'A significant severe-weather event will be likely' in U.S. this week, meteorologists say

A new robust storm system moving into the Pacific Northwest Sunday is expected to spawn severe storms in the South this week.

The U.S. Storm Prediction Center has issued an early forecast warning, calling for "a significant severe-weather event" across parts of the lower Mississippi River Valley Tuesday.

"This storm system will have the potential to produce severe thunderstorms capable of tornadoes, damaging straight-line winds, and also large hail," Bill Bunting, chief of forecast operations at the Storm Prediction Center, told CNN Weather. "The details regarding areas most at risk from tornadoes will become clearer as Tuesday approaches and smaller-scale trends become more evident."

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A Level 3 out of 5 enhanced risk for severe storms has been issued across the region including Memphis. However, the larger severe storm threat Tuesday could affect more than 25 million people from eastern Texas to southern Indiana.

The prediction center also noted in its forecast discussion Sunday morning "a categorical upgrade will be possible in later outlooks," meaning the severe storm threat has the potential to increase to a Level 4 or 5 out of 5 as Tuesday approaches and the forecast becomes more clear.

Regardless, storms this week are expected to be strong and potentially damaging. Be sure to keep up-to-date on the latest forecast as it develops over the next couple of days.

NIGHTTIME TORNADOES ADD AN EXTRA DANGER

The system is likely to spawn tornadoes and damaging winds across the lower Mississippi River Valley after dark Tuesday, which can be particularly dangerous.

"Unfortunately, it does appear that the severe storm threat will continue into the evening and overnight hours on Tuesday," Bunting said.

The time of day when a tornado occurs also makes a big difference in the fatality rate. Nocturnal tornadoes are more dangerous because many people are asleep and unaware they need to be seeking a safe location.

While the greater tornado threat for this particular event exists during the day, there is still the possibility for a few rotating storms through the evening hours.

The areas most at risk for nocturnal storms Tuesday include southern Illinois down through Louisiana.

"Another challenge with nighttime tornadoes, especially in the fall and winter, is that storms typically move very quickly, at times 50 or 60 mph," Bunting said. "This means that you must make decisions quickly and take shelter based on information contained in the severe thunderstorm or tornado warning, and not wait until the storm arrives."

Bunting pointed out another concern will be the potential for flooding, thanks to heavy rainfall resulting from repeated rounds of thunderstorms.

Widespread rainfall is expected to fall in the one to two-inch range, but some isolated locations could see up to four inches.

SECOND 'SEVERE WEATHER SEASON' IN NOVEMBER

While tornadoes in the U.S. can happen in any month of the year, they are most common in the spring time thanks to the clash of cold and hot air as the seasons change. The same merging of temperatures also occurs in the autumn, which is why you will often see a secondary "severe season" later in the year.

"While severe storms occur with less regularity in the fall and winter than during the spring/summer months, major severe weather outbreaks have occurred during this time of the year," Bunting noted. "One only has to think back to last December and the record number of tornadoes for the month and the tragically high number of deaths."

Earlier this month, a Level 3 severe storm risk was issued three days ahead of time, before later being upgraded to a Level 4 risk. Dozens of tornadoes were reported across Texas, Oklahoma, Louisiana, and Arkansas causing damage to numerous homes and businesses.

If Tuesday's threat is increased to a Level 4 risk, it would be the first time there have been two Level 4 risks issued anywhere in the U.S. in the month of November.

"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 sees the most tornadoes (7) in the month of November on average, followed by Alabama (6), Louisiana (5), and Mississippi (5).

"Severe thunderstorms in the fall and winter can be extremely impactful, and may sometimes catch people off guard as thunderstorms tend to occur less frequently during the cooler months," Bunting added. "Now is the time to revisit, or develop, a severe weather emergency plan for you and your family."

Know where you will go if severe weather hits, and make sure flashlights work and cell phones are fully charged in case you lose power.