EVALUATION OF A HYDROMETEOROLOGICAL FORECAST SYSTEM FOR THE METROPOLITAN AREA OF SÃO PAULO A. J. Pereira Filho 1 , R. Hallak 1 , F. Vemado 1 1 University of São Paulo, DCA/IAGUSP, apereira@model.iag.usp.br ABSTRACT A Hydrometeorological Forecast System (HFS) has been implemented for Metropolitan Area of São Paulo (MASP) to mitigate the effects of severe rainfall events. This work shows an evaluation of ARPS high resolution precipitation forecasting and MXPOL rainfall nowcasting over the MASP for the rainy seasons of 2008 and 2009. Significant improvements of ARPS rainfall forecasting have been accomplished by using explicit cloud microphysics. On the other hand, the MXPOL higher sensitivity and polarimetric measurements have increased the leading time on the inset of heavy rainfall convective systems by monitoring boundary layer features associated with local circulation in the MASP. 1. INTRODUCTION The Metropolitan Area of São Paulo (MASP) is one of the largest urban environments of the planet with a population of nearly 20 million people. It is located within the Alto Tietê River basin. Flash floods in the summer and heavy pollution in the winter are a common place (Pereira Filho et al, 2005). There are hundreds of flood prone areas in the MASP. Studies of convective events have shown higher rainfall accumulations and higher probabilities of heavy precipitation in the MASP. Such events are related to its heat island and sea breeze circulation. Heavier rainfall events tend to produce higher density lighting flashes, strong wind gusts, shear, and turbulence. Most significant rainfall episodes are related to squall lines (spring), fronts (winter) and thermal convection (summer) or a combination of them, especially during the warmer months. Recently, the State Government invested in new observing platforms such as a mobile polarimetric radar (Pereira Filho et al., 2007) with higher spatial and temporal resolution measurements as well as computing platforms to diagnose and to predict severe weather. This work evaluates these two components of the hydrometeorological forecast system tailored for the MASP. 2. DATA AND METODOLOGY 2.1. The ARPS model Pereira Filho et. al. (2005) describes the computational system used to run the ARPS model in the operational switch. In short, the system consists of 16 P4/3.6GHz processors. The ARPS model is run by 48 hours in two different resolution grids, 12 km and 2 km (nested grid). Both grids are basically configured in the same way, using complete physics and the global forecasting system (GFS) analyses data as initial and boundary conditional. More details can be found in Pereira Filho et. al. (2005).