Looking at forecast charts by global Numerical Weather Prediction models one can safely say that the southwest monsoon is set to arrive in Kerala by May 30, 2017. Our estimate is it will hit Goa by June 4 and Mumbai by June 7-8.
The monsoon onset we are talking about as decided by parameters set by the indian Meteorological Department. If we were to go by amount of rainfall alone, heavy thunderstorms will start lashing Kerala in the next 3-4 days.
According to accumulated rainfall forecast charts by the European agency, ECMWF, by June 1, parts of Kerala will have received a whopping 334 millimetres of rain. Monsoon is expected to cover coastal Karnataka by June 1 itself.
After the Kerala onset, the rains will clamber up the latitudes quickly and reach Mumbai by June 8. Fortunately for Indian monsoon this year there have been no depressions/tropical cyclones in either the Arabian Sea or Bay of Bengal which suck away its energy and throw it in another direction, thus slowing down its progress.
There is little possibility of a storm in the seas except for a Bay of Bengal upper air cyclonic circulation which may intensify to a low pressure area/tropical storm by month end. But even that is expected to move into Bangladesh/Indian northeast, thus it will actually hasten monsoon's entry into eastern India.
According to the ECMWF forecast rains will be knocking on Goa's doorsteps by June 2.
Monsoon arrival means massive changes in upper atmosphere winds, some 12-13 kilometres high in the air. This June 1 forecast map shows the monsoon winds are already blowing over peninsular India. The perennial easterly jet stream has been pushed northwards into Tibet. This occurs every year during monsoon time.
There is sobering news for those expecting a bumper harvest this year. In it's latest forecast the Bureau of Meteorology says the Indian Ocean Dipole may not be so positive in August. In fact the European model predicts a weak IOD this year.
Similarly NASA and the ECMWF says El Niño may rear it's ugly head by August. Only NOAA paints a rosier picture. Be prepared for poor rains in August-September unless of course the picture changes by then.