Santacruz, Mumbai recorded 121 mm in just 3 hours from morning 0830 am to 1130 am and still continued to record 207 mm till 1430 hrs IST which already crossed the definition of extremely heavy rain. While Colaba recorded only 56 mm at 1430 hrs IST.
The highest rainfall ever recorded in the month of September are:
Colaba – 548 mm on 10th September 1930
Santacruz – 318 on 21st September 1981
Under the influence of low pressure over Northwest Bay of Bengal the monsoon wind currents have increased forming trough of low pressure along west coast. Satellite pictures shows intense cloud patch in and around Mumbai and along west coast.
Why such situation occurs is explained in the article HOW MONSOON TROUGH AFFECTS MONSOON OVER INDIA
Above satellite image source: IMD