South Mumbai and Central areas received very heavy showers since early morning. Later, during the day, the showers spread towards the North.
On studying the developments, I would say it is an extension Northwards of the strenghtening of the off shore trough.
If we recollect, (check blog of 13th Sept), Vagaries had estimated heavy rains in coastal Karnataka on wednesday, and extending up to Goa on Thursday. This was calculated on the basis of the off shore trough strengthening.
Well, it seems from the detailed pressure chart, that the "loop" showing a trough is off Mumbai on Thursday morning. It is seen on both, the 925 mb and 850 mb charts. More prominent in 925 mb.
This is an extension of the trough a bit more Northwards ( showing prominently around Mumbai in the 925 mb chart).
Answer to Queries: This has nothing to do with BB-7. Such developments can be shortlived. Mumbai could possibly go back to "status Quo" soon.
All lakes( supplying water to Mumbai) except Vaitarna are overflowing. Vaitarna is 98% full. Total water content is 12.95 lmlitres against capacity full of 13.0 lmlitres.