Severe Storms

Since 1980, there have been 186 recorded severe storms causing at least $1 billion in damage, totaling $455 billion by 2023. Over a third of the storm events have occurred in the past five years.

• As baseline temperatures rise, the threat of severe storms increases. Warmer air holds more moisture, enabling stronger thunderstorm events and impacting more communities as developments push into storm-threatened regions.

A severe storm produces wind gusts of at least 58 mph, hail one inch or larger in diameter, and/or tornadic activity (NOAA https://www.weather.gov/bgm/severedefinitions). Flash flooding is also associated with severe storm threats. There are about 100,000 thunderstorms each year in the US. About 10% of these reach severe levels (NOAA https://www.weather.gov/key/tstmhazards#:~:text=Approximately%2016%20million%20thunderstorms%20occur,hail%20or%20larger%20or%20tornadoes).

Thunderstorms form when warm, moist air rises into cold air and condenses into a rain event with lightning occurring from within the cumulonimbus cloud (a.k.a anvil clouds). Thunder comes from lightning, so, all thunderstorms have lightning as a convective feature. Lightning occurs as the negative charges (electrons) in the bottom of the cloud are attracted to the positive charges (protons) on the ground (UCAR https://scied.ucar.edu/learning-zone/storms/thunderstorms). Temperatures in the air of a lightning channel may reach as high as 50,000 °F, five times hotter than the sun (NWS https://www.weather.gov/safety/lightning-science-thunder#:~:text=The%20lightning%20discharge%20heats%20the,air%20cools%20and%20contracts%20quickly).

Hail forms when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere and freeze (NOAA NSSL https://www.nssl.noaa.gov/education/svrwx101/hail/). Hailstones grow by colliding with liquid water freezing onto the hail. With rising baseline temperatures due to extreme weather, smaller hail pellets begin to melt before reaching the surface causing larger hailstones to become the main hail event as extreme weather progresses.

Tornadoes are narrow, violently rotating columns of air that extend from the base of a thunderstorm to the ground. About 1,200 tornadoes occur in the US annually (NOAA NSSL https://www.nssl.noaa.gov/education/svrwx101/tornadoes/) although recent years have been overactive.

Damaging Winds are often called “straight-line” winds to differentiate the damage they cause from tornado damage.  https://www.nssl.noaa.gov/education/svrwx101/wind/

• Damaging Wind Gusts range between 58 mph and 74 mph (between 50 knots and 64 knots) causing minor damage. Very Damaging Wind Gusts range between 75 mph and 91 mph (between 65 knots and 79 knots) causing moderate damage.

CISA’s Role in Increasing Severe Storm Resiliency

CISA continues to be a major purveyor of forecast data for partners before, during, and after severe weather events to ensure the widest distribution of products, dashboards, and reports for pre-event planning and mitigation methodologies.

CISA produced the Emergency Communications and Extreme Weather Factsheet to assist in hardening telecommunications and critical power infrastructure to be more resilient in the face of worsening storms.

CISA also produced the Resilient Power Best Practices for Critical Facilities and Sites guide to support emergency and continuity managers with guidelines, analysis, background material, and references to increase the resilience of backup and emergency power systems during all durations of power outages.

CISA is a key member in the NCS4 (National Center for Spectator Sports Safety and Security) Lightning Safety Task Force and contributing author for severe storm protective response industry policy.

Critical Infrastructure Impacts

The threat to critical infrastructure owners and operators comes from the initial storm damages due to lightning, large hail, high wind damages, tornadoes, and local flooding events then the secondary impacts such as delayed power restoration, riverine flooding from runoff, delayed or damaged supply chain operations, sanitation threats, water sector degradation, structural instability delaying return to operations, and impacts throughout the community from debris, evacuations, and outages.

As developments in hazardous areas continue and atmospheric instability increases, critical infrastructure sites will see increased damage reports that include:

• Uprooted objects, including trees, furniture and pieces of houses causing damage along the storm path.
• Hail damaged vehicles, glass, siding, exposed metal, and most roofing types resulting in microfractures, dents, mineral loss, or material separation.
• Power outages leading to equipment cost increases for utilities and supply chain delays extending outage periods.
• “Blown” transformers or breakers on the lines or trigger a safeguard shutdown of the energy equipment due to lightning strikes.
• Damaged or downed powerlines. Most can withstand winds around 55mph, while severe storms can produce winds over 75 mph.
• Damaged surface soils, fences, barns, livestock, homes, exposed pipes, water quality, and various supporting infrastructure to agricultural operation as the scale of impact can range multiple states.
• Rapid surges of water runoff with short notice due to downpouring rain events causing upstream rivers to runover into local streams or low-lying areas.
• Damage to transportation includes derailed trains, damaged roadways, miles-wide debris fields, downed live powerlines, potential aviation damage and delays, and impacts across trucking and standard vehicle movements.
• Pipelines damage from soil slips in flash floods and straight-line and tornadic wind damage, reducing flows or damaging equipment.
• In December 2021 (a rare time for severe tornadoes), tornadoes ripped through 6 states killing scores of people and causing billions of dollars in damages. Factories and warehouses collapsed and over 15,000 structures were damaged. (NYTimes, https://www.nytimes.com/live/2021/12/13/us/tornadoes-kentucky-illinois).
• Severe storms are expected to increase risk of moderate disruptions to many National Critical Functions (NCFs), especially the Provide Medical Care and Provide Housing. (RAND, https://www.rand.org/pubs/research_reports/RRA1645-7.html, p.24).