The average rainfall for Mumbai (India) is usually 23 inches (Approximately 584 mm) according to Accu weather data. However during the period 11 June to 23 June 2015, heavy thunderstorms and rain has produced an incredible 797 mm of rain (Approximately 31 inches) at the city's weather station.
This has led to flooding and local disruption of the city. Generally the monsoon is welcome in a country that requires the rain for its crops such as rice production. Too little rain and too much rain creates issues at either end of the spectrum. The monsoon storms for June 2015 is currently bringing too much rain. As shown, the rainfall for Mumbai for this period is provided on a daily basis (Note - It appears that inches are used to measure rainfall so these have been converted to mm).
June 11 - 0.42 inches (10.66 mm).
June 12 - 4.5 inches (114.3 mm).
June 13 - 0.59 inches (15 mm).
June 14 - 2.13 inches (54 mm).
June 15 - 0.43 inches (11 mm).
June 16 - 0.94 inches (24 mm).
June 17 - 1.23 inches (31.2 mm).
June 18 - 5.31 inches (134 mm).
June 19 - 5.39 inches (136.9 mm).
June 20 - 4.52 inches (114.8 mm).
June 21 - 4.02 inches (102 mm).
June 22 - 0.51 inches (12.9 mm).
June 23 - 1.46 inches (37 mm).
This amounts to 797 mm during the period. The bulk of this being 19.24 inches or 488 mm fell between June 18 and June 21 which is approaching half a metre of rain in 4 days.
The reason has been the tropical storms. The attached satellite photo (Acquired from CIMSS June 24 2015) shows one such storm just north of Mumbai currently impacting Gujarat State and affecting cities such as Bhavragar, Jamnagar and Rajkat. The storms such as this one have produced the intense rains and local flooding. This particular storm being one of many is being monitored but unlikely to develop into any tropical cyclone. This shows an active start to the monsoon across many parts of India following the recent deadly heatwave.