Severe and Anomalous Weather in Recent Decades

Severe storms and meteorological events in and around the United States have increased concern about climate change and sustainable development in the last few decades. Highly publicized tornadoes, nor’easters, and other snow and ice storms, hurricanes, and cyclones have cost billions in monetary damage and have led to significant human casualties. Scientists, emergency professionals, and political leaders are working to understand these phenomena and how they affect developed areas. Such studies could help introduce systems and protocols that warn citizens and protect against weather-related disasters.

Tornadoes

The tornado is an unpredictable and incredibly destructive weather phenomenon. It is a vortex of violently circulating winds associated with severe storms. Tornadoes are formed when warm and cold air systems intersect to form powerful storm clouds known as "mesocyclones." The mix of warm and cold air, resulting in strong updrafts and downdrafts, causes the air near the surface to spin.

Scientists are learning more about how tornadoes form and move. This knowledge has helped meteorologists and emergency personnel alert at-risk residents, giving them time to take shelter before a tornado arrives. However, science has yet to reach a point where tornadoes can be accurately predicted. In the mid-2020s, however, scientists can predict when an outbreak of tornadoes is likely to occur and can track them and the thunderstorms that accompany them using radar.

In 1971, University of Chicago researcher Tetsuya Fujita developed what would be called the Fujita scale, which assesses the strength of a tornado based predominantly on the amount of damage it causes. For example, an EF-0 tornado (with winds ranging between 64 and 116 kilometers, or 40 and 72 miles per hour) may strip shingles from a house, knock over television antennae, and cause tree damage. Meanwhile, an EF-5 tornado, the most powerful type on record, can contain sustained winds between 400 and 500 km (about 250 to 310 mi) per hour or more, destroying homes and large structures and causing wide swaths of damage.

In the early twenty-first century, several severe tornadoes (categorized EF-3 or higher) have touched down in the United States. For example, on May 22, 2011, a seemingly unremarkable storm system formed over southeastern Kansas. This storm quickly intensified into a supercell (a thunderstorm that contains a mesocyclone), generating small tornadoes. A massive, wedge-shaped funnel cloud (a cone-shaped, circulating cloud emanating downward from a mesocyclone that, when it reaches the ground, becomes a tornado) dropped from the storm and headed from the southwest in a northeasterly direction—the general trajectory on which tornadoes travel—to the city of Joplin, Missouri. At its apex, the EF-5 wedge (which contained multiple vortices) was approximately 1 km (0.62 mi wide), cutting a swath of devastation through the city's southern area of 49,000 people. About 160 people were killed, and another 1,150 were injured by the twister, which caused approximately $3 billion in damage; almost seven thousand homes were destroyed.

The Joplin tornado was preceded by another major storm, an EF-4 tornado that leveled a large portion of Tuscaloosa, Alabama, in April 2011. Nearly three hundred people were killed, thirty-two of whom were in Tuscaloosa, where the tornado cut a swath nearly 1.5 km (1 mi) wide in some areas.

The Joplin and Tuscaloosa tornadoes were among the 1,691 confirmed tornadoes that touched down in 2011, 59 of which were deemed “killer tornadoes” by the National Oceanic and Atmospheric Administration (NOAA). Since the start of the twenty-first century, this total number has been exceeded three times: 2004 had about 1,800 tornadoes in the United States, and 2008 had about 1,700 tornadoes. In 2019, there were 1,529 confirmed tornadoes. The year 2017 marked a nearly thirty-year low in the number of tornadoes in the US, with only 1,030 preliminary reports. In 2021, the United States saw 1,545 tornadoes; it saw 1,384 in 2022; in 2023, the US experienced 1,518 tornadoes, the twelfth most on record and higher than the ten-year average of 1,155. March 31, 2023, saw the third most tornadoes recorded in a single day, with 138 tornadoes.

Hurricanes and Typhoons

Hurricanes and typhoons are some of the most destructive natural forces on Earth. Like tornadoes, these massive storms (which are both identified scientifically as cyclones) feature high winds but also include extremely large amounts of precipitation. This combination of wind and water is potentially devastating for any area in its path.

Cyclones are tropical storms forming over the warm waters of the Atlantic and Pacific oceans near the Earth’s equator. The warm, moist air in these areas rises upward, creating areas of low pressure beneath the developing system. Air from nearby systems with higher pressure rushes into these lower-pressure areas. This air is quickly warmed and sent into the growing clouds (formed as the rising, moist air cools off). This constant updraft causes the clouds to start spinning counterclockwise if the system is above the equator and clockwise if below the equator.

The spinning eventually creates an eye in the center of the storm; the eye is a region in which the sky is clear, and the winds are calm. Winds continue to build outside the eye as more air rushes into the low-pressure system through the eye and along the ocean surface. Once winds in the storm reach speeds of 63 km (39 mi) per hour, the system is classified as a tropical storm. At 120 km (74 mi) per hour, the system becomes a cyclone or hurricane in the Atlantic and eastern Pacific oceans and a typhoon in the central and western Pacific.

There are five categories in which hurricanes and typhoons are classified. According to the Saffir-Simpson scale, the weakest of these storms, category 1, has sustained winds of between 119 and 153 km (74 and 95 mi) per hour, while the strongest type, category 5, has sustained winds of more than 252 km (157 mi) per hour. At the beginning of the annual Atlantic and Pacific cyclone seasons (roughly June 1 through November 30 and April through December, respectively), a list of names (developed by the World Meteorological Organization) is available for each storm once it reaches tropical storm status.

Hurricanes Florence and Michael, which reached the Carolinas and Florida, respectively, in 2018, were at their strongest category 4 storms. Hurricanes Katrina and Irma, which reached the Gulf region in 2005 and 2017, respectively, were category 5 storms, as was Hurricane Camille in 1969. (This storm’s final wind speed could not be determined because the storm had destroyed all available measuring devices.)

Because they are oceanic storms, hurricanes and typhoons will quickly weaken once they reach land. However, even a weak hurricane can cause massive damage, generating a storm surge (high water caused by a cyclone’s wind and low pressure) and triggering coastal flooding. Such a storm can also produce several inches of rain quickly, causing coastal and inland flooding.

Using computer modeling, remote sensor technologies, other meteorological tools, and information about major airflow patterns such as the Global Wind Oscillation, scientists work to predict the number of cyclones that will occur in a given season. Prediction is important because it helps scientists, public safety officials, and private citizens take stock of emergency measures.

Severe Winter Storms

Some forms of severe and anomalous weather occur in the winter, producing heavy snow, ice, and high winds. Blizzards, for example, are massive winter storms in which sustained winds of 56 km (35 mi) per hour or higher are coupled with snowfall.

The more common type of condition associated with blizzards is heavy snowfall of 30 centimeters (1 foot) or more. However, ground blizzards, in contrast, do not produce as much snow; instead, they have strong winds that kick up snow already on the ground. Whether the snow falls from the sky or is blown from the ground, one of the most common features of a blizzard, in addition to high winds, is extremely poor visibility of 0.5 km (0.25 mi) or less. The severe winds and poor visibility characteristic of a blizzard make these storms extremely dangerous.

Some unusual winter weather patterns do not produce a large amount of snow; rather, they produce rapid temperature changes, which cause rain that already has fallen to quickly freeze. Ice storms occur when warm, moist air typically driven from the south comes into contact with colder weather patterns. Such systems may produce freezing rain, which is rain that freezes once it contacts a colder surface. One of the best-known examples of a severe storm is a system that accumulated nearly 7 cm (3 in) of solid ice in northern New York, New England, and parts of Canada in 1998. That storm snapped trees and telephone wires, caused power outages for millions of people, and caused more than $3 billion in damage. It also led to the death of nearly forty people.

Another severe weather system, the nor’easter, continues to garner study, especially because it affects more people and has a wider geographic effect, particularly in the northeastern United States, than tornadoes and hurricanes. A nor’easter is a strong storm that involves the interplay of cold air from Canada with the warm air of the Atlantic Ocean. The two fronts create a slow-moving, counterclockwise storm with high winds and heavy precipitation.

Nor’easters are frequently known as winter storms, although they occur year-round. One of the most famous examples of a nor’easter is the so-called perfect storm of October 1991, which caused damage from as far south as Florida and north as Maine. This storm was so powerful that a hurricane formed inside the larger nor’easter. This hurricane was never named, becoming one of only eight unnamed cyclones since the naming practice was introduced in the 1950s; a more pressing need was to track the major storm and the devastation it caused.

Climate Change

Many of the devastating tornadoes, nor’easters, and hurricanes since the early 1990s may be attributed to several atypical (although not unnatural) factors. Two critical phenomena, El Niño and La Niña, are well-known contributors to weather patterns.

El Niño, a warming trend in the eastern tropical Pacific, is known to contribute to the creation of storms with heavy precipitation. La Niña, in contrast, is a period marked by cooler water temperatures that bring colder, drier air along the jet stream (the band of air currents that proceeds from the west to the east) and causes periods of cooler air in the United States. El Niño and La Niña are cyclical events caused by the interaction of the atmosphere and the surface of the ocean in the tropical Pacific.

The longtime emission of greenhouse gases into the atmosphere has also caused the atmosphere to increase in temperature, on average, across the globe. Such climate changes are theorized to foster El Niño and La Niña conditions more frequently than in previous centuries. Such shifts could lead to more severe droughts and severe hurricanes, tornado-producing storms, flooding, and blizzards.

Since 2004, many scientists have attributed high-profile and devastating storms and other extreme weather events to this trend of climate disruption. Researchers continue to seek connections between global climate change and severe weather, including heatwaves, droughts, extreme precipitation, tropical cyclones, wildfires, and major storms. The collaborative World Weather Attribution (WWA) Project, for example, began linking the occurrence of specific extreme weather events with climate change effects in 2014. Some challenges in making those attributions include assessing the scope of a given weather event, gathering sufficient data, accounting for typical natural variations like El Niño, and determining the intensity in a scenario without greenhouse gas emissions. Another approach to climate-change attribution is to assess how climatic changes worsened a given event.

Climate researchers say measurements from meteorological sensors used in climate modeling could also be used for early warning systems. These data may allow warnings to be issued several months ahead of major events such as droughts, enabling local governments and residents to prepare for and even avert disaster.

Heat waves, droughts, storms, and flooding are all forms of severe weather that have increased in intensity and frequency due to global climate change. Other aspects of global climate change, such as rising sea levels, are believed to intensify severe weather further, causing issues such as increased coastal flooding during storms. As the frequency of billion-dollar-damage storms increases in the United States, climate change's impact on severe weather's economic and social effects has become increasingly apparent. 

Principal Terms

El Niño: a meteorological condition in which the waters of the eastern, tropical Pacific Ocean are warmed by the atmosphere

Fujita scale: scale that rates the severity of tornadoes based on the amount of destruction they cause

La Niña: meteorological condition in which the waters of the eastern, tropical Pacific Ocean are cooled by a lack of radiation from the atmosphere

nor’easter: severe storm in which storm fronts combine off the Atlantic seaboard, resulting in a circulating, high-precipitation storm

Saffir-Simpson scale: system used to categorize the strength of a hurricane or typhoon based on wind speed

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