Monroe County will also have a curfew from 8 p.m. Sunday to 6 a.m. Monday. Mobile homes will be damaged or destroyed.
Deaths were reported across Mississippi, Texas and Louisiana.
The National Weather Service rated the tornado EF-3 with winds of about 140 miles per hour (225 kph). The children, ages 3 and 8, were killed when a tree fell on a auto Saturday.
The Storm Prediction Center has placed most of the state, including central Maryland, under an enhanced risk for severe storms.
One person was killed in northeastern MS when a tree crashed onto his trailer, Monroe County Road Manager Sonny Clay said at a news conference, adding that a tornado had struck.
A microburst "is a localized area of strong straight line winds that descends from a thunderstorm and is less than 2.5 miles in diameter", the weather service said.
Strong storms, including at least one tornado, swept through parts of the Midwest and the Mid-Atlantic region, damaging homes and businesses and causing injuries after storms in the South killed at least eight people. As a result, the National Weather Service has issued a wind advisory for all 21 counties in New Jersey, as well as for New York City. Know where to go in your home if you are included in a Tornado Warning and make sure you have a way of receiving those very important weather warnings, either by cell phone or weather radio.
The roof of a hotel in New Albany, Mississippi, was damaged, although the cause was unclear. No injuries were reported, and news footage showed shattered windows and rooftop debris.
In Pollok, Texas - about 150 miles (240km) southeast from Dallas - two children were killed when their family auto was hit by a falling pine tree.
The Alto Police Department also confirmed a woman who was injured last night at Caddo Mounds has died.
In Alabama, the Jefferson County Sheriff's Office reported that a county worker had died in a hospital after being hit by a vehicle near Hueytown.