The southwest monsoon remains active over south, central and eastern India on September 20.
The IMD has warned of heavy rain in Kerala, Karnataka, Tamil Nadu, Andhra Pradesh and Telangana.
Heavy to very heavy rain is likely over Odisha and Chhattisgarh.
Thunderstorms are forecast over Delhi, Uttar Pradesh and Bihar.
Landslide risk exists in hilly areas.
Lucknow is expected to reach about 34 degrees Celsius with a minimum near 26 degrees, and rain in Uttar Pradesh may extend into September 21.
Patna is forecast to see a maximum of about 34 degrees Celsius and a minimum near 27 degrees Celsius, with widespread thunderstorms across Bihar.
Heavy rain in hilly areas may cause landslides and disrupt travel, and lightning and hail are possible in the hills.
The overall pattern is driven by an active southwest monsoon over central, eastern and southern India during the season’s withdrawal phase.
The weather department advises people to avoid open fields, trees and electric poles during thunderstorms and to check local bulletins before travelling.
The IMD has issued a heavy rainfall warning for Sunday, September 20, for parts of the Andaman and Nicobar Islands, Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, Puducherry and Telangana.
Heavy to very heavy rainfall is forecast for parts of Odisha, Chhattisgarh, Gangetic West Bengal and the Andaman and Nicobar Islands.
Fairly widespread monsoon showers with thunderstorm and lightning activity are expected over Bihar, Jharkhand and east Madhya Pradesh.
Thunderstorms and lightning are also forecast over Marathwada, sub-Himalayan West Bengal, east Rajasthan, Konkan and Goa, and parts of the Northeast.
Reports counting the states affected by rain, thunderstorms and strong winds on Sunday put the number at 18, including Delhi, Uttar Pradesh, Bihar and Rajasthan.
Strong surface winds are very likely over parts of Andhra Pradesh and Telangana, while Tamil Nadu may see gusts of 40 to 50 kmph with isolated heavy rain.
Rain clouds during the monsoon in Goa (representative image), Wikimedia Commons, CC BY-SA 2.0
