A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.
Widespread severe weather is likely to move east into the Middle and Lower Mississippi River Valley on Friday, NWS said.
Severe storms also could affect parts of eastern Missouri, central and southern Illinois, southwest Indiana, western Kentucky, Tennessee, Georgia and the Florida panhandle through early Saturday.
The combination of increasing moisture and strengthening winds in conjunction with daytime heating and uplift along and ahead of the cold front would create conditions favorable for long-lived rotating thunderstorms called supercells.
The supercells and their attendant threat for tornadoes, high wind and large hail likely will be most concentrated over parts of Mississippi and Alabama later today through early tonight, NWS said in a statement.
Supercells are long-lived thunderstorms which exhibit quasi-steady structure including a rotating updraft.
Elsewhere severe storms also may occur from parts of eastern Missouri and the lower Ohio valley southward through the Tennessee valley to the central and eastern gulf coast.
The agency said a potent upper level disturbance now over Kansas will move east into Missouri this evening and into Illinois early Saturday as a band of very strong jet stream winds on its south side overspreads the lower Mississippi and Tennessee valleys.
At the same time, a broad current of increasingly warm and humid air will spread north across the central and eastern gulf coast region ahead of the strong cold front associated with the upper level system.
Additional severe weather may arise from the remnants of yesterday's severe storms in Oklahoma and Arkansas as that activity continues east into the Mississippi and Tennessee valleys, NWS said.
On April 15, a tornado killed at least two people and destroyed the only school in the small southeastern Oklahoma town of Tushka.