Kolkata’s rainfall has climbed to a 30% surplus for the monsoon season after three days of heavy showers pushed September’s total to more than double its normal level, according to data shared by the India Meteorological Department’s regional office.
Between September 1 and 25, the Alipore observatory, the official rain gauge for the city, recorded 547.9mm of rain. The normal rainfall for the period, calculated from 30 years of readings beginning in 1990, is 265.2mm.
The September total has exceeded the rainfall usually recorded during July and August, which are generally Kolkata’s wettest months. A Met Office official attributed the sharp increase to at least three days of heavy rain during the month.
The unusually wet September has also changed Kolkata’s overall monsoon position. Between June 1 and September 25, the city received 1,677.7mm of rain, or 30% above the seasonal normal for the period. This followed a rainfall deficit in July and a minor surplus in August.
The southwest monsoon reached north Bengal on June 9 and south Bengal on June 11. According to the Met Office, rainfall over south Bengal is strongly influenced by weather systems developing over the Bay of Bengal. The first quarter of this year’s monsoon was relatively dry because such systems were absent, but rainfall increased as more systems formed over the Bay.
The official said around 10 systems had developed over the Bay so far this season. Between mid-June and early October, around 10 to 12 such systems typically form over the Bay of Bengal, according to the Met Office.
The latest spell was linked to a system that made landfall in Andhra Pradesh as a deep depression between Wednesday night and the early hours of Thursday. It brought more than 120mm of rain to Kolkata over three days before moving towards Uttar Pradesh. By Saturday morning, the system had weakened into a depression and was losing strength.
A Met bulletin said the depression over northeast Madhya Pradesh and adjoining north Chhattisgarh was moving north-northwestwards at 13kmph. At 8.30am on Saturday, it was centred about 70km south-southwest of Sidhi, 90km south-southeast of Rewa, 120km southeast of Satna, 180km south of Prayagraj and 250km south-southeast of Fatehpur.
The system was expected to move towards southeast Uttar Pradesh across northeast Madhya Pradesh and weaken gradually into a well-marked low-pressure area. The Met Office forecast light to moderate rain and thundershowers at a few places in both north and south Bengal over the following four to five days.
The rainfall pattern contrasts with the broader national monsoon picture. India’s cumulative monsoon rainfall deficit stood at around 15% as the southwest monsoon season approached its conclusion. In Kolkata, however, the concentration of rain in September has moved the season’s cumulative total into surplus territory. The monsoon usually begins retreating from south Bengal around October 10, according to the Met Office.
