Last modified 25 Sep 7

This list of Frequently Asked Questions (FAQ) has been compiled from questions asked of the SPC as well as basic tornado research information and countless scientific resources. More material will be added, time permitting.

The Tornado FAQ is not intended to be a comprehensive guide to tornadoes. Instead, it is a quick-reference summary of tornado knowledge, which will link you to more detailed information if you desire. Recent books from your local library or a major university library are still the deepest resource for learning about tornadoes and other severe storms; so if you are doing your own research or school reports, please visit the library in person. There are many good websites with tornado information also. Some of them are linked from the answers below. None of the links to outside websites implies any kind of commercial endorsement on the part of the SPC. The intent here is to direct you to the best tornado info available. There is also a partial list of technical scientific references related to tornadoes for those with some meteorological education and training.

NOTE: All images found in FAQ pages on this site must be public domain and not copyrighted. However, credit MUST be given to National Oceanic and Atmospheric Administration/Department of Commerce unless instructed to give credit to the photographer or other source. Complete Tornado FAQ content is found at the mirror-backup site.

What is a tornado? According to the Glossary of Meteorology (AMS 2000), a tornado is "a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud." Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice, however, to classify and define tornadoes. For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also disgreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. It is well-known that a tornado may not have a visible funnel. Also, at what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.
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How do tornadoes form? The classic answer -- "warm moist Gulf air meets cold Canadian air and dry air from the Rockies" -- is a gross oversimplification. Many thunderstorms form under those conditions (near warm fronts, cold fronts and drylines respectively), which never even come close to producing tornadoes. Even when the large-scale environment is extremely favorable for tornadic thunderstorms, as in an SPC "High Risk" outlook, not every thunderstorm spawns a tornado. The truth is that we don't fully understand. The most destructive and deadly tornadoes occur from supercells -- which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX program suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone (the occlusion downdraft). Mathematical modelling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind these theories are given in several of the Scientific References accompanying this FAQ.
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What direction do tornadoes come from? Does the region of the US play a role in path direction? Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked. [A tornado can double back suddenly, for example, when its bottom is hit by outflow winds from a thunderstorm's core.] Some areas of the US tend to have more paths from a specific direction, such as northwest in Minnesota or southeast in coastal south Texas. This is because of an increased frequency of certain tornado-producing weather patterns (say, hurricanes in south Texas, or northwest-flow weather systems in the upper Midwest).
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Does hail always come before the tornado? Rain? Lightning? Utter silence? Not necessarily, for any of those. Rain, wind, lightning, and hail characteristics vary from storm to storm, from one hour to the next, and even with the direction the storm is moving with respect to the observer. While large hail can indicate the presence of an unusually dangerous thunderstorm, and can happen before a tornado, don't depend on it. Hail, or any particular pattern of rain, lightning or calmness, is not a reliable predictor of tornado threat.
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How do tornadoes dissipate? The details are still debated by tornado scientists. We do know tornadoes need a source of instability (heat, moisture, etc.) and a larger-scale property of rotation (vorticity) to keep going. There are a lot of processes around a thunderstorm which can possibly rob the area around a tornado of either instability or vorticity. One is relatively cold outflow -- the flow of wind out of the precipitation area of a shower or thunderstorm. Many tornadoes have been observed to go away soon after being hit by outflow. For decades, storm observers have documented the death of numerous tornadoes when their parent circulations ( mesocyclones) weaken after they become wrapped in outflow air -- either from the same thunderstorm or a different one. The irony is that some kinds of thunderstorm outflow may help to cause tornadoes, while other forms of outflow may kill tornadoes.
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Do tornadoes really skip? Not in a literal sense, despite what you may have read in many older references, news stories, or even damage survey reports. By definition (above), a tornado must be in contact with the ground. There is disagreement in meteorology over whether or not multiple touchdowns of the same vortex or funnel cloud mean different tornadoes (a strict interpretation). In either event, stories of skipping tornadoes usually mean

There was continuous contact between vortex and ground in the path, but it was too weak to do damage;

Multiple tornadoes happened; but there was no survey done to precisely separate their paths (very common before the 1970s); or

There were multiple tornadoes with only short separation, but the survey erroneously classified them as one tornado.

How long does a tornado last? Tornadoes can last from several seconds to more than an hour. The longest-lived tornado in history is really unknown, because so many of the long-lived tornadoes reported from the early 1900s and before are believed to be tornado series instead. Most tornadoes last less than 10 minutes.
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How close to a tornado does the barometer drop? And how far does it drop ? It varies. A barometer can start dropping many hours or even days in advance of a tornado if there is low pressure on a broad scale moving into the area. Strong pressure falls will often happen as the mesocyclone (parent circulation in the thunderstorm) moves overhead or nearby. The biggest drop will be in the tornado itself, of course. It is very hard to measure pressure in tornadoes since most weather instruments can't survive. A few low-lying, armored probes called "turtles" have been placed successfully in tornadoes. This includes one deployment on 15 May 2003 by engineer/storm chaser Tim Samaras, who recorded pressure fall of over 40 millibars through an unusually large tornado. On 24 June 2003, another of Tim's probes recorded a 100 millibar pressure plunge in a violent tornado near Manchester, SD (National Geographic report). Despite those spectacular results, and a few fortuitous passes over barometers through history, we still do not have a database of tornado pressures big enough to say much about average tornado pressures or other barometric characterstics.
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What is a waterspout? A waterspout is a tornado over water -- usually meaning non-supercell tornadoes over water. Waterspouts are common along the southeast U.S. coast -- especially off southern Florida and the Keys -- and can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn small boats, damage ships, do significant damage when hitting land, and kill people. The National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.
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How are tornadoes in the northern hemisphere different from tornadoes in the southern hemisphere? The sense of rotation is usually the opposite. Most tornadoes -- but not all! -- rotate cyclonically, which is counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes (clockwise-spinning in the northern hemisphere) have been observed, however -- usually in the form of waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell's mesocyclone. There have been several documented cases of cyclonic and anticyclonic tornadoes under the same thunderstorm at the same time. Anticyclonically rotating supercells with tornadoes are extremely rare; but one struck near Sunnyvale, CA, in 1998. Remember, "cyclonic" tornadoes spin counter-clockwise in the northern hemisphere, and clockwise.
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What is a multivortex tornado? Multivortex (a.k.a. multiple-vortex) tornadoes contain two or more small, intense subvortices orbiting the center of the larger tornado circulation. When a tornado doesn't contain too much dust and debris, they can sometimes be spectacularly visible. These vortices may form and die within a few seconds, sometimes appearing to train through the same part of the tornado one after another. They can happen in all sorts of tornado sizes, from huge "wedge" tornadoes to narrow "rope" tornadoes. Subvortices are the cause of most of the narrow, short, extreme swaths of damage that sometimes arc through tornado tracks. From the air, they can preferentially mow down crops and stack the stubble, leaving cycloidal marks in fields. Multivortex tornadoes are the source of most of the old stories from newspapers and other media before the late 20th century which told of several tornadoes seen together at once.
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What is the original F-scale? Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and Pearson 1973) for winds, including tornadoes, which was supposed to relate the degree of damage to the intensity of the wind. This scale was the result. The original F-scale should not be used anymore, because it has been replaced by an enhanced version. Tornado wind speeds are still largely unknown; and the wind speeds on the original F-scale have never been scientifically tested and proven. Different winds may be needed to cause the same damage depending on how well-built a structure is, wind direction, wind duration, battering by flying debris, and a bunch of other factors. Also, the process of rating the damage itself is largely a judgment call -- quite inconsistent and arbitrary (Doswell and Burgess, 1988). Even meteorologists and engineers highly experienced in damage survey techniques often came up with different F-scale ratings for the same damage. Even with all its flaws, the original F-scale was the only widely used tornado rating method for over three decades. The enhanced F-scale takes effect 1 February 2007.
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What is the Enhanced F-scale? The Enhanced F-scale (simple table or detailed 95 page PDF) is a much more precise and robust way to assess tornado damage than the original. It classifies F0-F5 damage as calibrated by engineers and meteorologists across 28 different types of damage indicators (mainly various kinds of buildings, but also a few other structures as well as trees). The idea is that a "one size fits all" approach just doesn't work in rating tornado damage, and that a tornado scale needs to take into account the typical strengths and weaknesses of different types of construction. This is because the same wind does different things to different kinds of structures. In the Enhanced F-scale, there will be different, customized standards for assigning any given F rating to a well built, well anchored wood-frame house compared to a garage, school, skyscraper, unanchored house, barn, factory, utility pole or other type of structure. In a real-life tornado track, these ratings can be mapped together more smoothly to make a damage analysis. Of course, there still will be gaps and weaknesses on a track where there was little or nothing to damage, but such problems will be less common than under the original F-scale. As with the original F-scale, the enhanced version will rate the tornado as a whole based on most intense damage within the path. There are no plans to systematically re-evaluate historical tornadoes using the Enhanced F-scale. A full PDF document on the Enhanced F-scale is online.
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So if the original F-scale winds are just guesses, why are they so specific? Excellent question. Those winds were arbitrarily attached to the damage scale based on 12-step mathematical interpolation between the hurricane criteria of the Beaufort wind scale, and the threshold for Mach 1 (738 mph). Though the F-scale actually peaks at F12 (Mach 1), only F1 through F5 are used in practice, with F0 attached for tornadoes of winds weaker than hurricane force. Again, F-scale wind-to-damage relationships are untested, unknown and purely hypothetical. They have never been proven and may not represent real tornadoes. F-scale winds should not be taken literally.
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I heard the Oklahoma City tornado was almost "F6." Is that a real level on the original F-scale? Only in untested theory. Fujita plotted hypothetical winds higher than F5; but as mentioned in the previous answer above, they were only guesses. Even if the winds measured by portable Doppler radar (32 meters above ground level, roughly 302 mph) had been over 318 mph, the tornado would still be rated "only" F5 since F5 is the most intense possible damage level. On the Enhanced F-scale, there is no such thing as "F6."
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What is a "significant" tornado? A tornado is classified as "significant" if it does F2 or greater damage on the Enhanced F scale. Grazulis (1993) also included killer tornadoes of any damage scale in his significant tornado database. It is important to know that those definitions are arbitrary, for scientific research. No tornado is necessarily insignificant. Any tornado can kill or cause damage; and some tornadoes rated less than F2 probably could do F2 or greater damage if they hit a well-built house during peak intensity.
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Big fat tornadoes are the strongest ones, right? Not necessarily. There is a statistical trend (as documented by NSSL's Harold Brooks) toward wide tornadoes having higher F-scale damage. This can be out of more strength or out of greater opportunity for targets to damage -- or some blend of both. However, the size or shape of any particular tornado does not say anything conclusive about its strength. Some small "rope" tornadoes can still do violent damage of F4 or F5; and some very large tornadoes over a quarter-mile wide have produced only weak damage of F0 to F1.
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Can't we weaken or destroy tornadoes somehow, like by bombing them or sucking out their heat with a bunch of dry ice? The main problem with anything which could realistically stand a chance at affecting a tornado (e.g., hydrogen bomb) is that it would be even more deadly and destructive than the tornado itself. Lesser things (like huge piles of dry ice or smaller conventional weaponry) would be too hard to deploy in the right place fast enough, and would likely not have enough impact to affect the tornado much anyway. Imagine the legal problems one would face, too, by trying to bomb or ice a tornado, then inadvertantly hurting someone or destroying private property in the process. In short -- bad idea!
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How does cloud seeding affect tornadoes? Nobody knows, for certain. There is no proof that seeding can or cannot change tornado potential in a thunderstorm. This is because there is no way to know that the things a thunderstorm does after seeding would not have happened anyway. This includes any presence or lack of rain, hail, wind gusts or tornadoes. Because the effects of seeding are impossible to prove or disprove, there is a great deal of controversy in meteorology about whether it works, and if so, under what conditions, and to what extent.
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What does a tornado sound like? That depends on what it is hitting, its size, intensity, closeness and other factors. The most common tornado sound is a continuous rumble, like a closeby train. Sometimes a tornado produces a loud whooshing sound, like that of a waterfall or of open car windows while driving very fast. Tornadoes which are tearing through densely populated areas may be producing all kinds of loud noises at once, which collectively may make a tremendous roar. Just because you may have heard a loud roar during a damaging storm does not necessarily mean it was a tornado. Any intense thunderstorm wind can produce damage and cause a roar.
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Where can I get tornado pictures? Photographic prints of tornadoes are sold by a number of storm chasers and by the NSEA Concession. You can see many interesting free weather images at http://www.photolib.noaa.gov/noaa_products/. There are also several stock photography agencies specializing in, or peddling on the side, weather photos which include tornadoes. A search engine can help you find online stock photo outfits and tornado photographs. For digital online photos, many tornado-related websites display images; but since all personal photography is legally copyrighted upon creation. Photos on this site and all National Oceanic and Atmospheric (NOAA) agencies, including the National Weather Service, are public domain and free to download, though credit to the agency and/or source is required.
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Where can I get video of tornadoes? Public-domain videos of National Severe Storms Lab tornado intercept footage are available for a reproduction fee through a video transfer service used by NSSL. Many production companies, TV stations and storm chasers have made videotapes of tornadoes available for sale as well. Try web search engines and storm chaser pages. This FAQ will not endorse any particular commercial tornado video source or tour operation.
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Do hurricanes and tropical storms produce tornadoes? Often, but not always. There are great differences from storm to storm, not necessarily related to tropical cyclone size or intensity. Some landfalling hurricanes in the U.S. fail to produce any known tornadoes, while others cause major outbreaks. The same hurricane also may have none for awhile, then erupt with tornadoes...or vice versa! Andrew (1992), for example, spawned several tornadoes across the Deep South after crossing the Gulf, but produced none during its rampage across South Florida. Katrina (2005) spawned numerous tornadoes after its devastating LA/MS landfall, but only one in Florida (in the Keys). Though fewer tornadoes tend to occur with tropical depressions and tropical storms than hurricanes, there are notable exceptions like TS Beryl of 1994 in the Carolinas. Some tropical cyclones even produce two distinct sets of tornadoes -- one around the time of landfall over Florida or the Gulf Coast, the other when well inland or exiting the Atlantic coast.
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What's the nature of tornadoes in hurricanes and tropical storms? Hurricane-spawned tornadoes tend to occur in small, low-topped supercells within the outer bands, NNW through ESE of the center -- mainly the northeast quadrant. There, the orientation and speed of the winds create vertical shear profiles somewhat resembling those around classic Great Plains supercells -- the shear being in a shallower layer but often stronger. Occasionally a tornado will happen in the inner bands as well, but the large majority still form outside the hurricane force wind zone. Because tornado-producing circulations in hurricane supercells tend to be smaller and shorter-lived than their Midwest counterparts, they are harder to detect on Doppler radar, and more difficult to warn for. But hurricane-spawned tornadoes can still be quite deadly and destructive, as shown by the F3 tornado from Hurricane Andrew at La Place LA (1992, 2 killed) and an F4 tornado at Galveston TX from Hurricane Carla (1961, 8 killed).
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Do tropical cyclones produce waterspouts? Yes. Waterspouts -- tornadoes over water -- have been observed in tropical systems. We don't know how many of them happen in tropical cyclones, but a majority probably are from supercells. The similarity in Doppler radar velocity signatures over water to tornado-producing cells in landfalling hurricanes suggests that it may be common -- and yet another good reason for ships to steer well clear of tropical cyclones.
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Does tropical cyclone strength or size matter for tornadoes? Often, but not always. Relatively weak hurricanes like Danny (1985) have spawned significant supercell tornadoes well inland, as have larger, more intense storms like Beulah (1967) and Ivan (2004). In general, the bigger and stronger the wind fields are with a tropical cyclone, the bigger the area of favorable wind shear for supercells and tornadoes. But supercell tornadoes (whether or not in tropical cyclones) also depend on instability, lift and moisture. Surface moisture isn't lacking in a tropical cyclone, but sometimes instability and lift are too weak. This is why tropical systems tend to produce more tornadoes in the daytime and near any fronts that may get involved in the cyclone circulation. It is also why SPC won't always have tornado watches out for every instance of a tropical cyclone affecting land. For more details, there is a set of articles on tropical cyclone tornadoes listed in the Scientific References section. For more information on hurricanes, go to the Tropical Cyclone FAQ by Chris Landsea, Neal Dorst and Erica Rule.
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Who forecasts tornadoes? In the U.S., only the National Weather Service (NWS) issues tornado forecasts nationwide. Warnings come from each NWS office. The Storm Prediction Center issues watches, general severe weather outlooks, and mesoscale discussions. Tornadoes in Canada are handled by the Meteorological Service of Canada. Very few other nations have specific tornado watch and warning services.
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How do you forecast tornadoes? This is a very simple question with no simple answer! Here is a very generalized view from the perspective of a severe weather forecaster: When predicting severe weather (including tornadoes) a day or two in advance, we look for the development of temperature and wind flow patterns in the atmosphere which can cause enough moisture, instability, lift, and wind shear for tornadic thunderstorms. Those are the four needed ingredients. But it is not as easy as it sounds. "How much is enough" of those is not a hard fast number, but varies a lot from situation to situation -- and sometimes is unknown! A large variety of weather patterns can lead to tornadoes; and often, similar patterns may produce no severe weather at all. To further complicate it, the various computer models we use days in advance can have major biases and flaws when the forecaster tries to interpret them on the scale of thunderstorms. As the event gets closer, the forecast usually (but not always) loses some uncertainty and narrows down to a more precise threat area. [At SPC, this is the transition from outlook to mesoscale discussion to watch.] Real-time weather observations -- from satellites, weather stations, balloon packages, airplanes, wind profilers and radar-derived winds -- become more and more critical the sooner the thunderstorms are expected; and the models become less important. To figure out where the thunderstorms will form, we must do some hard, short-fuse detective work: Find out the location, strength and movement of the fronts, drylines, outflows, and other boundaries between air masses which tend to provide lift. Figure out the moisture and temperatures -- both near ground and aloft -- which will help storms form and stay alive in this situation. Find the wind structures in the atmosphere which can make a thunderstorm rotate as a supercell, then produce tornadoes. [Many supercells never spawn a tornado!] Make an educated guess where the most favorable combination of ingredients will be and when; then draw the areas and type the forecast.
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That sounds really hard. What hardware and software tools do you use to help you forecast tornadoes? The most important hardware for forecasting at the Storm Prediction Center is the human hand. Numerous hand-drawn analyses of surface and upper-air data are still performed at SPC every day so forecasters can be intimately familiar with the weather features. SPC forecasters also use high-performance computer workstations (mainly running Unix and Windows 98), with a huge variety of software to display the things we need to help us forecast severe weather. The variety of those things is enormous: many kinds of computer model displays, satellite image loops, radar displays, wind profiler and radar-wind plots, data from surface weather stations, upper air data from balloons and planes, lightning strike plots, weather data tables, multiple-source overlays, and more. It may sound trite; but by far, the most important software in the tornado forecast process is within the human brain. The forecaster must use it to sort all that information, toss out what is not needed, properly interpret what is needed, and put it into a coherent form -- all on a time deadline.
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What is needed to be a good tornado forecaster? It all starts with...

Motivation: Almost all severe storms forecasters are passionate about violent weather, with an intense desire to learn about and become better at predicting it. For many, this dates back into childhood -- a first-hand encounter with violent storms, images on TV or in books and magazines, or even a deep attraction to storms which goes back too far to recall. Others start out in other fields or college majors, then became fascinated with severe weather. In any case, this desire leads to...

Education: Consistently good severe storms forecasters have a solid educational background in atmospheric science which allows them to understand "textbook" concepts of thunderstorm formation. They don't stop with their college education, either. They constantly re-educate themselves in the latest discoveries about severe thunderstorms and tornadoes -- reading scientific journal articles on cutting-edge research, perhaps doing some research themselves. The understanding of storms which results lets the forecaster think of "conceptual models" -- visualizations of what the storms will do and how.

Flexibility: Because the atmosphere doesn't read textbooks or science journals, the forecaster must adapt those "classroom" ideas to an endless variety of day-to-day situations which may look a lot different. He or she also should be able to recognize when and why a forecast is not working out, and make the right adjustments. These skills come from...

Experience: In meteorology, history never repeats itself exactly. But certain types of situations do recur, allowing the forecaster to set a mental benchmark for what to expect. From there, he or she can better decide what data will be most important to examine, and what data will not be as relevant to the situation. Experienced forecasters are able to learn how bad forecasts went wrong and how good forecasts worked each time, building a more complete mental warehouse of severe storm forecast knowledge as time passes. When the experience is continually blended with motivation, flexibility and more education, he or she will keep improving as a forecaster.

What is the tornado forecast for next spring? Are there going to be tornadoes in Iowa the week of next October 5? We just don't know. Tornado forecasting today and tomorrow is quite difficult already. Specific severe weather forecasting more than days in advance is little more than guessing, or using tornado climatology for the forecast area and time of year. For that reason, there is no such thing as a long range severe storm or tornado forecast. There are simply too many small-scale variables involved which we cannot reliably measure or model weeks or months ahead of time; so no scientific forecasters even attempt them. Our farthest convective outlook is for day-3, and can be found on the SPC Forecasts page. Perhaps, someday, the density of weather observations and atmospheric modelling capabilities will advance enough to allow us to do severe storms forecasting many days out with some degree of accuracy better than a coin toss. We are a long, long way from that kind of forecasting!
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What is the role of Doppler radar in tornado forecasting? Each NWS forecast office uses output from at least one Doppler radar in the area to help to determine if a warning is needed. Doppler radar signatures can tell warning meteorologists a great deal about a thunderstorm's structure, but usually can't see the tornado itself. This is because the radar beam gets too wide to resolve even the biggest tornadoes within a few tens of miles after leaving the transmitter. Instead, a radar indicates strong winds blowing toward and away from it in a way that tells forecasters, "An intense circulation probably exists in this storm and a tornado is possible." Possible doesn't mean certain, though. That is why local forecasters must also depend on spotter reports, SPC forecast guidance on the general severe weather threat, and in-house analysis of the weather situation over the region containing thunderstorms, to make the best-informed warning decisions.
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What was the first successful tornado forecast? Nobody knows when was the first time someone claimed a tornado would occur in an area, and it happened. But the first documented, successful tornado forecast by meteorologists was on March 25, 1948, by Air Force Capt. (later Col.) Robert Miller and Major Ernest Fawbush. After they noticed striking similarities in the developing weather pattern to others which produced tornadoes (including the Tinker AFB, OK, tornado several days before), Fawbush and Miller advised their superior officer of a tornado threat in central Oklahoma that evening. Compelled from above to issue a yes/no decision on a tornado forecast after thunderstorms developed in western Oklahoma, they put out the word of possible tornadoes, and the base carried out safety precautions. A few hours later, despite the tiny odds of a repeat, the second tornado in five days directly hit the base. For more insight into this event, Charlie Crisp has transcribed the late Col. Miller's recollections of the event; and they are now online.
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What is the history of tornado forecasting? It's too long and eventful to summarize here; but there is an online guide at NSSL, as well as a timeline of SELS and SPC, and a history of the SPC that provide insight into how tornado prediction has evolved. There is also an entire book devoted to the subject: Scanning the Skies : A History of Tornado Forecasting by Marlene Bradford (hardcover - March 2001). Some libraries, bookstores and online book sellers carry this comprehensive and detailed history work.
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Was tornado forecasting once banned in the U.S.? Yes. Before 1950, at various stages of development of the Weather Bureau, the use of the word "tornado" in forecasts was at times strongly discouraged and at other times forbidden, because of a fear that predicting tornadoes may cause panic. This was in an era when very little was known about tornadoes compared to today, by both scientists and the public at large. Tornadoes were, for most, dark and mysterious menaces of unfathomable power, fast-striking monsters from the sky capable of sudden and unpredictable acts of death and devastation. As the weather patterns which led to major tornado events became better documented and researched, the mystery behind predicting them began to clear -- a process which still is far from complete, of course. In 1950, the Weather Bureau revoked the ban (PDF) on mentioning tornadoes in forecasts.
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How has SPC performed with tornado forecasting? By most measures, SPC (formerly SELS, NSSFC) has improved its tornado forecasting over the past few decades. There are many ways to objectively gauge forecast performance -- for example, verifying tornado watches with tornado reports and both watch types by all severe reports. The general trend from 1985-2003 has been for a greater percentage of tornado watches to contain tornadoes.
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How is tornado damage rated? The most widely used method worldwide, for over three decades, was the F-scale developed by Dr. T. Theodore Fujita. In the U.S., and probably elsewhere within a few years, the new Enhanced F-scale is becoming the standard for assessing tornado damage. In Britain, there is a scale similar to the original F-scale but with more divisions; for more info, go to the TORRO scale website. In both original F- and TORRO-scales, the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are only judgmental estimates. Because:

Nobody knows the "true" wind speeds at ground level in most tornadoes, and

The amount of wind needed to do similar-looking damage can vary greatly, even from block to block or building to building,

...damage rating is (at best) an exercise in educated guessing. Even experienced damage-survey meteorologists and wind engineers can and often do disagree among themselves on a tornado's strength.
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Who surveys tornado damage? What's the criteria for the National Weather Service to do a survey? This varies from place to place; and there is no rigid criteria. The responsibility for damage survey decisions at each NWS office usually falls on the Warning-Coordination Meteorologist (WCM) and/or the Meteorologist in Charge (MIC). Budget constraints keep every tornado path from having a direct ground survey by NWS personnel; so spotter, chaser and news accounts may be used to rate relatively weak, remote or brief tornadoes. Killer tornadoes, those striking densely populated areas, or those generating reports of exceptional damage are given highest priority for ground surveys. Most ground surveys involve the WCM and/or forecasters not having shift responsibility the day of the survey. For outbreaks and unusually destructive events -- usually only a few times a year -- the NWS may support involvement by highly experienced damage survey experts and wind engineers from elsewhere in the country. Aerial surveys are expensive and usually reserved for tornado events with multiple casualties and/or massive degrees of damage. Sometimes, local NWS offices may have a cooperative agreement with local media or police to use their helicopters during surveys.
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How can a tornado destroy one house and leave the next one almost unscratched? Most of the time, this happens either with multiple-vortex tornadoes or very small, intense single-vortex tornadoes. The winds in most of a multivortex tornado may only be strong enough to do minor damage to a particular house. But one of the smaller embedded subvortices, perhaps only a few dozen feet across, may strike the house next door with winds over 200 mph, causing complete destruction. Also, there can be great differences in construction from one building to the next, so that even in the same wind speed, one may be flattened while the other is barely nicked. For example, a flimsy, unanchored mobile home may be obliterated while all surrounding objects suffer little or no damage.
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How do tornadoes do some weird things, like drive straw into trees, strip road pavement and drive splinters into bricks? The list of bizarre things attributed to tornadoes is almost endless. Much of it is folklore; but there are some weird scenes in tornado damage. Asphalt pavement may strip when tornado winds sandblast the edges with gravel and other small detritus, eroding the edges and causing chunks to peel loose from the road base. Storm chasers and damage surveyors have observed this phenomenon often after the passage of a violent tornado. With a specially designed cannon, wind engineers at Texas Tech University have fired boards and other objects at over 100 mph into various types of construction materials, duplicating some of the kinds of "bizarre" effects, such as wood splinters embedded in bricks. Intense winds can bend a tree or other objects, creating cracks in which which debris (e.g., hay straw) becomes lodged before the tree straightens and the crack tightens shut again. All bizarre damage effects have a physical cause inside the roiling maelstrom of tornado winds. We don't fully understand what some of those causes are yet, however; because much of it is almost impossible to simulate in a lab.
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I've heard about tornadoes picking up objects and carrying them for miles. Does this happen? Who does research on it? Yes, numerous tornadoes have lofted (mainly light) debris many miles into the sky, which was then carried by middle- and upper-atmospheric winds for long distances. The vertical winds in tornadoes can be strong enough to temporarily levitate even heavy objects if they have a large face to the wind or flat sides (like roofs, walls, trees and cars), and are strong enough to carry lightweight objects tens of thousands of feet high. Though the heaviest objects, such as railroad cars, can only be airborne for short distances, stories of checks and other papers found over 100 miles away are often true. The Worcester MA tornado of 9 June 1953 carried mattress pieces high into the thunderstorm, where they were coated in ice, before they fell into Boston Harbor. Pilots reported seeing debris fluttering through the air at high altitude near the thunderstorm which spawned the Ruskin Heights MO tornado of 20 May 1957. There is a research group at the University of Oklahoma which studies tornado debris flight. If you personally know of a case of tornado debris carried long-distance, they have a hotline you can call to report it.
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How does the damage from tornadoes compare to that of hurricanes? The differences are in scale. Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes typically cause much more damage individually and over a season, and over far bigger areas. Economically, tornadoes cause about a tenth as much damage per year, on average, as hurricanes. Hurricanes tend to cause much more overall destruction than tornadoes because of their much larger size, longer duration and their greater variety of ways to damage property. The destructive core in hurricanes can be tens of miles across, last many hours and damage structures through storm surge and rainfall-caused flooding, as well as from wind. Tornadoes, in contrast, tend to be a few hundred yards in diameter, last for minutes and primarily cause damage from their extreme winds.

Where can I find free pictures of tornado damage? We have some public domain images of typical examples of F0 through F5 tornado damage linked from this FAQ's F-scale page. Otherwose, public-domain tornado damage pictures are scattered among various National Weather Service websites. Because web addresses change so often, we don't maintain a listing of them here; but you can start your search at this map of all NWS websites. Browse around for damage survey photos in severe weather and tornado event sections of local NWS office pages; and please make sure the photos are not copyrighted before using them. If there are any doubts, or to get permission to use copyrighted material, e-mail the webmaster at that office. SPC has some public-domain images and descriptions of Spencer, SD tornado damage available here. If you want hardcopies for research projects, the best bet is to download and print public-domain images from a high-quality color printer. Even when using public-domain images, you should give proper credit to the source. Historical archives at local and college libraries might have public-domain hardcopy prints of historical tornado damage in your area.
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NOTE: Historical tornado information used here comes from both the SPC database and Grazulis (1993).

How many tornadoes hit the US yearly? About one thousand. The actual average is unknown, because tornado spotting and reporting methods have changed so much in the last several decades that the officially recorded tornado climatologies are believed to be incomplete. Also, in the course of recording thousands of tornadoes, errors are bound to occur. Events can be missed or mis-classified; and some non-damaging tornadoes in remote areas could still be unreported.
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How many tornadoes have there been in the US this year and how does it compare to previous years? Killer tornadoes? Such tornado report totals are in an online table of monthly tornado stastistics at the SPC. Remember, those are preliminary numbers which may be amended at any time.
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How many people are killed every year by tornadoes? How do most deaths happen in tornadoes? On average, tornadoes kill about 60 people per year -- most from flying or falling (crushing) debris.
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What is tornado season? Tornado season usually means the peak period for historical tornado reports in an area, when averaged over the history of reports. There is a general northward shift in "tornado season" in the U.S. from late winter through mid summer. The peak period for tornadoes in the southern plains, for example, is during May into early June. On the Gulf coast, it is earlier during the spring; in the northern plains and upper Midwest, it is June or July. Remember: tornadoes can happen any time of year if the conditions are right! If you want to know the tornado peak periods for your area, Harold Brooks of NSSL has prepared numerous tornado probability graphics, which include distribution during the year.
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What is Tornado Alley? Tornado Alley is a nickname in the popular media for a broad swath of relatively high tornado occurrence in the central U.S. Various Tornado Alley maps which you may see can look different because tornado occurrence can be measured many ways -- by all tornadoes, tornado county-segments, strong and violent tornadoes only, and databaes with different time periods. Most recently, Concannon, et al., have prepared a "Tornado Alley" map using significant tornado data. Remember, this is only a map of greatest incidence. Violent or killer tornadoes do happen outside this Tornado Alley every year. Tornadoes can occur almost anywhere in the U.S., including west of the Rockies and east of the Appalachians -- and even in Canada and overseas.
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Does El Ni񯠣ause tornadoes? No. Neither does La Ni񡮠 Both are major changes in sea surface temperature in the tropical Pacific which occur over the span of months. U.S. tornadoes happen thousands of miles away on the order of seconds and minutes. El Ni񯠤oes adjust large-scale weather patterns. But in between those and tornadoes, there are way too many variables to say conclusively what role El Ni񯠨or La Ni񡩠has in changing tornado risk; and it certainly does not directly cause tornadoes. A few studies have shown some loose associations between El Ni񯠹ears and regional trends in tornado numbers from year to year; but that still doesn't prove cause and effect. Weak associations by year may be as close as the El Ni񯭴o-tornado connection can get -- because there are so many things on the scales of states, counties and individual thunderstorms which can affect tornado formation. For more detailed information, see The Relationship between El Ni񯬠La Ni񡠡nd United States Tornado Activity, a research paper by Schaefer and Tatom.
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What city has been hit by the most tornadoes? Oklahoma City. The exact count varies because city limits and tornado reporting practices have changed over the years; but the known total is now over 100. Mike Branick of the Norman NWS has prepared a detailed listing of 112 OKC-area tornadoes, updated through 2003. Another way to measure tornado count and avoid the vagaries of political boundaries is to use tornado hits within a radius; though this method will include tornadoes in some nearby communities. Frank Tatom has compiled such a list, using affected land area within a 20 mile radius of downtown of each city with at least 100,000 people. Through 2003, Oklahoma City also leads by this measure, followed by Huntsville AL.
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I noticed the word "preliminary" used a lot in the SPC tornado stats, and "final" too. What do those mean? Tornado data usually reaches SPC first from local storm reports (LSRs), warnings or other bulletins sent by local NWS forecast offices. Such reports are usually sent within the first day or two after a severe weather event, before all the information on a tornado is known. In fact, some tornado information might not be known for many weeks or months -- for example, if someone who was injured dies from his injuries a long time afterward. That is why we call all tornado data "preliminary" until the National Climatic Data Center publication Storm Data is completed. Storm Data contains the "final" information on all severe weather events.
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How many tornadoes have there been in my state or county? The actual number is unknown, because it is likely that (throughout the course of history) many tornadoes were either not reported or erroneously categorized. Recorded tornadoes are listed and described in the National Climatic Data Center publication Storm Data. NCDC is developing an interactive online severe weather database which you can use to search your state and/or county for tornado and other severe weather reports. NOTE: NCDC tornado data is not for whole tornadoes, but for county-segments; and there are still some incorrect county codings or other errors as docmented by Doug Speheger of NWS Norman.
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What are county-segments in NCDC tornado data? A county-segment is that portion of a tornado's path within a single county. If a tornado stays in one county, then a "tornado" is the same as a "segment." But this also means that tornadoes are counted twice when they cross into another county, three times when they enter a third county, and so forth. The reason for county-segment tornado recordkeeping is that the National Weather Service verifies tornado warnings by county. So when you look at NCDC tornado databases or the NWS Natural Hazard Statistics (based on Storm Data), you are not counting tornadoes, but instead county-segments of tornado tracks. This causes inflation of the tornado totals often reported by media and others who do not notice this important distinction.
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Where else can I obtain climatological tornado data? Besides the NCDC online lookup, an increasing number of local NWS offices have posted tornado stats for their regions. [Reminder: NCDC and most local data are broken down by county path segments and not whole tornadoes.] State-by-state value-added data can also be obtained from the Tornado Project databases.
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Why do F-scale ratings differ between SPC and others' tornado records? The F-scale is based on one's subjective judgment of damage intensity, and hundreds of people have rated tornadoes for Storm Data over the years. Currently, damage ratings for the "official" database are made by meteorologists at each local forecast office where tornadoes are reported. Sometimes, first-hand damage surveys are done, but because of travel budget constraints, ratings often must be made from spotter, chaser and/or media accounts. In order to assign F-scales to tornadoes from before about 1978 (the year varies from state to state), NSSFC contracted with college students to cross reference the NSSFC file with newspaper articles. When information that conflicted with Storm Data were uncovered, a judgment call was made as to what most likely occurred; and if necessary, the NSSFC information was "corrected" in the final data base. Also, several "new" tornadoes (previously unreported ones) were uncovered and added to the record. There were enough of these changes that the NSSFC (now SPC) database sometimes has differences with Storm Data, even before accounting for the county segments of paths done at NCDC. Tom Grazulis (1993) sometimes changed the "official" tornado ratings for Tornado Project records based on his judgment of damage from historical tornado accounts in newspapers and photographs. So, with all this interpretation going on, it is easy to see how tornado records have become inconsistent for many events.
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Do countries other than the US get tornadoes? How many? How strong? We know that tornadoes have been documented from many other nations, but we don't have a solid record of their frequency or damage potential in most areas. Several European countries, including the United Kingdom, Romania and Finland, have begun keeping detailed tornado records in recent years, as does Canada. Tornado records even in those nations are not as detailed or comprehensive as those in the U.S., and can't be compared directly. Indeed, in most of the world, there is no systematic documentation of tornadoes, other than those that happen to cause great damage and death, or that are caught by chance on someone's camera. To judge where else tornadoes are most common, we have to use a mix of actual tornado reports with heavy statistical analysis of weather records that indicate conditions favorable for them. Such a blend of recorded and inferential study indicates that the U.S. remains tops in tornado production, with secondary tornado-prone areas including the Canadian Prairie Provinces, Bangladesh, Britain, northeastern Mexico, northern Argentina and southern Brazil, and portions of southern Russia. The Mexican maximum (northern Coahuila, east of the Serranias del Burro range) and Canadian tornado prone zones each are border-crossing extensions of U.S. conditions.
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Do we know of other F5 or EF5 rated tornadoes besides those in the U.S.?