A large tornado struck near Little Rock, Ark., on Friday afternoon, tearing down trees and destroying homes, according to the National Weather Service, whose meteorologists in the local office were forced to evacuate.

The tornado is part of a complex and dangerous storm system began hitting the Upper Midwest and South, forecasters said.

Images from Little Rock showed debris and damage to homes from the tornado, which KATV, a local TV station, described as “catastrophic,” saying it expected widespread damage. In addition to the tornado emergency for parts of Little Rock, forecasters also declared an emergency for parts of nearby Sherwood and Jacksonville, Ark. It was unclear whether there had been injuries.

Meteorologists at the National Weather Service office in Little Rock had to move to a tornado shelter on Friday afternoon, as it became clear that their office was in the tornado’s path. The Memphis office of the Weather Service planned to issue warnings and monitor the weather on their behalf, said Desiree Meadows, a meteorologist in Memphis. In the aftermath of the tornado, another tornado warning was issued for the Little Rock area.

The worst of the storm system may not yet have passed. Damaging wind gusts and very large hail are expected in the region through the evening, the Weather Service’s Storm Prediction Center said. Widespread and damaging thunderstorms are likely, and flash flooding is also possible, forecasters said.

“Keep in mind that these storms will be fast moving today,” said Ashton Robinson Cook, a meteorologist with the Weather Prediction Center. “So if you find yourself in the path of one, be prepared to take shelter immediately.”

In the evening, the storms are expected to move east through the Ohio and Tennessee Valleys, where they are expected to bring damaging winds and large hail, and may spawn strong tornadoes.

More than 28 million people were under tornado watches on Friday afternoon, the Weather Service said. A tornado warning was in place for an area including Peoria, Ill.

A tornado watch was issued for parts of central and eastern Iowa, western Illinois, northern and central Missouri and southwest Wisconsin until 8 p.m. Central time. The Weather Service said that the area was likely to see “numerous” strong tornadoes, including a few intense tornadoes, hail as large as 3 inches in diameter and widespread damaging wind gusts of up to 70 m.p.h.

Another tornado watch was issued until 8 p.m. Central time for most of Arkansas, southeastern Missouri, Southern Illinois, eastern Kentucky, eastern Tennessee and northeastern Mississippi. The Weather Service said there were likely to be several strong tornadoes, scattered hail up to tennis-ball size and scattered wind gusts of up to 70 m.p.h.

Parts of northwestern Louisiana, southern Oklahoma and northeastern Texas were under another tornado watch.

The storms could affect an area stretching from Louisiana to Wisconsin, including Little Rock, Memphis, St. Louis, Des Moines, Chicago and other major cities. In a large area of the Mississippi Valley, “at least a few long-track, strong to potentially violent tornadoes” were likely, the Prediction Center said.

Ahead of the storms, crews worked to clear debris from drains in areas prone to flooding. Officials encouraged residents to have a plan to take shelter, to prepare for possible power outages and to have multiple ways to get weather alerts, such as on their phones and on the radio.

Officials also said people should clear large or loose material from their property, including patio furniture, dead trees and hanging branches that could become “a dangerous projectile” during high winds.

The storms could affect parts of Mississippi that were devastated last week by tornadoes that left at least 26 people dead.

President Biden on Friday visited Rolling Fork, the Mississippi community hit hardest by the tornadoes last week. Tornadoes killed 13 people and destroyed homes and businesses in Rolling Fork and in surrounding Sharkey County.

At midday on Friday the Storm Prediction Center upgraded the risk of severe weather to “high” — the top level on a five-category scale — for parts of Iowa, Illinois and Missouri near the Iowa-Illinois border, as well as for another area covering parts of eastern Arkansas, northwest Missouri and southwest Tennessee, including Memphis.

The center last used the high designation for severe weather risk two years ago, on March 25, 2021, hours before powerful tornadoes struck Alabama and Georgia, killing at least six people.

The Storm Prediction Center also extended a “moderate” risk designation, the fourth level on the scale, to a large area across Illinois and into much of Indiana. The area also included parts of eastern Missouri and Arkansas, western Tennessee and Kentucky, northern Mississippi and northwestern Alabama. More than 18 million people are in the high and moderate risk areas.

The Weather Service advised residents to look out for safety alerts. If a tornado warning is issued, people should move to a safe place, preferably an interior room on the lowest floor of a sturdy building.

The Quad Cities will likely receive two rounds of severe weather on Friday, forecasters said. The first was expected to begin in the early afternoon and the second in the early evening. During both events, winds may reach up to 75 m.p.h., and some strong tornadoes may be possible.

The storms could also produce intense rainfall that leads to flash flooding. The area at greatest risk for flooding through Friday night extends from Arkansas to Kentucky and includes western Tennessee and northern Mississippi. This could be followed by thunderstorms with damaging wind gusts across the northern Appalachians and interior Northeast on Saturday.

Derrick Bryson Taylor and John Keefe contributed reporting.