Cyclone Mocha, classified as an Extremely Severe Cyclonic Storm (ESCS), followed an unusual northeastward trajectory while exhibiting a well-defined eyewall structure. It experienced rapid intensification (RI) before making landfall along the Myanmar coast. It caused heavy rainfall (~90 mm) and gusty winds (~115 knots) over the coastal regions of Bay of Bengal Initiative for Multi-Sectoral Technical and Economic Cooperation (BIMSTEC) countries, such as the coasts of Bangladesh and Myanmar. The factors responsible for the RI of the cyclone in lower latitudes, such as sea surface temperature (SST), tropical cyclone heat potential (TCHP), vertical wind shear (VWS), and mid-tropospheric moisture content, are studied using the National Ocean and Atmospheric Administration (NOAA) SST and National Center for Medium-Range Weather Forecasting (NCMRWF) Unified Model (NCUM) global analysis. The results show that SST and TCHP values of 30 °C and 100 (KJ cm⁻²) over the Bay of Bengal (BoB) favored cyclogenesis. However, a VWS (ms⁻¹) and relative humidity (RH; %) within the range of 10 ms⁻¹ and >70% also provided a conducive environment for the low-pressure system to transform into the ESCS category. The physical mechanism of RI and recurvature of the Mocha cyclone have been investigated using forecast products and compared with Cooperative Institute for Research in the Atmosphere (CIRA) and Indian Meteorological Department (IMD) satellite observations. The key results indicate that a dry air intrusion associated with a series of troughs and ridges at a 500 hPa level due to the western disturbance (WD) during that time was very active over the northern part of India and adjoining Pakistan, which brought north-westerlies at the 200 hPa level. The existence of troughs at 500 and 200 hPa levels are significantly associated with a Rossby wave pattern over the mid-latitude that creates the baroclinic zone and favorable for the recurvature and RI of Mocha cyclone clearly represented in the NCUM analysis. Moreover the Q-vector analysis and steering flow (SF) emphasize the vertical motion and recurvature of the Mocha cyclone so as to move in a northeast direction, and this has been reasonably well represented by the NCUM model analysis and the 24, 7-, and 120 h forecasts. Additionally, a quantitative assessment of the system indicates that the model forecasts of TC tracks have an error of 50, 70, and 100 km in 24, 72, and 120 h lead times. Thus, this case study underscores the capability of the NCUM model in representing the physical mechanisms behind the recurving and RI over the BoB.

The Mocha cyclone, an extremely severe cyclonic storm (ESCS), moved unusually northeastwardly with a well-organized eyewall structure, underwent rapid intensification (RI), and then entered into coastal areas of Myanmar. It caused heavy rainfall (~90 mm) and gusty winds (~115knots ) over the coastal regions of BIMSTEC countries, such as the coasts of Bangladesh and Myanmar. The factors responsible for the RI of the cyclone in lower latitudes, such as SST, TCHP, vertical wind shear (VWS), and mid-tropospheric moisture transport, are studied using the National Ocean and Atmospheric Administration (NOAA) SST and National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Global Model (NCUM) analysis. The results show that SST and TCHP values of 30℃ and 100 (KJcm-2) over BoB favored cyclogenesis. However, VWS (ms-1) and relative humidity (RH) within the range of 10 m/s and > 70% also provided a conducive environment for the cyclone to transform into the ESCS category. The physical mechanism of RI and recurvature of the Mocha cyclone have been investigated using forecast products and compared with CIRA and IMD satellite observations. Key results indicate that the dry air intrusion associated with a series of troughs and ridges at 500 hPa level due to the western disturbance (WD) during that time was very active over the northern part of India and adjoining Pakistan, which brought north-westerlies at 200hPa level. The Q-vector analysis and steering flow (SF) underscore that baroclinic instability plays a crucial role in allowing the Mocha cyclone to move in northeast directions, and it has been reasonably well represented by the NCUM model analysis and the 24-, 72-, and 120-hour forecast. Additionally, a quantitative assessment of the system indicates that the model forecasts of TC tracks have an error of 50, 70, and 100 km in 24-, 72-, and 120-hour lead times. Thus, the present case study underscores the strength of the NCUM model in representing the physical characteristics of the recurving and RI systems over BoB.