The quality of MM5 ensembles is evaluated for short-range probabilistic quantitative precipitation forecasts over Arizona during the Southwest monsoon. The sensitivity of different ensemble constructs is examined with respect to analysis uncertainty, model parameterization uncertainty, and a combination of both. Model uncertainty is addressed through different cumulus and planetary boundary layer parameterizations, and through stochastic forcing representative of a component of subgrid-scale uncertainty. A first-order autoregression model adds a stochastic perturbation to the Kain-Fritsch cumulus scheme and MRF planetary boundary layer scheme. A sensitivity study is also conducted to determine the MM5 planetary boundary layer parameterizations capable of simulating the structure of the pre-convective, monsoon atmospheric boundary layer. The results indicate that ensemble precipitation forecasts are skillful and may assist operational weather forecasters during the monsoon. The most skillful ensembles contain both analysis perturbations and mixed-model physics. The Blackadar or MRF planetary boundary layer schemes are recommended for MM5 simulations or forecasts of the Southwest monsoon.
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