Abstract: There have been floods in a number of areas of Cilacap Regency on Saturday, October 7, 2017. The flood was caused by extreme rainfall that occurred since Saturday (7/10) in the morning. The rainfall recorded at the Cilacap Meteorological Station falls into the extreme category because it reaches above 150 mm per day. These conditions indicate a significant atmospheric disturbance. Therefore, the incident was interesting to study related to the atmospheric conditions at that time. This study aims to describe atmospheric conditions on a global scale, regional scale, and local scale. The data used include the National Oceanic and Atmospheric Administration (NOAA), which includes Sea Surface Temperature (SPL) and its anomalies and Isobars; from the Bureau of Meteorology (BoM), the value of the Southern Oscillation Index (SOI) and the Madden-Julian Oscillation (MJO) phase diagram, reanalysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) which includes streamline, relative humidity, vorticity, and vertical velocity, surface data from Ogimet and Himawari 8 Satellite data from the Sub Division of BMKG Satellite Image Management. The results of the study showed that extreme rain that caused the flooding was due to the presence of active La Nina, active MJO, shear, and high humidity.

Abstrak: Telah terjadi banjir di sejumlah wilayah Kabupaten Cilacap pada hari Sabtu 7 Oktober 2017. Banjir tersebut diakibatkan oleh curah hujan ekstrem yang terjadi sejak Sabtu (7/10) dinihari. Curah hujan yang tercatat pada Stasiun Meteorologi Cilacap masuk dalam kategori ekstrem karena mencapai di atas 150 mm per hari. Kondisi tersebut menandakan adanya gangguan atmosfer yang cukup signifikan. Oleh karena itu, kejadian tersebut menarik untuk dikaji terkait dengan kondisi atmosfer pada saat itu. Kajian ini bertujuan untuk mendeskripsikan kondisi atmosfer pada skala global, skala regional, dan skala lokal. Data-data yang digunakan antara lain dari National Oceanic and Atmospheric Administration (NOAA), yang meliputi suhu permukaan laut (SPL) dan anomalinya serta Isobar; dari Bureau of Meteorology (BoM) yaitu nilai Southern Oscillation Index (SOI) dan diagram fase Madden-Julian Oscillation (MJO), data renalisis dari European Centre for Medium-Range Weather Forecasts (ECMWF) yang meliputi streamline, kelembaban relatif, vortisitas, dan vertical velocity, data permukaan dari ogimet serta data Satelit Himawari 8 dari Sub Bidang Pengelolaan Citra Satelit BMKG. Hasil kajian menunjukkan bahwa hujan ekstrem yang menyebabkan banjir tersebut dikarenakan adanya La Nina aktif, MJO aktif, pola shear, dan kelembaban udara yang tinggi.