Observations and Predictability Analysis of the 20 April 2015 Xanxere Tornado in Southern Brazil

During the mid-afternoon hours of 20 April 2015, the town of Xanxere, located in mid-western Santa Catarina (SC) state, southern Brazil, was struck by a (high-end) F2 tornado resulting in three fatalities and ninety-seven people injured. Over 2000 homes and buildings were damaged. Observations from METAR reports and automated weather stations (AWS) from the Brazilian National Meteorological Institute (INMET; acronym in Portuguese) are utilized to describe the prevailing surface conditions around the time of the tornado. Satellite imagery from the Geostationary Operational Environmental Satellite 13 (GOES-13) and a few scans from distant meteorological radars are also inspected. The synoptic- and meso-scale atmospheric conditions that led to the development of the Xanxere tornadic supercell are investigated using NCEP Climate Forecast System Version 2 (CFSv2) data and numerical output from Advanced Research WRF simulations (12-km, 3-km and 1-km horizontal grid spacing) using CFSv2 fields for initial and boundary conditions. In order to assess the predictability of the Xanxere tornado in the context of severe convective environments in southern Brazil, a comparison also is performed with a severe weather outbreak that occurred on 7 September 2009 under a much stronger synoptic forcing.

The Xanxere storm developed in a rather weakly forced large-scale environment, characterized by an approximately zonal 500-hPa flow and moderate wind speed at mid- and upper levels. At lower levels, a weak low pressure system extending from Paraguay to extreme northeastern Argentina and western SC induced a corridor of stronger northwesterly winds at 850-hPa from the Amazon Basin to western portions of SC. Within that region, NW-SE-oriented horizontal convective rolls were observed a few hours prior to the tornado development. GOES-13 imagery showed that a thick cirrus deck from a previous MCS that developed in southern Brazil earlier that day reduced surface heating in western SC holding afternoon temperatures and dew point depressions (thus, leading to low LCLs), as confirmed by surface observations. In addition, CFSv2 data suggest a possible role played by the MCS in locally modifying the low-level northwesterly flow over SC. Numerical output from ARW-WRF is used to assess the hypothesis that such interaction between synoptic- and meso-scale circulations in mid-western SC in the afternoon may have influenced convective initiation and may have affected the atmospheric environment leading to the formation of the tornadic supercell.