Predictability of Tropical Cyclone Genesis in the IPRC Regional Model Y. Wang1, R. Zhan2, C.-C. Wu3, and Yi Lu3 1IPRC, 2Shanghai Typhoon Institute, 3National Taiwan University The predictability of tropical cyclone formation over the western North Pacific was studied in an ensemble of four 20-year simulations of the region’s atmospheric circulation during tropical cyclone season conducted with the IPRC Regional Model (iRAM). Forced with the same observed sea surface temperature and lateral boundary conditions, the ensemble members differed only in their initial conditions. Analyses show the model has skill in simulating the number of tropical cyclones that were observed to form each month, but the individual simulations have a large spread. This indicates that the internal variability in the atmospheric circulation limits the predictability of cyclone formation in the western North Pacific. The ensemble mean was usually more accurate in hindcasting the number tropical cyclones during each of the 20 years than any individual realization.   Left: the average number of TCs that formed from July through October over 20 years in each of the simulations, in the ensemble mean, and in observations from the China Meteorological Agency. Right: the same variables but for the 4- month average for each of the 20 years.