Active Northeast Monsoon over India during 2015 - An Assessment of Real-Time Extended Range Forecast

Associated with strong El Nino, the southwest monsoon rainfall over India during June to September 2015 was deficit, while the northeast monsoon (NEM) during October to December 2015 over the southern peninsula was very active, particularly during November and early December. Associated with the active phases of NEM, southeast India, especially Tamil Nadu (TN) and Puducherry experienced unprecedented rainfall activity leading to devastating floods over TN, with the megacity of Chennai being the worst affected. The present study discusses the performance of operational extended range forecast (ERF) up to three weeks of this unprecedented NEM rainfall activity over the southern peninsula and TN using the bi-model average (BMA) ERF based on outputs from the Japan Meteorological Agency Ensemble Prediction System and National Centre for Environmental Prediction's (NCEP's) latest version of Climate Forecast System (CFSv.2) coupled model. The BMA forecast captured the likely delay in the onset of NEM over meteorological (met) subdivision TN and associated weak phase of monsoon during October 2015. Similarly, the BMA forecasts also captured the active phases of NEM during November and early December 2015. Although it is difficult to capture the actual magnitude of observed high-rainfall departure over a smaller domain of met-subdivision scale, the BMA-based ERF could capture the active phase of NEM over southern peninsular India, including the metsubdivision TN with a lead time of 1-2 weeks. Quantitatively, the excess NEM rainfall spells during 2015 and particularly that during 5-11 November and 12-18 November 2015 are reasonably well captured in the BMA forecast, although forecast rainfall departure was lower than the actual departure.