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The present study examines urban impacts on short-duration precipitation in an idealized coastal city adjacent to a mountain range using two-dimensional idealized simulations performed with a modified version of the Weather Research and Forecasting (WRF) model. A unique aspect of this study is that an ensemble simulation approach with 243 members is used with sensitivity experiments to reduce uncertainty arising from nonlinearity in the precipitation simulation and to evaluate the statistical significance. Results of the experiments reveal that urbanization increases the precipitation leeward of the urban area and decreases it in the area further inland. These anomalies are statistically significant at the 99% level. Additional sensitivity experiments show that the urban influence on precipitation is slightly sensitive to the mountain height, urban scale and soil moisture, but not to the roughness length. Our analyses indicate that these precipitation changes arise because urbanization slows the sea-breeze front, shifting the moisture convergence zone further inland. Additional analyses are done using the water-budget equation and a linear theory on the diurnal variations of the planetary boundary layer. Results show that changes in the sea breezes and moisture transport arise from a decrease in surface pressure over the urban area associated with an increase in the surface sensible heat flux.