Rise in severe convective storms increases risk

If you live anywhere in the United States, you likely have received a severe thunderstorm warning from the National Weather Service. Such a warning indicates there is imminent danger to life and property and that individuals should take shelter in a substantial building.

Thunderstorms are part of what are more broadly known as convective storms. Severe convective storms are among the most common and most damaging natural catastrophes in the United States.

Thunderstorms form when warm, moist air rises into cold air. The warm air becomes cooler, which causes water vapor to condense into small water droplets. This circuit of rising and falling air is called a convection cell. And the perils that can result run the gamut — strong wind gusts and straight-line winds, lightning strikes, hailstorms, flash flooding and — most fearsome of all — tornadoes.

Severe convective storms can develop quickly and strike with little warning. The Great Plains and the Midwest are known to be hotbeds of storm activity, but, in reality, convective storms occur everywhere in the United States.

Economic impact

Convective storms are among the costliest natural disasters in the United States and around the globe. During the 2022 Atlantic Hurricane Season, there were 14 named storms, eight hurricanes, and two major hurricanes. It was the third costliest Atlantic hurricane season on record. Severe convective storms resulted in a record $63 billion in economic losses in 2020, surpassing the previous record of $53 billion 2011, according to Aon. Severe convective storms caused $35 billion — nearly 70% — in insured losses worldwide in the first half of 2023, according to a Swiss Re Institute report.

Twisters

Tornadoes are among the most violent and destructive storms, with wind speeds that can reach more than 200 miles per hour. The United States averages more than 1,200 tornadoes each year. While many occur in sparsely populated regions east of the Rocky Mountains, tornadoes have been recorded in all 50 states. Summer is the peak season for tornadoes, but they can form any time of the year.

Lightning

Lightning is a giant spark of electricity that originates in clouds or air. Cloud-to-ground lightning strikes happen when the negative electricity originating in the atmosphere connects with the positive charge of an object below. When this atmospheric electrical charge is less than a hundred yards from the ground, objects such as trees or buildings send up sparks to meet it. When those sparks connect, the resulting channel creates a huge electric current surge that rapidly travels downward, producing the flashing bolt we see as lightning.

Lightning can strike anywhere outdoors, but especially on elevated and open areas, near water or tall, isolated objects, such as trees.

More than 23 million cloud-to-ground strikes occur each year in the United States, according to the National Weather Service.

Direct lightning strikes in residential and non-residential properties combined cause millions of dollars in fire damages every year, according to the Insurance Information Institute. The Lightning Protection Institute notes that total business losses related to lightning, either directly or indirectly, can run into the billions. In 2018 alone, almost $1 billion in lightning claims were paid to nearly 78,000 policy holders.

In the United States, two-thirds of the area burned by wildfires is caused by lightning.

Hail

Tornadoes garner most of the attention because of their destructive force. But hail may be the costliest of the convective storm perils. According to the National Severe Storms Laboratory, “Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere, where they freeze into ice.”

Most hailstorms are relatively harmless. A simple handheld umbrella can keep small ice pellets at bay. But when widespread hailstorms with “golf ball-sized” or “softball-sized” chunks of ice start falling from the sky, nothing on the ground is safe. Auto dealers are particularly vulnerable, as hail can damage an entire fleet of new cars on the lot. But homes, office buildings, manufacturing plants and crops can also suffer from significant losses.

Hailstones form when moist, warm air rises into the upper atmosphere on powerful convective wind currents, allowing water vapor to freeze around specs of dust. As ice particles grow heavier, they fall to lower altitudes where strong winds may lift them higher once again, collecting more layers of ice. The cycle continues until the hailstones are too heavy to remain airborne and fall with varying levels of force, depending on their size.

Flash flooding

Severe convective storms can lead to extremely heavy rainfall, which can create flash flooding.

Flash floods are among the most dangerous kind of floods because they combine the destructive power of a flood with incredible speed. Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it. They also occur when water fills normally dry creeks or streams or enough water accumulates for streams to overtop their banks, causing rapid rises of water in a short amount of time.

The intensity of the rainfall, the location and distribution of the rainfall, the land use and topography, vegetation types and growth/density, soil type, and soil water-content all determine just how quickly flash flooding may occur.

Urban areas are prone to flooding in short timespans because the impervious surfaces in the urban areas do not allow water to infiltrate the ground, and the water runs off to the low spots very quickly.

Fire and water

Large wildfires can sometimes lead to severe convective storms. When fires get big enough, they can create their own weather systems. Pyrocumulonimbus clouds are formed by the same forces that generate typical convective storms — heat from the ground, driving moisture upward until it forms clouds. This weather process may produce rain, but it may just as likely produce lightning and powerful winds that can increase the fire’s intensity.

It may be difficult to discern if convective storm activity is increasing, but it’s clear that losses from these storms is definitely on the rise.

Visit the Convective Storm Resource Hub for additional articles offering insights on how to help mitigate, respond to and recover from lightning strikes, hailstorms and tornados.