All About Tornadoes!



This page will attempt to focus on the pre-development, formation, maturity, and dissipating phases of a tornado. We'll also discuss what we know about those pre-storm atmospheric conditions which often breed tornado formation, what tornadoes look like, and other common features and characteristics associated with them. We'll also discuss what kind of impacts a twister can have, and how they are predicted. From the beginning of civilization to date, man has always marveled and wondered about those secrets that went into the evolution of a "Twister of Terror!".
The tornado, perhaps one of the least understood weather phenomena known to man since the earliest of times. Tornadoes have been around since the eighteenth(18th) century, ever earlier, but back in those years, man knew little about them, where they came from, or how they formed. In fact, many people only knew they were a bad windstorm associated with a thunderstorm. It wasn't until 1870 that the Army Signal Corps begin intense research efforts to document and understand severe local storms, like tornadoes. Leading the Corps in this endeavor was Sgt. John P. Finley. In the mid 1880's Finley organized a team of more then 2000 so called "reporters" over the central and eastern states to document tornadoes, and associated weather patterns and conditions. Using this data, Finley produced charts showing characteristic tornado producing patterns, which were used in tornado "alerts". But by the late 1880's, Finley's forecasts fell out of favor with the Corps, and eventually the "Weather Bureau" because they felt the use of the word "tornado" would provoke panic and undue fear with the public. As a result, a ban was placed on the use of the word tornado in their forecasts. Little progress was made in tornado forecasts into the early 1900's. By 1948, the Weather Bureau along with the Army Navy Corps formed a special unit which would begin forecasting severe local storms. This unit would become the WBAN or Weather Bureau Army Navy "Severe Weather Unit". However, our knowledge was still fairly limited in this field, and many forecasters were under trained with little experience. Many of the center's forecasts were often wrong, or not well forecasted. But, in 1950, that WBAN SWU unit moved from Washington D.C. to Kansas City Missouri, and was renamed SELS, or the "Severe Local Storms Warning Center".
From 1950 to present day, man's knowledge of twisters has grown from very little back in the Army Signal Corps of 1870, into modern day super computers which can process thousands of pieces of data in seconds, giving weather scientists, meteorologists, and climatologists the tools they need to predict these ferocious killer storms well in advance, giving people time to seek shelter.


Since 1870 when the Army Signal Corp began a study to identify those associated weather patterns in the atmosphere, which would lead to severe thunderstorm and even tornado development, the quest for knowing how these storms formed began to increase. But little progress was made going into the early 1900's. In 1948, the very first ever tornado forecast was made by two men of the U.S. Air Force. E.J. Fawbush and R.C. Miller, created based solely on data available at the time, a detailed tornado forecast for Tinker Air Force Base in Oklahoma. This idea worked, as the alert was read moments before the tornado struck, thus preventing any fatalities or injuries. Inspired by this theory, the U.S. Weather Bureau in 1950 created a special unit within their headquarters in Washington, D.C., to forecast severe storms and tornadoes. This unit would be referred to as their "Severe Weather Unit". It was from this point onward, that our understanding of such weather events slowly unfolded.
While we have learned extensively from recent years and decades from researchers, scientists, and storm chasers, and the data collected in the field to further study tornadoes and their causes, and to this day, scientists still don't know exactly how a tornado forms within the parent thunderstorm. However, we do know that the majority of tornadoes produced are generated from a low level feature within the parent thunderstorm which rotates about a vertical axis. This is known as the "Mesocyclone". Rotating wall clouds too, often proceed the development of a funnel cloud or tornado by minutes. But what specific pre storm environmental conditions favor tornadic development? This is still up for debate for some researchers, but scientists have concluded that three (3) parameters need to be present. Those are...

  • Moisture. This is the flow of near surface moisture laden air northward across the plains and into our coverage region. Moisture is the energy needed for convection to develop.
  • Instablilty. An unstable environment favors thunderstorms. The greater the instability, the more likely severe weather will occur.
  • Lifting. A lifting mechanism to get the warm buoyant air rising into the atmosphere, such as fronts, troughs, or a dry line.
  • As we know, warm and moist air is less dense, so it typically rises up into the atmosphere. This is aided by good surface heating by the sun. The further it rises, it begins to cool and condense into clouds which tower up into the atmosphere. The rising currents of air are called "updraft's". There are also currents of air which flows downward through a thunderstorm as well. These rain-cooled flows are called "downdraft's". In order for a thunderstorm to sustain itself, it needs both updraft's and downdraft's within it. In figure 1, we can see what a developing thunderstorm looks like. Notice the main storm tower and anvil spreading out at the top.

    FIG. 1

    For a tornado to possibly evolve, scientists know that another set of parameters must also be present within the pre storm environment. Those being moisture and shear. Deep rich low level currents of moisture from the Pacific, Gulf of Mexico, and subtropical Atlantic flow northward across the southern and central states, and eventually into the northern plains and our coverage region. As this moisture plume surges northward, it will often meet with, and collide with colder and drier air from Canada. When this occurs, a "battle zone" is created typically along and ahead of boundaries known as "fronts". This is where the atmospheric fireworks begins. While not shown in figure 2, we do see the flow currents of warm, moist, and unstable air northward.

    FIG. 2

    As the flow of deep-rich moisture laden air continues streaming northward, the environment begins to destabilize as the sun heats the surface of the earth. As previously mentioned, this warm and moist air mass is less dense then cold air, so it can rise freely into the atmosphere. As it does so, the rising air currents intersect with currents flowing in a different direction at a different height. This results in a "tube" or column of air which spins about a horizontal axis. As more rising warm air pushes upward, it begins to "pull" the rotating horizontal column upward, and eventually into a near vertical axis. Figure 3 shows this column of rotating horizontal flow, and figure 3a shows how it becomes stretched into a near vertical fashion.

    FIG. 3

    FIG. 3a

    The flow of winds crossing each other at different heights is known as "Directional Shear". Wind velocity changes with height as well. This is known as "Speed Shear". Wind shear is very important in the prediction of severe thunderstorms and tornadoes because the amount of shear present dictates the coverage and magnitude of any severe storms which do develop. Instability also plays a pivotal role in convective development, and we'll have more on this below. But figures 4 and 5 illustrates both speed and directional shear abit closer.

    FIG. 4

    FIG. 5

    All of the parameters mentioned to this point would be required for a thunderstorm to develop, but the primary factor which is also required is atmospheric "Instability". One of the primary ways of looking for such instability is using a parameter referred to as CAPE, or "Convective Available Potential Energy". This is expressed in joules per kilogram. Usually, the higher amount of CAPE, the more unstable the environment is, and the more likely storms become. CAPE is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, which makes it very valuable in predicting severe thunderstorms and tornadoes. The table below shows the levels of CAPE and what degree of severe weather could result.

    0-1000 J/KG Widely scattered thunderstorms possible. No severe storms likely.
    1000-2000 J/KG Scattered severe storms. Isolated tornadoes possible.
    2000-3000 J/KG Scattered to numerous severe thunderstorms, including super cells capable of all facets of severe weather.
    3000-4000 J/KG Numerous to widespread severe thunderstorms, including tornadic super cells capable of all facets of severe weather.
    4500+ J/KG Widespread violent super cells and tornadic super cells likely.

    It should be noted here, that CAPE is not the only way of determining atmospheric instability. There are also other parameters looked at and evaluated, such as EHI "Energy Helicity Index", or SREH "Storm Relative Environmental Helicity". The Energy Helicity Index (EHI) is a combination of two indexes. By itself, it is the best index available for tornado prediction since it combines both CAPE and Helicity. The CAPE is the amount of pure instability present from a parcel of air that rises from the lower "Planetary Boundary Layer". Helicity is the product of low level shearing (known as stream wise vorticity) and storm inflow directly into the stream wise vorticity. The Helicity is storm relative which means the Helicity is calculated from the storm's frame of reference. Some of the pitfalls with this include:

    a. Tornadoes are still possible in cases with a low EHI in cases where the CAPE or the shear is very large. This is especially true when the CAPE is low based and when mesoscale boundaries enhance the Helicity.
    b. Helicity is especially variable on the mesoscale, thus EHI may be much higher in particular areas.
    c. Make sure storms will develop in the first place in the high EHI environment (i.e. cap will break).

    The table below illustrates the EHI, and what is possible:

    Energy Helicity Index Severe Elements
    Greater then 1 Potential for super cells
    1 to 5 F-2 to F-3 tornadoes possible
    Greater then 5 F-4 to F-5 tornadoes possible

    However, their is also a parameter which plays a vital role in predicting severe thunderstorms and tornadoes. This is known by forecasters as CAP. The CAP is a layer of stable air parcels within the lower troposphere which basically impedes any convective processes from happening. The CAP is determined as the maximum temperature difference between a parcel of air and the surrounding actual temperature in the lower troposphere. The cap is a region of stable air in the troposphere that traps convective lifting that originates under the cap to remain under the cap. The cap will be present due to warm air aloft or warmer air that has been advected into the forecast area aloft. The cap when present will often be above the boundary layer and in the lower troposphere. The temperature lapse rate within the cap will be stable (inversion, isothermal layer, or weak temperature decrease with height). The cap is very important in that it can determine the timing that convection takes place, if convection even occurs at all. It will also influence the strength of the convection indirectly. The cap is famous for producing severe weather busts. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This is because the difference between a major severe weather outbreak and no storms can depend on if the cap weakens enough to allow boundary layer based convective instability release to take place. The cap will weaken due to daytime heating and uplifting dynamic forcing mechanisms. If there is not enough daytime heating and/or not enough dynamic uplift, the cap will not break. In some forecasting situations it will be easy to tell whether the cap will break or not, but in other situations, it is far from obvious.

    The following summarize how influential the cap can be.

    CAP CAP Strength
    0 No Cap Present
    0.1 - 1.9 Weak Cap
    2.0 - 4.0 Moderate Cap
    Greater then 4.1 Strong Cap

    When the CAP is less than 2.0, storms are likely to develop shortly, when the only parameter holding back convection is the CAP. When the CAP is greater than 4, help will be needed over the next few hours to break it. Some pitfalls with the CAP would include:

    a. Forecasters should pay attention to factors that can rapidly weaken a cap such as synoptic uplift, daytime heating, low level convergence.
    b. The CAP does not consider elevated convection. The CAP is a warm season warm sector index.
    c. The CAP Index is meaningless if there is zero CAPE in troposphere.

    The "Storm Relative Environmental Helicity" is a measure of the stream wise vorticity within the inflow environment of a convective storm. It is calculated by multiplying the storm-relative inflow velocity vector by the stream wise vorticity and integrating this quantity over the inflow depth. Geometrically, the storm-relative environmental helicity is represented by the area on a hodograph swept out by the storm-relative wind vectors between specified levels (typically the surface and 3 kilometers to represent the primary storm inflow). It is thought to be a measure of the tendency of a super cell to rotate.
    We fully realize that much of this is "Greek" to the average person, but at least you know the basics here. Thanks to the advances in modern research and data processing, better and more accurate parameters now allow forecasters a boost in their efforts to predict such severe weather, and in some cases, several days in advance! In figure 6 below, we illustrate a skew-T from a sounding. It shows how large instability developed as mid level temperatures (red line) drops off in the afternoon (The red line moves to the left). The green line is moisture.

    FIG. 6

    Their are three (3) stages of a thunderstorm's life cycle, the developmental stage, the mature stage, and the dissipation stage. We have already examined some of the factors which can breed severe thunderstorms, and yes, even tornadoes. We have covered a number of severe weather parameters used by forecasters to asses the existing state of our atmosphere and determine it's potential to produce severe storms. But any thunderstorm that develops will have these three stages of it's life cycle. Figures 7, 8, and 9 below illustrate the three stages.

    FIG. 7

    FIG. 8

    FIG. 9


    Not all tornadoes look or behave the same! The following are some basic characteristics of tornadoes.

    1). Tornadoes come in a variety of shapes and sizes, ranging from a slender rope like pendant hanging from the base of the thunderstorm to the surface, to a massive wedge like funnel which often can exceed a mile or more in width. Wedge type of tornadoes are considered the worst of all tornadoes (F-5) on the Fujita Tornado Intensity Scale.
    2). The funnel can hop, skip, or make a U turn on the ground.
    3). In most instances, thunderstorms will spawn only a single tornado at a time, but on rare occasions, multiple tornadoes can occur at the same time with the same parent thunderstorm.
    4). The winds wrapping around the funnel of a tornado have never been accurately measured because the wind instruments never survived the storm. This was in part why Dr. Ted Fujita of the Illinois State University in Chicago devised the Fujita tornado scale in 1971. This scale measures a tornado intensity based on the amount of damage it produced. The scale has five levels starting at F-0 and going to F-5. Most tornadoes were of the "weak" type (F-0 to F-2).
    5). Most tornadoes travel from the Southwest to the Northeast, but others have traveled from the Northwest to the Southeast.
    6). Tornadoes can move from speeds ranging from stationary to as fast as 70 mph.
    7). The appearance of a tornado has been described as black or grey in color, but some have said they observed a greenish tint just prior to a tornado.
    8). No area of the country is immune from tornadoes, and they can and have occurred at all hours of the day or night.
    9). Tornadoes have happened in other countries of the world, but are most common here in the lower 48 states.
    To be safe when a tornado threatens means being prepared ahead of time.


    At the top of this page in the ABSTRACT section, mention was made of how the process of forecasting severe weather was commenced by the Weather Bureau at it's headquarters in Washington, D.C. in 1950. The special unit was known as it's "Severe Weather Unit". Here, the duty of combing through numerous charts, analyzing data, and perform calculations on various datasets of diagnostics to asses the state of the atmosphere across the country. But, the staff was fairly young and inexperienced, so forecast errors happened a lot. Then, a decision was made by the Weather Bureau staff to relocate the SWU to a location more prone to severe storms. That location was Kansas City, Missouri. In April of 1954, the SWU moved into the Federal building in downtown Kansas City. A team of both existing and new staff, including staff from the Air Force, was selected to run the unit. A short time later, the SWU was renamed to the "Severe Local Storms Warning Center", or "SELS" for short. It was this center who, in 1956 began issuing on a daily basis, a text product known as a "Severe Weather Forecast" via the teletype network. This was done on an experimental basis. The purpose of this product was to inform the district offices of the potential for severe weather in their service areas. The message had two parts, the first part was highlighting the affected areas using standard Aviation 3 letter city identifiers. The degree of the severe threat was expressed as categorical risks (e.g. slight, moderate, and high risk). An outline where general (non-severe) storms was also included. The second part of the message contained a discussion of those elements and parameters being evaluated for possible thunderstorm development. Much of this discussion was done in contraction format, so only other meteorologists and forecasters knew what was being discussed. This product became operational in early 1957. About the same time, another experimental text message was begun by SELS. This was known as a "Severe Weather Bulletin". This message was transmitted by SELS when it became apparent that severe thunderstorms were likely to develop. Included in the message was the primary areas under immediate threat, the type of severe weather most likely to occur, the approximate time frame of occurrence, and some advice. These messages were issued consecutively numbered so as to keep track of the number of bulletins issued. These two products became the normal policy of SELS, and it had varied results among the public.

    In 1959, reactions from people across the country came pouring into the Weather Bureau regarding the overall sizes of their severe weather threat areas. The thought was that they were often "too large" and did not represent the actual areas where storms happened. The Weather Bureau launched an investigation of SELS and their forecasting methods. It was found that some of the staff lacked the proper experience for this kind of work. It was also found that a typical severe threat area covered around 40,000 square miles. These factors helped explain why such blunders were occurring. Yet this did not happen all the time either. The average watch area today is about 24,000 square miles. Later on that year, SELS began issuing smaller threat areas. Figure 10 illustrates the "typical" severe weather threat area by SELS in 1958.

    FIG. 10

    Forecast progress into the early 1960's continued at SELS as continued research led to more parameters which proved useful to the prediction of severe storms. Then, in April of 1963, a new IBM 1620 computer system was installed at SELS. This new computer system allowed forecaster to plot and analyze weather charts much quicker then by manually doing them as was the case before. In addition, this system allowed forecasters the added benefit of accessing diagnostic fields of convergence and divergence that were difficult or impossible to manually compute. Then, in November of 1965, another new computer system, the CDC 3100 was installed. This gave the additional benefit to automatically analyze and plot both surface and upper level data and charts. The district offices could also have access to this system for data tabulation and research work. These two pieces of technology of the time gave forecasters the badly needed break to help them prepare and issue more accurate forecast products. Forecaster confidence, skill, and experience also benefited their products. In 1966, changes were in store for the forecast and bulletin products. The severe weather forecasts were renamed to "Convective Outlooks", and the severe weather bulletins were renamed as "Tornado or Severe Thunderstorm WATCHES". This change was so that these products conformed better with similar products being issued by the NHC or "National Hurricane Center. In addition, SELS was renamed to the NSSFC or "National Severe Storms Forecast Center". This name change was done to better reflect the national scope of the agency. Figure 11 shows the National Severe Storms Forecast Center in 1967, a year after it's agency name change.

    FIG. 11

    In 1968, the National Severe Storms Forecast Center assisted with the creation of two additional agencies. The first, was a joint effort to launch an agency to conduct laboratory researches into the causes of severe local storms, and the environments which triggered them. Thus, the NSSL or "National Severe Storms Laboratory" was born. Later that same year, another agency, not part of NOAA, was also created. This agency was comprised of local storm spotter groups from every state, who took the time to go out and monitor weather and sky conditions whenever a watch was issued by the NSSFC for their areas. The idea here, was to report any kind of severe weather elements (i.e. large hail, damaging winds, or tornadoes), they observed to the Weather Bureau district offices. This agency was known as "SKYWARN". Then in early 1969, the NSSFC had issued a policy which made it clear the elements which defined a severe thunderstorm (those are mentioned above for reference). Because of the spotter groups, many folks in their respective communities became interested in and attended special group meetings where trained spotters and staff from area Weather Bureau district offices would explain what thunderstorms were, how they develop, and the elements of a severe thunderstorm. They also offered classes for those who wanted to become spotters free of charge. Sky warn still operates today, in conjunction with other groups. In 1970, the U.S. Weather Bureau was officially renamed as the "National Weather Service", and put under the Commerce department. Over the next 30+ year period, numerous advances in computer technology, local research and field studies of data collected in pre storm weather environments, and data from tornado "probes", allowed for vast improvements with existing forecast and watch products. While their were some occasions where false reports were received by the NWS generating needless warnings, the number of verified reports outnumbered the false reports. This along with the already badly aged WSR-57c radar systems became a serious problem with both watch and warning issuances. Under existing circumstances, the radar would indicate thunderstorms, but no way of knowing their intensity, unless reports came in of severe weather. This occasionally would result in "missed" warnings, or late warnings being issued. The average lead time for tornado warnings was only 3 minutes. Figures 12 and 13 shows this badly aged WSR-57c radar tower and indoor counsel unit.

    FIG. 12

    FIG. 13

    From severe weather bulletins to tornado and severe thunderstorm watches, and from severe weather forecasts to convective outlooks, the science of predicting tornadoes and severe thunderstorms has made great progress over the years, despite the many setbacks and pit falls along the way. With today's modern technology and improved radar and satellite systems, forecasters can get warnings to the public much quicker then back in 1980. All this was started in 1995, when a sweeping re-organization effort was launched of the National Weather Service. Gone were the older facilities, equipment, radar, and satellite. New facilities were built, complete with the latest WSR-88D or "Doppler Radar", GOES 13 and 15 weather satellites, and newer high speed computer workstations, known as AWIPS or "Advanced Weather Interactive Processing Station". All these newer tools allows for much quicker forecasts and warnings. Also that year, the NSSFC after more then 40 years of dedicated service in Kansas City, Missouri, was relocated to Norman, Oklahoma to join the agency it helped get started over 30 years back, the National Severe Storms Laboratory. A short time later, the National Severe Storms Forecast Center was renamed as the SPC or "Storm Prediction Center". Figure 14 shows the lead forecasters area during a shift briefing in 2012 at the Storm Prediction Center.

    FIG. 14


    It is the hope that you have benefited greatly from the data and information provided. However, their is still a lot more that couldn't be covered due to time and resource constraints. The information on this page is only the mere basics into what we know about tornadoes, the pre-storm environments conducive to their possible development, how forecasters have forecast their development in years and decades gone by, and how it's done today, and much more. Truly, the science behind tornado and severe thunderstorm prediction has improved markedly over the years since SELS was launched back in 1954. But, it was not only the forecasters who predicted such storms, but another elite group of spirited people whose duty was to go out into the field and report what destructive elements were occurring with the thunderstorms, and gather critical data of the pressure, winds, dew point, and temperature. These groups are the "Storm Chasers", and "Storm Spotters". For many years, and even to this day, the reports received by them provides the "ground-truth" information needed by forecasters at the NWS to issue the appropriate warnings to the public. Figure 15 shows a recent gathering of these chaser and spotters at the NSSL in Norman, Oklahoma in 2013.

    FIG. 15

    There is also some other things you and your community can do to help minimize the dangers of not only tornadoes, but the other destructive elements often associated with severe thunderstorms. If you don't know about severe weather safety, you can always go online on the Internet, and search for the closest NWS website near you. No doubt, they can provide you with all the necessary facts and information (publications are also available for download). You can also take our in-house severe weather safety quiz to find out what you know about safety. This quiz is available online at this page. Don't worry, this is NOT a "pass" or "fail" test! It's purpose is so you can find out what you really know about severe weather safety. If you fail the quiz, we also have a comprehensive safety page where you can learn more about safety. Their is a big difference between a severe weather "WATCH", and a severe weather "WARNING". Many folks still tend to get these terms confused with each other. The table below illustrates the difference between the two terms.

    Severe Thunderstorm WATCH Atmospheric conditions are favorable for severe thunderstorm development over your area. Continue on with normal activities. Keep watch on local weather developments. Be ready to respond to a warning. Monitor NOAA all hazards weather radio, or commercial radio and TV for further information.
    Tornado WATCH Atmospheric conditions are favorable for severe thunderstorm and tornado development over your area. Continue on with normal activities. Keep watch on local weather developments. Be ready to respond to a warning. Monitor NOAA all hazards weather radio, or commercial radio and TV for further information.
    Severe Thunderstorm WARNING Severe thunderstorms are imminent, or now occurring in your area. Cease all outdoor activities at once, and move quickly inside your house, or other safe sturdy building. Inside, stay away from windows and doors. Unplug all electrical items before the storms arrive, and don't use the telephone during the storm. Keep a watchful eye for signs of rotation within the clouds, or for a possible funnel cloud. If you observe any of these, get to your basement or storm cellar at once! Severe thunderstorms can and occasionally do produce tornadoes.
    Tornado WARNING A tornado has been sighted by trained spotters, or is being indicated by Doppler radar in your area. Move quickly to your basement, or underground storm cellar. If your home does not have a basement or cellar, move quickly to the lowest level of the building. Then seek cover in any "interior" small room or hallway. Closets or bathrooms make good choices. In mobile homes, get out and move to a safe sturdy shelter. Act quickly, as seconds count!

    It's important to develop a "Severe Weather Safety Plan" well ahead of time, so that everyone in your family knows what to do, and where to go for safety should the need arise. To ensure that everyone knows the plan, and what to do, conduct monthly "drills" on a fair weather day. While such drills are not required by any local, state, or federal laws, they will help those in your family know what to do, and where to gather. During a severe thunderstorm or tornado watch, it's important to remember that severe weather is not currently happening, but could develop over the next few hours. If you plan to be outdoors, keep an eye on the sky occasionally. Watch for any cumulus clouds which tower up into the atmosphere. If you observe such towering clouds, that means a storm is developing. Stay up to date with the latest weather information. When a severe thunderstorm warning is issued, and you are outdoors attending a public or sports related function, out at the beach, or boating, you should head for a safe sturdy building quickly. If your boating, head for harbor at once. Then get inside a safe building. Beach pavilions are not safe shelter areas during a thunderstorm!
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