Extended range forecast for the genesis of a cyclonic storm over north Indian seas during the month of May

A synoptic method for extended range forecasts for the genesis of a depression/cyclonic storm over North Indian Seas (Arabian Sea and Bay of Bengal) during the pre-monsoon month of May has been proposed. Synoptic indications on likely development of a low-pressure area, available in advance, could serve as a watch on the development of a cyclonic storm. The method has been developed making use of data of vector wind, relative humidity (RH) and Sea Surface Temperature (SST) anomaly fields. An examination of 850 hPa vector wind anomaly field for the month of April during the 30 year period of 1948-1977 has shown that the following synoptic situation is conducive for development of a low pressure area over North Indian Seas during May, (i) dominant anti-cyclonic flow along Bay of Bengal from 5 degrees N, along Arabian Sea from 10 degrees N, cyclonic flow from 5 degrees N southwards between 40 degrees E-70 degrees E and a trough along 100 degrees E from 15 degrees N northwards, (ii) +RH along Arabian Sea and Bay of Bengal up to 15 degrees N between 40 degrees E-80 degrees E and between 90 degrees E-100 degrees E respectively and (iii) warmer SST from north of 5 degrees N along Arabian Sea and Bay of Bengal in comparison with the region 40 degrees E- 120 degrees E from 40 degrees S. An examination of the vector wind anomaly field for the period 1978-2018 has shown the following synoptic situation, as one of the factors, for the development of more number of cyclonic storms over Bay of Bengal as compared to that over Arabian Sea: cyclonic flow between 45 degrees E- 60 degrees E from 20 degrees N southwards at 850 hPa, between 50 degrees E-65 degrees E at 700 hPa level and between 60 degrees E-65 degrees E at 500 hPa across Arabian Sea. Cyclonic flow between (i) 80 degrees E-95 degrees E/10 degrees N-5 degrees N at 850 hPa level, (ii) Trough between 80 degrees E-90 degrees E at 700 hPa along 5 degrees N (iii) Trough along 110 degrees E from 10 degrees N southwards at 500 hPa and (iv) Trough along 100 degrees E southwards from 10 degrees N at 300 hPa over Bay of Bengal and Andaman Sea. A low pressure area develops over Bay of Bengal/Arabian Sea under the influence of (i) a Trough of low at mean sea level over equatorial Indian Ocean and adjoining south Bay of Bengal/Andaman Sea, or (ii) a cyclonic circulation /cyclonic flow from 850 hPa to 300 hPa levels.