By Karina Tsui, CNN

(CNN) — Parts of the South and Midwest were pummeled by relentless rain and tornadic storms on Saturday as a prolonged period of life-threatening flood risk reached its peak.

The line of violent storms that swept from Texas to Ohio have left at least 16 dead since Wednesday, including a 5-year-old boy found in a storm-battered home in Little Rock, Arkansas, according to emergency officials. Local police discovered the child after responding to a medical call for service at the home Saturday.

In Kentucky, a 9-year-old boy was swept away by floodwaters while walking to his school bus stop in Franklin County on Friday, police said. In nearby Nelson County, a 74-year-old was found dead in a fully submerged vehicle on Saturday, according to the sheriff.

Tennessee alone has reported at least 10 storm-related deaths. Flash flood warnings covered multiple counties Saturday, and a flood watch was issued for western parts of the state through Sunday morning.

Tens of millions were under flood watches and severe storm alerts in the central US on Saturday, according to the Storm Prediction Center. Active alerts remain in parts of Louisiana, Arkansas, Tennessee, Kentucky and Mississippi through Sunday morning.

Tens of thousands of homes and businesses in the region were without power as of early Sunday, according to PowerOutage.us. The hardest-hit state was Arkansas, with nearly 75,000 in the dark overnight.

The possibility of “generational” flooding that the National Weather Service (NWS) warned about this week stems from a stagnation in the current weather pattern that’s caused the string of storms to repeatedly hit the same areas in the central and southern US.

By Sunday, the “stuck” pattern will finally start to break, and the storms will begin to push eastward.

Storm threats will remain in effect for Florida Panhandle, Alabama and Georgia on Sunday, while the flood threat for Arkansas, Tennessee and Kentucky should lessen by Sunday afternoon.

Saturday was the third consecutive day of level 4 of 4 risk of flooding rain across several states in the Mississippi Valley. The three-day stretch of the highest possible flood threat is almost unheard-of outside hurricane season.

Rising flood waters sweep through communities and major roadways

Parts of several major highways through Tennessee and Kentucky have been closed because of rising water levels, with the NWS urging drivers to “turn around” if they encounter flooded roadways.

Sections of Interstate 40 in Memphis, Tennessee have been closed due to standing water, while portions of north- and southbound lanes on Interstate 69 in Daviess County, Kentucky were temporarily closed because of high water, CNN affiliate WTHR reported.

Videos obtained by CNN show roads swelling amid heavy rain in Cordova, Tennessee, about 25 miles east of Memphis.

Other videos show rescue crews from the Tennessee Department of Transportation assisting drivers as they navigate flooded roadways in Memphis, with some driving on the shoulder of the I-40 near the Danny Thomas Boulevard exit to bypass flooded lanes.

At least 390 roads were closed across Kentucky on Saturday morning due to flooding, mudslides and rockslides, Gov. Andy Beshear said. The governor also declared a state of emergency for the western part of the state, citing potentially record rainfall in areas unaccustomed to flooding.

The Ohio River, which traverses through Louisville, Kentucky, has risen more than 5 feet over the last 24 hours and is expected to rise significantly higher over the next two to three days, Mayor Craig Greenberg said Saturday afternoon.

Videos obtained by CNN show rushing water covering part of a state highway in Howell County, Missouri, west of West Plains. The highway patrol told CNN it was “just one example of many other low water crossing areas” and urged motorists to stay off roads especially in the dark.

In Mammoth Spring, Arkansas, a train was stopped on a bridge due to multiple active weather warnings in the area early Saturday morning, when heavy flood waters washed out the bridge and derailed multiple cars, according to BNSF Railway. BNSF personnel were on site coordinating with Mammoth Spring State Park to clear the incident and repair the bridge Saturday afternoon.

Some of the train cars were fully underwater, with nonstop rain from the early hours of the morning complicating recovery efforts.

President Donald Trump on Saturday approved Arkansas’ request for an Emergency Declaration for Direct Federal Assistance to help the state with its response to the storms, tornadoes, and flooding, Gov. Sarah Huckabee Sanders said.

Such extreme flooding across states serving as major cargo hubs will dramatically impact interstate commerce and lead to shipping and supply chain delays, Jonathan Porter, chief meteorologist at AccuWeather, told the Associated Press.

Ten days of tornadoes

Saturday marks the tenth day in a row that tornadoes have formed in some part of the country, and the seventh straight day a tornado has occurred somewhere east of the Mississippi River.

Damage was reported in northeast Texas Friday following a series of reported tornadoes. In Wood County, one person was injured and at least one house was damaged by toppled trees, according to emergency officials.

On Saturday, the Mississippi Department of Transportation released video showing a possible tornado moving through the New Albany area on Interstate 22.

Damage survey teams from the NWS have given preliminary ratings to at least 31 tornadoes in eight states since the outbreak began Wednesday. So far, they have found three tornadoes of at least EF3 out of EF5 strength in Missouri, Tennessee and Arkansas.

Additional surveys could take days to complete, and some have been held up by the continuing hazardous weather.

CNN’s Anastasios Stefanidis, Sarah Dewberry, Dalia Faheid, David Williams and Sharif Paget contributed to this report.