We’re approaching the end of winter and the inevitable climb toward summer. The annual ritual of warmth flushing out the cold makes the atmosphere pretty restless, allowing dangerous storms to bubble up on a regular basis. It’s critically important for everyone to know what to do when bad weather strikes, and staying safe begins with knowing the six terms meteorologists use most often to describe life-threatening weather.
1. Severe Thunderstorm
The defining type of spring weather in the United States is severe thunderstorms. A severe thunderstorm is one that produces hail the size of quarters or larger, wind gusts that reach 58 mph or stronger, or a tornado.
Severe thunderstorms can seriously injure or kill you if you’re caught outside during one, or if you’re in a building that can’t withstand the fierce winds or damaging hail. After all, the wind gusts within a severe thunderstorm can cause as much damage as a tornado. Hail is no laughing matter, either—even small pieces can easily shatter windows, especially when it falls during strong winds.
Tornadoes are the best-known type of severe weather. These destructive forces can range in shape from a thin rope to a wedge that consumes the landscape. Most tornadoes are small and don’t cause much damage, but a handful of tornadoes every year are strong enough to level entire neighborhoods.
One of the most dangerous things about a tornado is that you can’t always see one coming. We can see tornadoes because of the condensation funnel, or the cloud that forms as a result of the low pressure within the rotating column of air. Some tornadoes—especially when they just start to form—don’t have that condensation funnel, making them hard to spot until it’s too late. To make matters worse, many tornadoes that form in humid areas are often wrapped in heavy rain, making one impossible to see until it’s right on top of you.
One of the most common terms you’ll hear during the spring and summer is watch, either a severe thunderstorm watch or a tornado watch. A watch means that conditions are favorable for dangerous thunderstorms to develop over the next couple of hours, and they’re issued to give people as much time as possible to seek safety. If you’re ever placed under a watch, it means that you should keep an eye out for urgent weather updates through the day.
Severe thunderstorm and tornado watches are issued for wide areas on a county basis, sometimes covering entire states. You should always know what county you’re in and what counties surround you—it could save your life one day.
Where we’re talking about severe weather, an advisory is more urgent than a watch but not as urgent as a warning. They’re issued for less dangerous conditions that may cause disruption or inconvenience if people aren’t adequately prepared. In spring, you may hear of advisories issued for high winds, dense fog, or potential flooding.
A warning is an urgent alert that means your immediate safety is at risk. When a severe thunderstorm develops, meteorologists at the National Weather Service will issue a severe thunderstorm warning that tells you about the imminent arrival of large hail or damaging winds. A tornado warning means that meteorologists detect rotation in a thunderstorm or that someone has spotted a tornado on the ground.
Warnings are issued ahead of individual storms using polygons on a map that cover areas most likely to see dangerous weather. Most severe thunderstorm and tornado warnings are issued 15 to 30 minutes before the storm arrives, but sometimes there’s little or no warning ahead of time. If you’re ever placed under a warning, you have to act quickly to protect your life.
Sometimes when a large tornado is barreling toward a city, the National Weather Service will issue an extremely rare “tornado emergency” instead of a tornado warning. The enhanced language is designed to drive home the point that you need to seek safety, stat.
6. Storm Prediction Center
The Storm Prediction Center (SPC) is a branch of the National Weather Service that’s devoted to predicting severe thunderstorms. The agency issues thunderstorm forecasts up to seven days out. The risk for severe weather is conveyed using a five-category scale that ranges in severity from “marginal risk” to “high risk.” Days featuring a high risk—a 5 on the scale—are extremely rare, saved for the worst severe weather outbreaks that could produce widespread destruction and claim dozens if not hundreds of lives.
You should check the SPC at least once a day during severe weather season, and more often if hazardous weather is on your doorstep.
A version of this story ran in 2016; it has been updated for 2023.