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After you've completed this section, you should be able to describe the conditions under which a thunderstorm would be considered "severe," and be able to interpret the meaning of a severe thunderstorm watch, severe thunderstorm warning, tornado watch, and tornado warning.
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All thunderstorms can be dangerous because they produce lightning. But, some thunderstorms pack more of a punch than others, bringing a variety of serious weather hazards. In order to help the public prepare for the risks associated with thunderstorms, the National Weather Service formally classifies a storm as "severe" if it produces at least one of the following:
- a tornado (a rapidly rotating column of air in contact with the ground and a cloud above)
- hail at least one inch in diameter (the size of a quarter)
- straight-line winds of at least 50 knots (58 miles per hour)
Why were these particular criteria chosen? Well, chances are, if a thunderstorm produces at least one of the above, it's more likely to damage property and / or endanger lives. Note that the formal definition of a severe thunderstorm doesn't cover all thunderstorm hazards; it says nothing about lightning or flash flooding (although some other nations do, indeed, include flash flooding in their definition of severe thunderstorms). So, just because a thunderstorm doesn't meet the official definition of "severe" doesn't mean it can't produce damage or endanger lives.
Still, formally classifying thunderstorms as severe helps forecasters alert the public on days when the risks from thunderstorms are heightened. In the United States, a branch of the National Weather Service called the Storm Prediction Center (SPC), headquartered in Norman, Oklahoma is responsible for monitoring conditions that could favor severe thunderstorms anywhere in the country.
If SPC identifies an area with favorable conditions for an organized outbreak of severe thunderstorms, they will choose to issue either a severe thunderstorm watch or a tornado watch for that region. What do these watches mean?
- A severe thunderstorm watch means that severe thunderstorms are possible and that you should be prepared.
- A tornado watch means that severe thunderstorms with multiple tornadoes or particularly strong tornadoes (in addition to other severe weather threats) are possible and that you should be prepared.
Severe thunderstorm and tornado watches are fairly large, typically covering tens of thousands of square miles (roughly the size of a state or perhaps parts of several states), and are in effect for several hours (perhaps as many as six to eight hours), but watches can be canceled, re-issued, or modified as conditions dictate. I like to think of severe thunderstorm and tornado watches as an initial "heads up" of sorts, aiming to get the public's attention that there might be severe weather (damaging straight-line winds, large hail, or tornadoes) in the coming hours, so that they can stay alert and be prepared.
To see an example of a tornado watch, check out the image below showing Tornado Watch #511, issued by SPC on November 5, 2017 at 12:10 PM EST, in effect until 7 PM EST (nearly seven hours). Watch areas, like this one, typically resemble large parallelograms, but are really issued by county. All of the counties shaded in red were part of the tornado watch, which highlighted a swath covering parts of Indiana and Ohio, including cities like Indianapolis, Dayton, Toledo, and Cleveland.
Text description of the Tornado Watch #511 Map.
The image is a weather hazard map issued by NOAA’s Storm Prediction Center, focusing on a Tornado Watch (Watch #511) affecting a large region of the Midwestern United States. The overall design uses a light, neutral background map with county boundaries outlined in thin lines, while the areas under alert are highlighted prominently in shades of red. The graphic has a clean, data-focused appearance typical of official meteorological products.
At the center of the image, a broad, irregular region is shaded in deep red, indicating the active tornado watch area. This shaded region stretches diagonally from central and southern Indiana northeastward into northern Ohio, forming a band that covers numerous counties. The red shading varies slightly in tone but remains consistently bold, making the affected area stand out strongly against the pale background. The watch area includes and surrounds several labeled cities, such as Indianapolis, Terre Haute, Lafayette, Fort Wayne, Dayton, Toledo, Findlay, and Mansfield, indicating that both densely populated and rural areas fall under the alert.
Surrounding the highlighted region, the map shows neighboring states and cities in lighter tones, providing geographic context. Major cities outside the watch area—such as Chicago to the northwest, Detroit to the north, Cleveland to the northeast, and Louisville to the south—are labeled in small black text. State boundaries are subtly drawn, and a grid of county lines is visible across the entire map, emphasizing the precise coverage of the warning. Lake Michigan appears in the upper-left portion as a large light-gray body of water, helping orient the viewer geographically.
In the lower-left corner, the NOAA logo appears in blue and white, reinforcing the official source of the information. Along the bottom edge of the image, a text banner provides key details about the alert. The text reads: “Tornado Watch #511 – Valid from 1210 PM until 700 PM EST”, followed by a timestamp on the right side indicating when the graphic was last updated (“2017/11/05/1715 UTC”). The text is displayed in a combination of red and black, matching the urgency of the map’s color scheme.
While no forecasters are perfect, the forecasters at SPC are very good at what they do, and this case was no exception. The tornado watch box was meant to raise awareness of the possibility of multiple tornadoes across Indiana and Ohio, and the severe weather reports from November 5(opens in a new window) show that more than a dozen tornadoes occurred in the area covered by the watch (each red "T" marks a tornado report, while each blue "W" marks a report of damaging straight-line winds).
A more zoomed out look at the severe weather reports from November 5(opens in a new window) shows that tornadoes and damaging wind also occurred across northwestern Pennsylvania, and severe hail occurred across parts of Missouri and Illinois. Given the large areas that can experience severe weather in a single day, it's not uncommon for several severe thunderstorm and / or tornado watches to be in effect simultaneously. In fact, SPC issued three tornado watches, along with two severe thunderstorm watches on this date, represented by the red and blue parallelograms, respectively on this animation of SPC watches superimposed on radar(opens in a new window) spanning 23Z on November 5 to 0015Z on November 6. As you can see, SPC covered the area where severe weather occurred quite well!
When severe thunderstorms are occurring, or when radar imagery reveals signs of imminent severe weather, a local forecast office of the National Weather Service will issue a severe thunderstorm warning or tornado warning. What do these warnings mean?
- A severe thunderstorm warning means that forecasters have identified a thunderstorm capable of producing at least one-inch diameter hail or wind gusts of at least 50 knots (58 miles per hour) based on its radar characteristics (or storm spotters are reporting that at least one of these things is occurring). Seek shelter immediately!
- A tornado warning means that forecasters have identified a thunderstorm capable of producing a tornado (possibly in addition to other severe weather) based on its radar characteristics (or based on spotter reports). Seek shelter immediately!
If a severe thunderstorm warning or a tornado warning has been issued for your area, imminent danger to lives and / or property from severe weather may exist currently or very soon. Take shelter and any other necessary precautions immediately!
Severe thunderstorm warnings and tornado warnings are more "urgent" than watches. Warnings also are much smaller than watches, usually spanning about the size of a single county, or maybe portions of several counties, and are also in effect for much shorter time periods (usually about an hour or less). Ideally, warnings are issued far enough in advance to give people in potentially-affected areas enough time to seek shelter and take other safety measures (at least a few minutes before severe weather strikes). Within the tornado watch from November 5, 2017 (shown above), a number of individual tornado warnings were issued, such as the one shown below, for a small area of north-central Ohio. Note the relatively small size of the tornado warning (compared to a watch) in the inset map on the lower left. This particular warning was in effect for just 32 minutes (it was issued at 4:58 PM local time and expired at 5:30 PM local time).
Text description of the Tornado Warning Graphic.
The image is a weather alert graphic titled “Tornado Warning,” presented in a clear, high-contrast layout designed to quickly communicate urgent information. A wide red banner spans the top of the image, with the title “Tornado Warning” written in large white text. The background behind the rest of the graphic is a dark gray, which helps the lighter text and map details stand out. The overall design resembles an official emergency notification panel used by meteorological agencies.
On the left side of the graphic, a vertical panel provides detailed warning information. Near the top, the text reads “Valid Until 5:30 PM EST Sunday November 5, 2017,” indicating the expiration time of the alert. Below this, a section labeled “Threat Information” appears with a thin red underline. Two hazards are listed: “TORNADO – Radar Indicated,” accompanied by a simple white tornado icon, and “HAIL – Penny Sized Possible,” shown with a small icon of a falling hailstone. Further down, another section labeled “Potential Exposure” lists numerical details: “Population: 35,450,” “Schools: 16,” and “Hospitals: 1.” A small information icon appears next to these statistics. At the bottom of the left panel, a small inset map shows a portion of the Great Lakes region, including parts of Michigan, Ohio, Pennsylvania, and West Virginia, with a red-highlighted area indicating the broader region affected.
The right side of the image is dominated by a detailed map showing a specific area under warning. The map includes labeled towns such as Norwalk, Collins, Wakeman, Clarksfield, Olena, North Fairfield, New London, and Holiday Lakes, along with nearby locations like Milan, Berlin Heights, and Huron closer to the top. Major roadways are visible, including Interstate 80 and Route 20, as well as smaller connecting roads. A large, irregular red polygon overlays the central portion of the map, clearly marking the tornado warning area. This shaded region covers multiple towns, indicating where the greatest risk is currently present.
The map uses muted earth tones for land and pale blue for nearby water along the northern edge, with thin lines representing roads and boundaries. The red warning polygon sharply contrasts against these subdued colors, drawing immediate attention. In the lower-right corner, faint official seals or logos are visible, reinforcing that the graphic comes from an authoritative weather source. Overall, the image combines text, symbols, and geographic detail to communicate both the severity and the exact location of the tornado risk in a clear, accessible format.
Ultimately, the severe weather (either damaging straight-line winds, large hail, or a tornado) that occurs within a warning area often only affects a small fraction of the area covered by the warning, so a severe thunderstorm warning or tornado warning is not a guarantee that you will personally be impacted by severe weather. A tornado did occur within this particular warning(opens in a new window), but its damage path never exceeded 400 yards wide (less than one-quarter of a mile). So, most people in the warning area were not affected by the tornado, which is typical.
Most warnings are issued based on analysis of Doppler radar data, and in the case of tornado warnings, that's a double-edged sword. On the good side, Doppler radar has helped forecasters give earlier advanced warning of tornadoes. Before the Doppler radar era (which began nationwide in the early 1990s), tornado warnings were issued only about three minutes before the actual tornado occurred, on average. But, with Doppler radar's ability to detect rotation within thunderstorms, forecasters can give more advanced warning (10-15 minutes, on average), giving the public more time to seek shelter.
But, not all thunderstorms that exhibit rotation actually form tornadoes (in fact, most don't), and that leads to many "false alarm" tornado warnings (warnings issued in cases where a tornado never occurs). Statistics show that about 75 percent of tornado warnings are actually false alarms (opens in a new window). Weather forecasts have improved greatly over the years, but small-scale, fast-changing weather events still provide huge challenges to forecasters, and the precision and accuracy of warnings is an area where there's still lots of room for improvement. The new capabilities of dual-polarization radar (to detect debris from tornadoes) may be able to lower the false-alarm percentage in time, but there's significant concern that the high false-alarm rate will lead to public complacency about tornado warnings. Still, at the end of the day, it's wise to take all warnings seriously and take appropriate precautions immediately. If you prepare and seek shelter and then no severe weather occurs, you're safe (although maybe inconvenienced or annoyed). But, if you ignore the warning and then you're ill-prepared for severe weather that does occur, you may end up injured or dead.
Being able to interpret severe thunderstorm watches and warnings is a critical life skill so that you can identify situations when you may need to take quick actions to seek shelter. I highly recommend having a trusted source of weather information that allows you to get watches and warnings at all times (such as NOAA Weather Radio(opens in a new window), or a reliable weather mobile app). Receiving and acting on critical warnings could save your life! Up next, we're going to start looking at the types of thunderstorms that are the most prolific severe weather producers. Read on!