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Severe thunderstorm outbreak, tornadoes predicted for South

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Forecasters are warning residents across the South to brace for a potential outbreak of severe thunderstorms Tuesday afternoon and night. Storms could well be violent, unleashing damaging winds as well as tornadoes — some long-lasting and strong.

“Multiple rounds of severe thunderstorms — some capable of long-tracked tornadoes with EF3+ damage potential — will be possible this afternoon into tonight over parts of the lower Mississippi Valley region and Mid-South,” the National Weather Service Storm Prediction Center wrote Tuesday morning. EF3+ tornadoes are the most intense on the 0 to 5 Enhanced Fujita scale for tornado strength.

Severe storms could develop over a sprawling zone that includes more than 40 million people from near Houston into western Ohio. But central Mississippi faces the highest odds of particularly dangerous weather. That’s where the National Weather Service Storm Prediction Center has declared a rare level 4 out of 5 risk for severe storms. This area could see storms come in repeated waves, meaning a “long duration severe weather event” from early Tuesday afternoon to Wednesday morning, according to the Weather Service forecast office in Jackson, Miss.

The danger will be amplified as predicted tornadoes are expected be fast-moving and sometimes wrapped in rain, reducing lead time and making them hard to see. Additionally, the threat of tornadoes may continue after dark, when they are known to be most deadly. Some of the areas most at risk have many poorly-constructed homes that are vulnerable to tornado damage.

A number of tornado watches and warnings will probably be issued amid the rapidly evolving weather situation. Watches connote an elevated possibility of severe weather over a broad region and necessitate a heightened state of awareness. A more targeted warning is issued when a tornado is suspected to be imminent or occurring, and is a dire, immediate call to action that requires immediate sheltering in a strong building or underground.

The threat of severe weather will continue into parts of the Southeast on Wednesday, but storm intensity should ease some.

Areas affected: The level 4 out of 5 risk of severe weather includes most of central Mississippi, extreme northeastern Louisiana and extreme northwest Alabama. In Mississippi, it includes Jackson, Meridian, Clinton, Pearl and Vicksburg.

Surrounding that region is a level 3 out of 5 risk of severe weather for much of the Lower Mississippi Valley. This zone includes Monroe and Alexandria, La.; Hattiesburg, Miss; and Huntsville, Ala.

A level 2 out of 5 slight risk covers Nashville, New Orleans, Birmingham, Mobile and Shreveport, La.

Houston, Indianapolis and Chattanooga are within a level 1 out of 5 risk zone.

Timing: Scattered showers will stream north across the risk areas during the late morning into early afternoon as moisture builds and the atmosphere becomes saturated. Severe thunderstorms will rapidly develop during the mid- to late afternoon, continuing through the evening and first-half of the overnight. After midnight, they may congeal into a line that gradually weakens as it pushes east.

Hazards: The specific hazards associated with thunderstorms will be predicated on whether storms are able to remain isolated as lone, rotating thunderstorms or supercells. If rotating storms remain discrete, then tornadoes will be possible, with the risk of isolated strong and/or long track tornadoes. Otherwise, damaging straight-line winds and hail, perhaps up to the size of hen eggs, will accompany thunderstorms. Overnight, any lines of storms that develop will contain damaging winds and a low-end risk for brief, transient tornadoes.

Where storms pass over the same areas repeatedly, flash flooding may ensue. Flood watches have been posted for portions of southeast Louisiana, and southern Mississippi and Alabama with up to four inches of rain possible.

The setup: A potent high-altitude disturbance ejecting out of the western United States will swing a strong cold front east across the Plains, Mid-South and Mississippi and Tennessee valleys. Ahead of it, warm, humid air will waft north from the Gulf of Mexico, providing ample fuel for storms. Enhanced rising motion ahead of the disturbances will further bolster thunderstorm risk, while an uptick in wind shear, or changing winds with height induced by an overhead jet stream, will encourage said storms to rotate.

Nighttime tornado risk: Indications are that the greatest severe weather risk may exist between 4 and 9 p.m. Considering that the sun sets in most locales by 5:30 p.m., that means several hours of a significant nocturnal tornado threat. Nighttime tornadoes are disproportionately more deadly.

Storm characteristics: Thunderstorms in the South are often more swiftly moving than their U.S. Great Plains counterparts. Tuesday night’s storms could be moving to the northeast at highway speeds. That means that, by the time conditions begin to deteriorate, only a few moments will exist before an immediate, life-threatening tornado risk arrives. That also may lead to less advance warning. In addition, storms will have low cloud bases, which means any tornadic circulation may be impossible to see (assuming it’s even during the daylight hours). Moreover, tornadoes may be rain-wrapped and enshrouded in heavy precipitation.

An especially vulnerable zone: Mississippi has the fourth-greatest per capita density of mobile homes in the nation. It’s estimated that nearly a sixth of all housing units in the Magnolia State are mobile and/or manufactured homes. These homes often fail structurally even in minimal tornadic winds.

Complicating matters is the urban planning. Unlike in Florida, where mobile home communities may form entire neighborhoods, many mobile homes in Mississippi and, in particular, Alabama, are isolated — the next house might be hundreds of feet or more away. That makes it impossible to install communal storm cellars that are realistically accessible at a moment’s notice.

Even more concerning is the road network of the Deep South. Unlike on the Plains, where one-mile grids are the standard, the roadways in Mississippi are labyrinthine and winding, offering few if any reasonable escape paths for those seeking to relocate from mobile homes ahead of a storm.

On severe weather days, advanced preparation is key. Ensure that, wherever you go, you are no more than five minutes from a sturdy tornado shelter — ideally something below ground and without windows. This includes if you are driving. Have multiple ways to be notified if a warning is issued for your location.

Possible limiting factors

Low-level lapse rates: There is a chance that storms will be tempered by weak low-level lapse rates, or the rate at which temperatures decrease with altitude in the sky. That means there’s less upward “oomph” to boost thunderstorm updrafts in the lowest few kilometers of the atmosphere, which could acutely reduce the threat of strong and/or violent tornadoes.

Lift: It’s unclear how much broad-scale “lift” will exist to foster upward motion in the first place. There is a distinct possibility that lift on the broad scale is meager. In that case, there may be fewer storms overall, but we’d probably see one to two “hot spots” of enhanced storm activity. Those would become strips of surface convergence where air gathers, that would focus a tornado risk. Thunderstorms would probably ride along that repeatedly and “train,” possibly resulting in the threat of multiple tornadoes over a localized area.