Flash Flood event over central Argentina: a case study Paola Salio (1), Yanina Garcia Skabar (2) and Matilde Nicolini (1) (1) Departamento de Ciencias de la Atmósfera y los Océanos. Centro de Investigaciones del Mar y la Atmósfera. CONICET – UBA (2) Servicio Meteorológico Nacional. Cátedra de Climatología y Fenología Agrícolas. FA – UBA. A primary goal of the present work is: to describe the synoptic and mesoscale characteristics of the environment associated to a flash flood over the central region of Argentina, with a special emphasis in the relationship between the behaviour of convective precipitation and the evolution of the low level jet. In order to achieve this objective a numerical simulation is performed considering a version of the Brazilian Regional Atmospheric Modeling System (BRAMS), that include a microphysics scheme and explicit convection in the finest resolution grid and estimations of precipitation are considered in order to detect convective and stratiform precipitation areas. Methodology: Estimation of Convective and Stratiform Precipitation - IR Brightness Temperature (235 K contour) to determine precipitation areas - hourly CMOPRH Precipitation Estimation - FORTRACC technique to following the Mesoscale Convective System Stratiform precipitation < 7.5 mm h -1 Convective precipitation >= 7.5 mm h -1 Evolution of the environmental conditions March ZMarch ZMarch ZMarch ZMarch Z Upper pannel: Reflectivity (shaded), ageostrophic wind (barbs) and vertical velocity (contour). Lower Pannel: Equivalent Potential temperature (shaded), ageostrophic wind (barbs) and meridional ageostrophic wind (contour) Reflectivity 850 hPa March Z March Z 235 K covered area in km 2 by the mesoscale convective systems Convective and Stratiform Precipitation estimated by the combination of CMORPH and IR area CMORPH BRAMS Grid 3 3 hr - Accumulated Precipitation and reflectivity Stratiform, convective and total precipitation estimated for all systems that affect the central and northern region of Argentina between March 21 to April 1, Large Scale Conditions 200 hPa Streamlines and wind intensity 500 hPa Geopotential and 24-hour tendency 850 hPa Geopotential and 24-hour tendency March 27 12Z Vertically Integrated Moisture Flux Large scale is dominated by:  a strong trough centered on 75°W  a strong trough centered on 75°W, that remains stationary over the whole studied period.  a cut-off  Associated to the trough a cut-off low remain stationary over the coast of Chile generating large scale upper motion over central Argentina.  lowover northwestern Argentina.  The availability of moisture is favoured by the presence of a low over northwestern Argentina.  moisture flux presents an anticiclonic gyre  The moisture flux presents an anticiclonic gyre and supply moisture from the Atlantic Ocean, this situation differs from the classical low level jet situation associated with convection. Methodology: BRAMS simulation BRAMS 3.2 Brazilian Regional Atmospheric Modeling System version 3.2 Simulation period : March 2007 from 25 to 31 at 12UTC Two-way grid interactive nesting technique Number of atmospheric levels: 30; vertical cordinate: shaved eta Horizontal Resolution: Grid km Grid km, Grid km GDAS analyses from NOAA/NCEP as initial and boundary conditions. Model includes topography data (1km resolution) terrain land use (1km resolution), soil types (50km resolution) weekly sea surface temperatures daily soil moisture heterogeneous fields from USP/CPTEC Parameterizations: Shallow cumulus:Sousa y Silva; Deep convection: Grell; Explicit convection on Grid 2 and 3 Radiative: Chen and Cotton; Horizontal diffusion:Smagorinsky; Vertical diffusion: Mellor-Yamada; Microphysics: 7 water species, bulk water scheme Observed, Estimated and Modeled Precipitation Observed CMORPH estimations Total acumulated precipitation between Z to Z BRAMS Grid 2BRAMS Grid 3 Observed precipitation over central Argentina (red), CMORPH (left) and models forecast by (right) different grids accumulated between March 26 to 31, Grid 1 (light blue) represent the precipitation forecast for 50 km resolution, Grid 2 (yellow) for 12.5 km and Grid 3 (bordeaux) for km respectively.  CMORPH estimation denotes an excelent performance over the studied are. Values of precipitation are overestimated close to Buenos Aires.  The accumulated precipitation simulated by BRAMS results in most locations very close to the observed values, but in some locations as in Rosario simulated rain is significant overestimated, reaching nearly twice the observed value on grid 2 and 3.  Although the model evidences some spatial and temporal misplacements in maximum precipitation respect to CMORPH estimation, accumulated precipitation during the whole period are properly represented by the model. Convective and Stratiform Activity: Mesoscale convective activity tend to generate during the beginning of the night and decay during the day. The maximum extension of the systems varies from small systems to the bigger one on March 29 at 8Z that cover all area, and shows also developments over northwestern Argentina. Strong convective rates overpass by three times the total stratiform precipitation. Systems during the rest of the period present an equivalent total stratiform and convective precipitation but, in general, convective maximum precipitation occurs before the stratiform precipitation. Environment The thermodynamic environment is characterized by strong CAPE, low CINE and the presence of a deep flow from the north that shows a LLJ profile. The relationship between the LLJ and the development of convection, during an extreme of convective precipitation shows:   deep layer of unstable air toward the storm.   an increase in the northerly flow at the moment of the developing of the storm. AKNOWLEDGMENTS: This research is supported by UBA grant X266, ANPCyT grant Nº PICT 07 – and CONICET PIP N° First author would like to thank to the Organization Committee of 15 ICCP for the support to participate in the conference