Parts of Mississippi could see tennis-ball sized hail and 60 mph wind gusts, the National Weather Service in Jackson said
. Isolated flash flooding was also a threat.
Severe weather was also expected to menace the coastlines of Alabama and the Florida panhandle, the weather service said
The new round of severe weather comes hours after southern Mississippi was battered by fierce winds.
Preliminary damage assessments conducted in Hattiesburg, Mississippi on Saturday showed the tornado packed winds, at times, between 136 and 165 mph, making it an EF3 -- which can cause severe damage, the National Weather Service said.
EF0 is the weakest point on the Enhanced Fujita Scale and EF5 is the strongest. The Fujita scale
is used to estimate the wind speed of a tornado by the damage the tornado causes.
The tornado left four dead, and more than 20 injured in Forrest County, the Mississippi Emergency Management Agency said. Most of the damage was centered around the cities of Hattiesburg and Petal.
The team found evidence of at least 15 miles of damage, the weather service said on Twitter.
Officials warned of downed power lines and debris spread over large areas, and urged people to avoid traveling.
Gov. Phil Bryant declared a state of emergency and "ordered all available resources toward rescue and recovery."
Hattiesburg has sustained a lot of damage, the National Weather Service said in an advisory. It's the largest city in Forrest County with about 46,000 people.
William Carey University's Hattiesburg campus on the outskirts of downtown was among the properties affected.
Students at William Carey were reporting minor injuries, the university said. Dormitories and other buildings were damaged, the school said.
Volunteers began cleaning up the damage Saturday afternoon.
Reporters and editors from CNN affiliate WDAM posted to Twitter pictures of severe damage in Hattiesburg and in Petal, a nearby city of about 10,400 people.
Parts of an AT&T store and a loan shop collapsed at a shopping center in Petal, one of the images showed.