LESS THAN four days into September, Mumbai has already surpassed its September average rainfall of 327.1 mm. The Santacruz observatory recorded 488.7 mm rain in Mumbai from September 1 till 2.30 pm on September 4.
The all-time record for rainfall in September is 920 mm, recorded in 1954.
The Met department issued an orange alert with a forecast of “heavy to very heavy” rainfall at some places for Thursday for Mumbai and its neighbouring areas — Palghar, Thane, Raigad, Ratnagiri and Nashik.
According to India Meteorological Department (IMD), Mumbai can expect intermittent rain or showers with possibility of “heavy to very heavy” rainfall at isolated places in the city and suburbs, on Thursday.
“The latest satellite images indicate persistence of the weather system (low pressure) over Arabian Sea. Rainfall will continue in Mumbai and surrounding areas,” said K S Hoslikar, deputy director general, IMD Mumbai.
In Mumbai, the southwest monsoon spans across four months from June to the end of September that still leaves plenty of potential for more rain this month.
The normal monthly average rainfall for September between 1960 and 2010 was 327.1 mm for Santacruz weather observatory and 304.6 mm for Colaba observatory.
Last monsoon, the average rainfall in September was 73.1 mm, whereas the 24-hour highest rainfall was 15.6 mm, recorded on September 2, 2018.
The city has earlier received extremely heavy rainfall in September — 303.7 mm rainfall on September 20, 2017, and 318.2 mm rain on September 23, 1981.
In the nine hours between 8.30 am and 5.30 pm on September 4, the Santacruz observatory recorded 214.4 mm rain, which falls in the “extremely heavy” rain category. The Colaba observatory recorded 70.6 mm in the same time frame. Over 33 hours, from 8.30 am on Tuesday to 5.30 pm on Wednesday, Mumbai recorded 332.7 mm rainfall.
According to the weather department, 15.6 mm to 64.4 mm rain is considered “moderate”, 64.5 mm to 115.5 mm “heavy”, 115.6 mm to 204.4 mm “very heavy” and more than 204.5 mm “extremely heavy”.
“It is not completely unusual to get heavy rain in September — the city has received more than 300 mm rainfall on a day during September 2017 and 1981. However, the intensity of rain (more than 200 mm in nine hours at the Santacruz observatory) comes under high-impact category. Two weather systems — a low pressure over eastern India and another weaker one in the vicinity of Mumbai strengthened the offshore trough along the coast of Maharashtra. This combo of weather conditions enhanced rainfall in Mumbai and adjoining areas,” said Akshay Deoras, independent meteorologist and PhD researcher at the department of meteorology, University of Reading, UK.
The Met department had earlier said due to lack of weather systems in Arabian Sea or Bay of Bengal, there was a break in active monsoon conditions over the past 15 days but the rain returned from the weekend onward.
On Wednesday, IMD issued a timely red alert for Mumbai, Thane, Palghar and Raigad. A red alert warning denotes the likelihood of extremely heavy rainfall and it means that the authorities, in this case, Mumbai Police, Brihanmumbai Municipal Corporation (BMC), Mumbai Fire Brigade, National Disaster Response Force (NDRF), Railways, should be prepared for all eventualities.
The IMD said 100 out of 150 weather stations in the city recorded over 200 mm rain in the 24 hours ending at 8.30 am on Wednesday.
Between 8 am and 6 pm on Wednesday, Vikhroli recorded the highest rain at 209.84 mm, followed by Marol (195.06 mm), Vile Parle (186.16 mm), Kandivali (179.55 mm) and Dindoshi (178.82 mm).
According to the district forecast by the IMD, the rainfall activity is likely to reduce from September 6, that is, from Friday, in Mumbai and neighbouring areas.
Deoras added, “Monsoon withdrawal is likely to commence from Western Rajasthan around mid-September. Usually, the monsoon withdraws from Maharashtra in the first fortnight of October.”
Earlier this season, three spells of extremely heavy rain days — July 1 and 2, when 375.2 mm was recorded in 24 hours, 219.2 mm from July 26 to July 27 and 204 mm from August 3 to August 4 — were “extreme rainfall” events that took the record past the season’s average.