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(CNN) --
The Storm Prediction Center is warning that "a regional outbreak of tornadoes is possible in the afternoon and evening this Tuesday in parts of the Lower and Middle Mississippi Valley."
The prediction center issued a level 4 of 5, moderate risk, of severe storms for the night and afternoon of this Tuesday.
Moderate risk will affect 1.8 million people from northeast Louisiana to southern Tennessee, including parts of greater Memphis and Greenville, Mississippi.
"Long track and/or significant tornadoes are anticipated, with the most favorable corridor located from far northeast Louisiana northeast through northwestern Mississippi," the forecast center noted.
Long-track tornadoes are those that stay on the ground much longer than a typical tornado and can cause greater damage.
Significant tornadoes are those with force EF2 or greater, which can cause winds of 178 km/h or greater.
This image shows the risk of severe weather in the United States on Tuesday.
Green and yellow colors represent the lowest risk, while red and orange indicate high risk.
In addition to potentially severe tornadoes, Tuesday's storms could bring hurricane-force wind gusts of up to 120 km/h and hail.
On Sunday, the storm center began warning of a "significant severe weather event" in parts of the lower Mississippi River valley.
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In total, more than 25 million people are under severe storm threat Tuesday from southeast Texas to Alabama and north to central Indiana, including Houston, New Orleans, Nashville, Baton Rouge, Shreveport, Little Rock and Jackson.
The reason for this bout of severe weather is a strong cold front, which will push its way across the Mid-South from the Plains on Tuesday.
The cold air will meet much warmer and more humid air than is currently in the south.
The clash of air masses will be one reason for the potential for monster storms.
The storms are expected to begin late Tuesday afternoon and continue through the night and into the early hours of the morning.
Nighttime tornadoes add an additional danger
The system is likely to generate tornadoes and damaging winds in the lower Mississippi River Valley after dark Tuesday, which can be especially dangerous.
"Unfortunately, it looks like the threat of severe storms will continue through late afternoon and into Tuesday night," Bill Bunting, head of forecast operations at the Storm Prediction Center, told CNN Weather.
The time of day a tornado occurs also makes a big difference in the fatality rate.
Nighttime tornadoes are more dangerous because many people are asleep and are unaware that they need to find a safe place.
What is the difference between a tornado, a tropical storm, and a hurricane?
Although the greatest tornado threat for this particular event exists during the day, there is still a chance of a few spinning storms during the evening hours.
Areas most at risk for overnight storms Tuesday include southern Illinois to Louisiana.
"Another challenge with nighttime tornadoes, especially in the fall and winter, is that the storms tend to move very quickly, sometimes 50 mph or 56 mph," Bunting said.
"This means you have to make quick decisions and take shelter based on the information contained in severe storm or tornado warnings, and not wait until the storm hits."
Bunting noted that another concern will be the possibility of flooding, thanks to heavy rains resulting from repeated rounds of thunderstorms.
Widespread rain is expected to fall in the 25-50mm range, but some isolated spots could record up to 100mm.
Second "adverse weather season" in November
Although tornadoes in the US can occur in any month of the year, they are more frequent in spring thanks to the clash of cold and warm air with the change of seasons.
The same temperature melting occurs in autumn as well, so a second "severe season" is often observed late in the year.
"Although severe storms occur less regularly in the fall and winter than during the spring/summer months, significant clusters of adverse conditions have occurred during this time of year," Bunting noted.
"Just think of last December and the record number of tornadoes for the month and the tragic number of deaths."
Earlier this month, a Level 4 moderate risk, severe storm spawned dozens of tornadoes in Texas, Oklahoma, Louisiana and Arkansas that damaged numerous homes and businesses.
This marks the first time two Level 4 hazards have been issued anywhere in the United States in the month of November since the storm center began using its five severe storm hazard categories in 2014.
"You can see that while the spring months are our busiest time climatologically, there is a secondary increase in tornado activity in November," the National Weather Service in New Orleans said.
Texas is the state that registers the most tornadoes on average in the month of November with 7, followed by Alabama with 6, and Louisiana and Mississippi with 5 each.
At least one dead and several people missing in Oklahoma after more than a dozen tornadoes hit 3 states, authorities say
"Severe fall and winter thunderstorms can be extremely severe, and can sometimes catch people off guard as they tend to occur less frequently during the colder months," Bunting added.
"Now is the time to revisit, or develop, a severe weather emergency plan for you and your family."
It's important to know where to head in case of bad weather, and to make sure flashlights are working and phones are fully charged in case of a power outage.
-- CNN meteorologists Allison Chinchar, Robert Shackelford, and Jennifer Gray contributed to this report.
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