U.S. state of Kentucky is awaiting President Joe Biden who is scheduled to survey tornado damage, meet with victims and deliver remarks there on Wednesday, manifesting that the federal government has attached much attention to the disaster that has claimed at least 90 lives across six states in the central parts of the country.

The president's trip will include a stop in Fort Campbell for a storm briefing, and stops in Mayfield and Dawson Springs to survey storm damage. Compared with Arkansas, Illinois, Missouri, Mississippi and Tennessee, Kentucky was the hardest hit with more than 80 deaths aged from 2 months to 98 years old, and over 100 people unaccounted for.

Meanwhile, in Edwardsville, Illinois, the Occupational Safety and Health Administration is investigating the deadly collapse at an Amazon warehouse, which was caused by a tornado on Friday that devastated the area and killed six Amazon workers. Most workers who died there were not sheltering in the "designated location," the company said.

More alarmingly, CBS reported on Tuesday that the cluster of deadly storms occurring on Dec. 10 to 11 is the latest sign that "tornado activity has waned in the Great Plains -- historically known as 'Tornado Alley' -- while picking up in the southeastern U.S., a manufacturing-rich region increasingly critical to the nation's supply chain."

Workers clear rubbles in Mayfield, Kentucky, the United States, on Dec. 14, 2021. (Photo by Angie Yu/Xinhua)

TORNADO FORMATION

Amid the catastrophe, the early signaling systems placed throughout the middle of the country mostly worked as promised, reported The Wall Street Journal on Tuesday. People received text alerts and radio alerts from local officials and forecasters, which would have given them enough time to shelter in place, as they have done for years during tornadoes.

However, the power, speed and duration of the tornado caught residents, businesses and cities by surprise and overwhelmed many of their safety procedures. The tornado's estimated wind speeds of over 200 miles an hour would make it an EF5, the highest level on the Enhanced Fujita scale rating system, according to the report.

While meteorologists continued to analyze data on the tornado, its length was likely record-setting. "It sent debris 30,000 feet into the air, the cruising altitude of many passenger jets, and scattered prom photos, handmade quilts and wrapped Christmas gifts scores of miles away from the tornado's path," said the report.

The twister rivaled the 1925 Tri-State tornado that killed 695 people as it traveled 219 miles across Missouri, Illinois and Indiana. Bill Bunting, chief of forecast operations at the National Weather Service's (NWS) Storm Prediction Center, was quoted as saying that a rare confluence of weather conditions created the tornado.

First was the development of a supercell, an enormous weather system generating severe thunderstorms and rotating upward currents of wind. That combined with an unusually steady muggy environment, with temperatures hovering in the low 70s over the course of several hours. Moreover, no other major storms formed to disrupt the tornado's formation or block its path, according to Bunting.

Workers repair the roof of a bank in Mayfield, Kentucky, the United States, on Dec. 14, 2021. (Photo by Angie Yu/Xinhua)

WHY USA

With about 1,200 tornadoes on average per year, "the United States has the dubious honor of residing at the top of the list," reported The Washington Post on Tuesday. According to the NWS Storm Prediction Center, there are more tornadoes in the United States than any other parts of the world, though the data from other nations is problematic to assess in sheer accuracy.

"Tornadoes tend to form where cold, dry air clashes with warm, humid air. These contrasts are maximized over the mid-latitudes, where the majority of Earth's tornadoes occur. A good-size portion of the Lower 48 sits smack-dab in the center of that not-so-sweet ordinate zone," said the report, adding that the broad expanse of the country also plays a significant role.

"A blend of recorded and inferential study indicates that the U.S. remains tops in tornado production, with secondary tornado-prone areas including the Canadian Prairie Provinces, Bangladesh, Britain, northeastern Mexico, northern Argentina and southern Brazil, and portions of southwestern Russia," U.S. meteorologist Roger Edwards wrote for NWS.

In the case of the United States, add in the proximity to the warm waters of the Gulf of Mexico, mid-level dry air sloping down from the Rockies, plus unfettered access to cold air from northern environs (particularly notable due to the flat topography of the Great Plains), a nearly perfect set of otherwise harmless ingredients suddenly became a volatile mix of atmospheric terror, according to the report.