the warm-season QPFs at the monthly to seasonal scales could be improved by using reasonable diabatic physics and land-surface parameterizations.

In this study, we use the PSU/NCAR MM5, coupled with the simplified Simple Biosphere (SSiB) scheme, to study the weakly to monthly predictability of precipitation that occurred during June 1998 over the Large-Scale Area - East (LSA-E). Major features of the model include: a) a two-way, triply nested-grid (45/15/5 km) technique with the finest 5 km resolution to cover the most part of the LSA-East; b) the simultaneous use of the most recent version of Kain-Fritsch (1990) convective parameterization and simple ice microphysics package; c) the simplified SSiB; and d) the MRF boundary layer scheme. The outmost mesh covers most of the North American continent. A total of 30 vertical layers is used. The model is Initialized at 0000 UTC 1 June and continuously integrated up to 30 June 1998. The initial and the outermost lateral and bottom boundary conditions are specified from NCEP's Eta analyses, including the initial soil surface temperature and canopy temperature.

In general, the model captures quite well all the rainfall events, with the distribution and magnitude similar to the observed. The simulated time series of the area-averaged maximum and minimum surface temperatures over the LSA-East are also in good agreement with the observed. The model captures almost all prominent rainfall streaks with reasonable timing, locations, duration and propagating speed, except for the rainfall to the west of 900W where a nocturnal maximum was observed. The eastward propagation of the rainfall originated from the north of the Mississippi River basin is well reproduced. Some regions appear to have a preferential period of precipitation in the diurnal cycle. Of our major interest is that the timing and intensity over the LSA East are generally consistent with the observed.