India, Sept. 26 -- A storm originating in the Bay of Bengal brought surplus rain to eastern India this week and is now moving towards north-west India, where it is expected to bring more rain to Chhattisgarh, Madhya Pradesh and western Uttar Pradesh even as the monsoon withdraws. This is not an isolated case. Such storms have repeatedly brought rain to these regions this monsoon, offsetting the dryness seen across much of the country and bucking a long-term trend. Three charts explain the unusual pattern.

The storms being discussed here are depressions or deep depressions, which are close enough to cyclones to be tracked alongside them by the India Meteorological Department (IMD). Like most storms, they are low-pressure regions with wind...