The storm has been diagnosed as a Depression by IMD. This means that it is not formally named yet – the name “Nisarga” is not actually used yet on the IMD web site that I can see, even though it is all over the media already – but the formal track and intensity forecasts are now being produced.
Below are the current forecast maps. On the top one, the cone of uncertainty gives an indication of the range of possible tracks the center of the storm could take. On the bottom one, areas likely to be hit by winds of various strengths are indicated. To put these into words, the storm is forecast to pass almost directly over Mumbai with peak sustained surface winds of 60 knots, or about 110 kilometers per hour. This is a Severe Cyclonic Storm in the Indian scale, or a strong Tropical Storm (but not quite a Hurricane) on the US scale.
The track forecast is not good news. The Mumbai metropolitan area has 20 million people, is very vulnerable to cyclones at the best of times (not having had one ever, for all practical purposes), and is currently a hotspot of the coronavirus pandemic. On the other hand, the intensity forecast is good news, in that those models that yesterday were predicting much more powerful storms are no longer doing so. So this no longer looks like the worst case scenario, given current information. That said, a few things to keep in mind:
1) These forecasts are still just forecasts. Things could change.
2) Even 60 kt (110 kph) winds can cause significant damage. Winds will be stronger high above the ground, such as at the tops of tall buildings of which Mumbai has many.
3) There is still significant flood risk, both from rain, and possibly storm surge as well (particularly if the surge peaks at high tide). I have not seen any predictions of surge yet.
Please be prepared. Monitor the forecast and advice of local officials closely.