A storm system sweeping over large areas of the US South and Midwest resulted in at least 16 weather-related deaths by early Sunday, with overnight tornado and flash flood warnings setting up more severe weather that forecasters say could cause rising waterways for days to come.

Many of the impacted areas already are heavily waterlogged by days of severe storms that spawned deadly tornadoes. New tornado warnings were issued overnight in Alabama and Mississippi, along with flash flood warnings for several counties in Kentucky, Mississippi and Tennessee.

Saturday included more of the torrential rain and flash flooding that has pounded the central US, rapidly swelling waterways and prompting emergencies from Texas to Ohio. The 16 reported deaths since the start of the storms included 10 in Tennessee alone.

The National Weather Service said dozens of locations in multiple states were expected to reach what the agency calls “major flood stage”, with extensive flooding of structures, roads, bridges and other critical infrastructure possible.

A 57-year-old man died Friday evening after getting out of a car that washed off a road in West Plains, Missouri. Flooding killed two people in Kentucky including a 9-year-old boy swept away that same day on his way to school and a 74-year-old whose body was found Saturday inside a fully submerged vehicle in Nelson County, authorities said. Also Saturday, a 5-year-old died at a home in Little Rock, Arkansas, in a weather-related incident, according to police. No details were immediately provided.

Tornadoes earlier in the week destroyed entire neighbourhoods and were responsible for at least seven of the deaths.

There were 521 flights cancelled and more than 6,400 flights delayed within the US or coming into or leaving the country on Saturday, according to FlightAware.Com, which reported 74 cancellations and 478 delays of US flights early Sunday. Interstate commerce also could be affected. The extreme flooding across a corridor that includes the major cargo hubs in Louisville, Kentucky, and Memphis could lead to shipping and supply chain delays, said Jonathan Porter, chief meteorologist at AccuWeather.

The outburst comes at a time when nearly half of NWS forecast offices have 20 per cent vacancy rates after Trump administration job cuts, twice that of just a decade ago.

Louisville Mayor Craig Greenberg said Saturday that the Ohio River rose 5 feet (about 1.5 metres) in 24 hours and would continue to swell for days.