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ABSTRACT This study shows a synoptic climatology of warm fronts in Southeastern South America ( SESA). Data from Climate Forecast System Reanalysis ( CFSR) was used to identify warm fronts from 1979 to 2010. The identification method was based on the magnitude of meridional gradient of 850- hPa equivalent potential temperature ( θe) and 850- hPa wind fields. Composites of the most important atmospheric variables were constructed from 1 day before until 1 day after the formation of the warm front. An average frequency of two warm fronts per month is observed, with higher frequencies in austral winter. Most warm fronts precede the formation of extratropical cyclones over Uruguay and form because of the southward movement of previous cold/stationary fronts. Warm fronts form on average around southern Paraguay, northeastern Argentina and western part of southern Brazil and Uruguay, coupled to the eastern edge of the Chaco Low ( CL) and the Northwestern Argentinean Low ( NAL) where north/northwesterly flow predominates. An upper-level wave of wavenumber eight supports warm frontogenesis. Location and intensity of synoptic systems associated with a warm front event differ from winter to summer. Elevated instability is commonly present near warm fronts, and the average warm-front slope is 1 : 110, agreeing with other studies. Instability indices increase after the warm-front passage, leading to greater rainfall 1 day after the warm front forms. [ABSTRACT FROM AUTHOR]