6 Storms and Severe Weather
Learn how thunderstorms, tornadoes, hurricanes, and other severe weather develop, what hazards they bring, and how to respond to watches, warnings, and safety guidance.
Thunderstorms: formation, hazards, and safety
A develops when moist, unstable air rises and forms a tall cloud. Rising air can build into a cumulonimbus cloud; sinking air and precipitation help drive the storm’s later stages. In the United States, a storm is considered severe if it produces a , wind gusts of at least miles per hour, or hail at least inch in diameter.
Thunderstorms may produce several hazards:
Lightning can strike outside the area where rain is falling.
Hail forms when strong updrafts carry water droplets into freezing parts of a cloud, where they grow into ice stones.
Damaging winds include downbursts: powerful currents of air that rush downward and spread out near the ground.
Heavy rain and flash flooding are especially possible when storms move slowly or repeatedly pass over the same place.
Even a non-severe can produce dangerous lightning and heavy rain.
At the first sound of thunder, move into a substantial enclosed building or a hard-topped vehicle with the windows closed. Stay sheltered for minutes after the last thunder. During damaging winds or hail, move inside and stay away from windows.
Tornadoes: formation, ratings, and shelter
A is a violently rotating column of air that extends from a to the ground. Many tornadoes develop from organized severe thunderstorms when changes in wind speed or direction with height—called —help create rotation. Tornadoes can also form in other ways, and not every severe produces one.
The Enhanced Fujita (EF) scale rates tornadoes from EF0 to EF5. Damage surveys are used to estimate intensity.
A means tornadoes are possible; be ready to act. A means a has been observed or indicated by radar; take shelter immediately.
In a sturdy building, go to a basement or a small interior room on the lowest floor, away from windows, and protect your head. If you are in a vehicle or mobile home, get to a sturdy shelter if possible. Do not shelter under an overpass.
Hurricanes: classification, hazards, and response
Hurricanes are powerful tropical cyclones that form over warm ocean water. In the Atlantic and eastern North Pacific, a tropical cyclone is called a when its sustained winds reach miles per hour, and a at miles per hour. The Saffir–Simpson Wind Scale classifies hurricanes by wind speed, but a category does not describe every danger a storm may bring.
hazards include:
: abnormal ocean water pushed onto the coast by a storm. It can cause destructive coastal flooding and may travel inland.
Heavy rain and inland flooding: flooding can occur far from the coast, even after a storm weakens.
Damaging winds, large waves, and tornadoes.
A means conditions are possible in the specified area; a means they are expected. Follow local instructions, including evacuation orders. If told to evacuate, leave promptly. If sheltering in place, use a sturdy building, stay away from windows, and never walk or drive through floodwater.
Other severe weather hazards
Flash floods can rise quickly after intense rain, including rain from thunderstorms or tropical storms. Move to higher ground, and never drive or walk through floodwater: the road may be damaged, or the water may be deeper and faster-moving than it appears.
Damaging wind events, including long-lived lines of thunderstorms, can bring widespread wind damage even when no forms. Secure loose outdoor objects and shelter indoors away from windows.
Winter storms can bring heavy snow, ice, and dangerous cold. These conditions can make travel hazardous and potentially interrupt power. Check forecasts before traveling and follow local emergency guidance.
Extreme heat can cause heat-related illness. Reduce strenuous activity during the hottest periods, drink water, and check on people who may be especially vulnerable.
Forecasts, alerts, and preparedness
A means hazardous weather is possible in an area; review your plan and be ready. A means the hazard is occurring or imminent; take protective action. The exact meaning and recommended action depend on the hazard, so read the alert and follow local officials’ instructions.
Monitor reliable weather alerts, know your safest nearby shelter, and keep essential supplies and a way to receive warnings if power or phone service is interrupted. For any storm, stay informed, follow official instructions, and avoid floodwater.