Videos: How Mississippi's tornadoes unfolded overnight and why they were so deadly
Tornadoes this strong are difficult to survive unless in a very well-built building
Rolling Fork, Mississippi: At least 23 people are dead in Mississippi following a terrifying Friday night in which large, destructive tornadoes tore across the state.
The violent twisters formed amid a severe weather outbreak that unleashed damage from Louisiana to North Carolina. They were fueled by record-setting heat and energised by howling jet stream winds.
The twisters formed from the same destructive storm system that barged into California's Bay Area on Tuesday and produced deadly flooding in both the Desert Southwest and the nation's midsection.
The terrible toll of the twisters can be linked to their utter ferocity, the vulnerability of the region they struck and for sweeping through at night, when they were hard to see coming.
How the tornadoes were unleashed
The rotating thunderstorm or supercell that spawned the deadly tornadoes swept across the entirety of Mississippi and continued through northern Alabama - an exceptionally long path for a single storm. Supercells contain rotating updrafts fueled by warm, unstable air near the ground and are twisted by changing winds with altitude.
The National Weather Service in Jackson, Mississippi, first issued a tornado warning for the storm as it entered Mississippi at 7:40 p.m.
Just before 8 p.m., it cautioned "a large and extremely dangerous tornado was located 7 miles west of Rolling Fork." By 8:03 p.m., the Weather Service said the storm was located near Rolling Fork. "This is a PARTICULARLY DANGEROUS SITUATION. TAKE COVER NOW!," it warned. Rolling Fork was one of several towns especially hard hit.
The wedge-shaped funnel was probably on the ground for the next 90 minutes covering 80 miles as it barreled across west central Mississippi at highway speeds. It compelled the Weather Service to declare multiple tornado emergencies, the most dire alert for twisters, as it carved a path through the towns of Anguilla, Louise, Midnight, Silver City, Tchula and Winona.
Along this path, tornado lofted debris 30,000 feet high, said Samuel Emmerson, a member of the radar research group at the University of Oklahoma, describing it on Twitter as an "extremely high-caliber" tornado.
Shortly after passing Winona, around 9:30 p.m., the tornado may have weakened or lifted for a time as the storm raced across northeast Mississippi.
However, it appeared to re-form and strengthen around 10:50 p.m. when the Weather Service office in Memphis declared yet another tornado emergency for the town of Amory and then the city of Smithville.
When radar displayed an unmistakable signature of debris immediately west of Armory, confirmation of the destructive twister, broadcast meteorologist Matt Laubhan for affiliate WTVA could not contain his emotion.
"Dear Jesus, please help them," he pleaded.
The storm crossed into northwest Alabama shortly after 11 p.m. and continued producing tornadoes until it reached the northeast part of the state around 12:45 a.m.
The Weather Service received at least 10 reports of tornadoes in northern Alabama from this particular storm. Around Hartselle, which is about 30 miles southwest of Huntsville, the agency logged reports of damage to homes, people trapped and a tractor trailer overturned.
Severe storm affected 7 Southern states
The storm that swept across Mississippi and northern Alabama was one of many that raged through the South Friday into early Saturday morning.
The Weather Service has logged more than 100 reports of severe weather from Louisiana to western North Carolina, including more than 80 instances of violent straight-line winds that toppled trees and power lines and damaged homes. Tens of thousands lost power.
The Weather Service reported one person was injured near Nashville when a tree fell on a house.
On Saturday, the storm system will push off the East Coast but could still set off some strong thunderstorms in parts of the Southeast and Ohio Valley.
The risk of severe weather and tornadoes is much lower compared with Friday, however. The Storm Prediction Center has placed the zone from southern Mississippi to South Carolina as well as eastern Ohio, northern West Virginia and western Pennsylvania in a Level 1 out of 5 risk of severe thunderstorms; Friday's risk in the Mid-South was a Level 4 out of 5.
As the storm front stalls across the South Sunday, the risk of severe weather could increase again between Louisiana and Georgia, with damaging winds and a couple more tornadoes possible, the Storm Prediction Center said Saturday.
A deadly legacy
The larger storm system that spawned Friday's tornadoes has a deadly legacy. When it slammed into California on Tuesday, at least five people were killed by trees toppled by winds up to 80 mph in the Bay Area. The same storm spawned the strongest tornado to hit the Los Angeles metro area since 1983.
As the storm exited California on its way toward the central states, it triggered severe flooding in central Arizona, where at least three people died after their vehicles were swept away by floodwaters, the Associated Press reported.
The storm unloaded more heavy rain that spurred flooding over an extensive swath of the central and eastern U.S. - stretching from eastern Oklahoma into northern West Virginia. The Associated Press reported a car was swept away by floodwaters in southwest Missouri, killing two passengers.
What caused the storm
When it first hit California, the storm was a powerhouse, intensifying rapidly as it made landfall in the Bay Area, meeting the meteorological criteria for a "bomb cyclone" because of its rapid drop in pressure. The lower the pressure, the stronger the storm.
Sweeping across the country, the storm has been energised by the roaring high-altitude winds of the jet stream - powered by the strong temperature contrasts of early spring.
Ahead of the storm, temperatures soared, setting numerous records across the South and Southeast, rising into the 80s. Tupelo, Miss., tied a record high of 85 degrees Friday.
This warm and humid air helped provide the fuel for Friday's storms. Abnormally high sea surface temperatures over the Gulf of Mexico, drawn northward ahead of the storm, intensified this warm, moist air mass.
As the winds from the south fueled the storms at ground level, the westerly winds of the jet stream were crashing south, generating extreme amounts of wind shear. This shear - or changing wind direction with altitude - created the spin in the atmosphere necessary for the violent tornadoes.
The Southeast has a disproportionate number of mobile homes, which cannot withstand strong tornadoes, compared with other parts of the country, which makes it particularly vulnerable.
"[T]he Southeast's mobile home residents are one of the most socioeconomically and demographically marginalized populations in the United States and are more susceptible to tornado impact and death," tornado researchers Stephen Strader and Walker Ashley wrote in a 2018 study.
The tornadoes were also particularly dangerous because they were occurring at night. When it's dark, the visual cues which sometimes prompt people to take action are absent and some are asleep. A 2008 study by Ashley found 39 percent of tornado fatalities occur at night even though just 27 percent of twisters are nocturnal.