Changing Thunderstorm Potential | Climate Central

KEY CONCEPTS

  • Severe weather events — thunderstorms that produce tornadoes, damaging winds, and/or hail — have accounted for nearly half of all billion-dollar weather disasters in the United States since 1980.

  • This week, we look at how a key indicator of thunderstorm severity – available convective potential energy (CAPE) – has changed with recent warming.

  • CAPE values ​​typically peak during the warmer months in the eastern United States. But since 1979, parts of the eastern United States have seen up to 10 to 15 additional days of high CAPE values ​​in spring and summer, the peak time for thunderstorms.

  • On a yearly basis, higher CAPE days have become more frequent in the eastern United States and less frequent in the west.

  • Severe weather is complicated. CAPE is only one ingredient in the recipe for severe weather, and it’s unclear how other ingredients might react to further warming.

Seasonal change GIF
Annual Change - Changing Storm Potential
Annual change

Severe weather events – thunderstorms that include tornadoes, damaging winds and/or hail – account for nearly half (47%) of the $323 billion weather disasters that have affected the United States since 1980. This week , we examine how a key indicator of storm severity – available convective potential energy (CAPE) – has changed with recent warming.

The severity of thunderstorms is a sliding scale. Warm, rising air is central to the formation of thunderstorms. The amount of energy available for this rising air is called available convective potential energy (CAPE).

  • Higher CAPE values ​​mean more energy available to move through the atmosphere, and therefore greater potential for thunderstorms.

  • CAPE values ​​vary by season and region, but there is no absolute threshold above which a thunderstorm is considered severe. Thunderstorms often have CAPE values ​​of 1000 J/kg or more, and values ​​above 4000 J/kg represent extreme atmospheric instability.

  • The likelihood of a certain CAPE value fueling severe weather depends on whether it is above typical CAPE values ​​for that location and time of year.

The potential for severe thunderstorms is highest in the spring and increasing. To understand how thunderstorm potential has changed with rising temperatures, Climate Central analyzed trends in CAPE values ​​in the United States from 1979 to 2021.

  • Storm potential depends on the season. The highest CAPE values ​​typically occur between April and September each year, the same months that have historically seen the most frequent, costliest and deadliest impacts from severe storms in the United States.

  • CAPE values ​​increased the most in spring and summer. Spring has seen the largest increases, with 10-15 days longer with CAPE values ​​above 1000 J/kg since 1979 in parts of the Ohio Valley (central) region and hotspots in Kentucky and Tennessee. During the summer, parts of the northeast have experienced 10-15 days or more with CAPE values ​​above 1000 J/kg since 1979.

The continental fracture (convective). Whether the thunderstorm potential increases or decreases since 1979 depends on which half of the country you are in.

  • The western United States has experienced fewer high spring CAPE days since 1979, especially in the lower Great Plains.

  • The eastern United States has seen more spring high CAPE days since 1979, especially in the Midwest and East Coast.

  • Considering the potential for even more severe storms (where CAPE ≥ 2,000 J/kg) over all seasons, the country remains divided.

Severe weather and the link with the climate. Assessing the impact of climate change on severe weather events is difficult because severe storms are complex, local, and short-lived.

  • CAPE quantifies air instability, one of many important factors in the formation of severe weather.

  • Tornadoes also need rotation. It is unclear how the wind shear that generates the tornado’s rotation responds to rising temperatures.

  • Still, the number of tornadoes during large outbreaks and the frequency of storm-producing weather patterns tend to increase.

POTENTIAL LOCAL STORY ANGLES

What does severe weather look like near you?
NOAA’s Storm Prediction Center has local tools for storm prediction, tornado monitoring, and convection mapping. They also feature an interactive severe weather data map dating back to 1992. For immediate severe weather monitoring, check out the National Weather Service’s customizable Severe Weather Monitor.

What is the impact of severe weather events on your community?
Thunderstorms are a common cause of weather-related power outages; if the storms rise, the blackouts rise. The risk of outage is local and depends on the integrity of the transmission line, utility company practices, etc. Track power outages across the country and get to know your local energy infrastructure. In addition, compound extreme events are expected to become more frequent. This complicates disaster risk and decision-making; for example, one may need to choose to shelter in the basement or attic during a simultaneous thunderstorm-tornado. Extreme weather conditions can lead to business closures, infrastructure damage and road closures which can be monitored through the National Weather Service tool.

LOCAL EXPERTS

The SciLine service, 500 Women Scientists or local university press offices may be able to connect you with local scientists who have expertise in severe weather and climate change. The American Association of State Climatologists is a professional scientific organization made up of all state climatologists.

NATIONAL EXPERTS

  • Harold Brooks, Ph.D.
    senior scientist,
    NOAA National Severe Thunderstorm Laboratory
    harold.brooks@noaa.gov
    Expertise: severe weather science

  • Richard Rood, Ph.D.
    Professor, Department of Climate and Space Science and Engineering and
    The School of Environment and Sustainability,
    University of Michigan
    rbrood@umich.edu
    Expertise: human impacts, adaptation, modelling, climate change and severe weather phenomena

  • Robert Trapp, Ph.D.
    Chef, Professor of Atmospheric Sciences
    University of Illinois Urbana-Champaign
    jtrapp@illinois.edu
    Expertise: severe weather science

METHODOLOGY

The daily maximum available convective potential energy (CAPE) from 1979 to 2021 was calculated for the contiguous United States using data from the North American regional reanalysis. The number of days when the CAPE reached or exceeded 1000 J/kg (generally considered the lower threshold for severe storms) was calculated for each season and added together to calculate the annual total. Linear regression analysis was used to calculate an estimate of total change.

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