Hazard guide · Severe storms

Understanding severe thunderstorms

Every thunderstorm produces lightning, and some bring damaging wind and hail too. This guide explains what forecasters mean by "severe," how to read the storm outlook days ahead, and how to stay safe when the power goes out.

Last reviewed October 2026

When a storm counts as severe

The National Weather Service issues a severe thunderstorm warning when a storm is producing, or is about to produce, at least one of these:

• Hail 1 inch across or larger, about the size of a quarter
• Wind gusts of 58 mph or more
• A tornado (which also triggers a tornado warning)

Heavy rain and frequent lightning alone don't make a storm severe, but both can still be dangerous.

Read the outlook days ahead

The NOAA Storm Prediction Center publishes a convective outlook up to eight days ahead. It rates the risk of severe storms on a five-level scale. Local TV stations and weather apps often show the same map.

The name "Slight" can be misleading. Level 2 days still produce damaging storms and tornadoes. Use any outlook in your area as a reason to charge devices, secure outdoor items and know where you'll take shelter.

Damaging wind

Most severe weather damage comes from straight-line wind, not tornadoes. A downburst happens when cold air rushes out of a storm and spreads across the ground, sometimes with gusts over 100 mph. A small, intense downburst is called a microburst.

A derecho is a long-lived line of storms that produces widespread wind damage across hundreds of miles. Derechos can knock out power to millions of people and leave it off for days, often during summer heat.

Hail

Hail forms when strong updrafts carry raindrops high into freezing air, where they grow in layers before falling. The stronger the updraft, the bigger the hail. Common size comparisons used in reports:

Large hail can break windows and injure people and animals outside. Park cars in a garage or under cover when severe storms are forecast.

Lightning

Lightning can strike 10 miles or more from the storm, often before rain arrives or after it seems to have passed. If you can hear thunder, you are close enough to be struck.

• Go inside a building with plumbing and wiring, or a hard-topped vehicle with the windows up. Picnic shelters, tents and dugouts don't protect you.
• Wait 30 minutes after the last thunder before going back outside.
• Indoors, stay off corded phones and away from plumbing, since lightning can travel through wiring and pipes.
• If someone is struck, call 911. Victims do not carry an electrical charge and are safe to touch, so start CPR if needed.

When the power goes out

• Keep the refrigerator and freezer closed. A closed fridge keeps food cold for about 4 hours. A full freezer holds its temperature for about 48 hours, and a half-full one for about 24.
• Throw out perishable food that has been above 40°F for 2 hours or more.
• Run generators only outdoors, at least 20 feet from doors, windows and vents. Carbon monoxide from generators kills people every year. Put battery-powered CO alarms in your home.
• Use flashlights instead of candles.
• Unplug electronics to protect them from a surge when power comes back.
• Stay away from downed lines and treat every one as live. Report them to your utility.

Sources: NOAA Storm Prediction Center; National Weather Service lightning and severe weather safety pages; CDC; Ready.gov.