Superstorm Encyclopedia: Tornadoes, Severe Thunderstorms, Hurricanes, Tropical Storms, Typhoons, Cyclones - Meteorology, Forecasts, Safety and Preparedness, History, Disaster Health Problems
U.S. Government, National Weather Service (NWS), National Oceanic and Atmospheric Administration (NOAA), National Hurricane Center (NHC), Federal Emergency Management Agency (FEMA), Centers for Disease Control (CDC)
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Copyright 2011 Progressive Management
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CHAPTER 1: TORNADO OVERVIEW, SCIENCE, METEOROLOGY
CHAPTER 2: TORNADO SAFETY AND PREPAREDNESS
CHAPTER 3: STORM OBSERVATION, SPOTTING, AND REPORTING
CHAPTER 4: HISTORIC TORNADOES, HISTORY OF FORECASTING
CHAPTER 5: BUILDING PERFORMANCE ASSESSMENT: OKLAHOMA AND KANSAS TORNADOES 1999
CHAPTER 7: HEALTH AND MEDICAL IMPACTS OF DISASTERS
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Chapter 1: Hurricane Overview, Science, and Meteorology
Chapter 2: Hurricane Safety and Preparedness
Chapter 4: Mariner’s Guide For Hurricane Awareness In The North Atlantic Basin
Chapter 5: Glossary of National Hurricane Center Terms
Chapter 6: Federal Emergency Management Agency (FEMA) Hurricane Documents
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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.
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CHAPTER 1: TORNADO OVERVIEW, SCIENCE, METEOROLOGY
Tornado Basics
What is a tornado?
A tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Because wind is invisible, you can't always see a tornado. A visible sign of the tornado, a condensation funnel made up of water droplets, sometimes forms and may or may not touch the ground during the tornado lifecycle. Dust and debris in the rotating column also make a tornado visible and confirm its presence. (Condensation funnel - A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.)
What is known?
Tornadoes are the most violent of all atmospheric storms.
There are two types of tornadoes: those that come from a supercell thunderstorm, and those that do not.
Tornadoes that form from a supercell thunderstorm are the most common, and often the most dangerous. A supercell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating. This rotating updraft - as large as 10 miles in diameter and up to 50,000 feet tall - can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.
SUPERCELL - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes , extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.
UPDRAFT - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
UPDRAFT - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
GUSTNADO - [Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones ); they are more likely to be associated visually with a shelf cloud than with a wall cloud.
LANDSPOUT - [Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.
Visualizing Updraft Rotation in Supercell and Non-Supercell Thunderstorms
To visualize the horizontal spinning effect caused by wind shear, roll a pencil along a table top with the palm of your hand. Now put it between your hands, tilt your hands vertically and slide your right hand away from you. You have just created a miniature rotating updraft! To illustrate the way non-supercell tornadoes are formed, put a pencil between your hands, and place your hands perpendicular to a table (the table represents the ground, your hands represent wind coming from different directions at ground level, and the pencil is a parcel of air that gets caught and starts spinning). The spinning at the surface is drawn into the updraft of a developing or mature thunderstorm and is stretched into a tornado.
SHEAR - Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.
DOWNDRAFT - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.
More Ideas About Supercell Tornadogenesis
Scientists are actively trying to prove or disprove a number of tornadogenesis hypotheses. It is complicated science that draws on information from observations, theory, and mathematical and physical models. These are some basic ideas (basic to a scientist, that is) about the processes that might cause tornadoes to form from supercells:
Dynamic Pipe Effect
Development of a tornado begins when horizontal winds coming together from different directions are strong 3-4 km above the ground and weak or absent near the ground. The result is that rotation first increases aloft. The young tornado will build downward by something called the dynamic pipe effect (DPE): air can not enter through the sides of this belt of rotating air, but can pass through its ends like a pipe. The partial vacuum created within the pipe draws weakly rotating air up into the pipe's lower end. This causes the air to spin faster and eventually become part of the pipe. New sections on the rotating pipe form at lower and lower altitudes through this same process until the pipe (tornado) is in contact with the ground.
Another type of tornado development occurs when converging horizontal winds have the same windspeed through all levels in the thunderstorm. Rotation increases all at once and spans several kilometers along the vertical pipe. The tornado, in this case, forms nearly independent from how high it is above the ground, and develops very rapidly from the ground, up.
Rear Flank Downdraft (RFD)
The Rear Flank Downdraft (RFD) may play a role in tornadogenesis. The RFD is a region of dry air pushed towards the ground by the thunderstorm on the backside of, and wrapping around a rotating updraft, and eventually the tornado. It is often visible as a clear slot wrapping around a wall cloud (a persistent lowering from a rain-free base of the main thunderstorm). On radar, the presence of a hook or a small feature hanging from the thunderstorm may indicate the presence of an RFD. Scientists think the RFD may play a significant role in determining the development of a tornado, how long it lasts, and how intense it is. Some scientists think that the RFD, by wrapping around the low-level rotating updraft, forces the rotation to concentrate and lower to the ground.
DRY LINE - A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.
Non-supercell tornadoes are circulations that form without a rotating updraft. One non-supercell tornado is the gustnado, a whirl of dust or debris at or near the ground with no condensation funnel, which forms along the gust front of a storm. Another non-supercell tornado is a landspout. A landspout is a tornado with a narrow, rope-like condensation funnel that forms when the thunderstorm cloud is still growing and there is no rotating updraft - the spinning motion originates near the ground. Waterspouts are similar to landspouts, except they occur over water. Damage from these types of tornadoes tends to be F2 or less.
How do tornadoes form?
Scientists have learned a lot about tornadogenesis from theoretical studies, field projects and physical models – but tornadogenesis – the way tornadoes form – has vexed researchers for decades.
SUPERCELL TORNADOGENESIS
A rotating updraft is a key to the development of a supercell, and eventually a tornado. There are many ideas about how this rotation begins. One way a column of air can begin to rotate is from wind shear – when winds at two different levels above the ground blow at different speeds or in different directions.
An example of wind shear that can eventually create a tornado is when winds at ground level, often slowed down by friction with the earth's surface, come from the southwest at 5 mph. But higher up, at 5000 feet above the same location, the winds are blowing from the southeast at 25 mph! An invisible "tube" of air begins to rotate horizontally. Rising air within the thunderstorm tilts the rotating air from horizontal to vertical – now the area of rotation extends through much of the storm.
Once the updraft is rotating and being fed by warm, moist air flowing in at ground level, a tornado can form. There are many ideas about this too.
We still have many questions. Scientists know from field studies that perhaps as few as 20 percent of all supercell thunderstorms actually produce tornadoes. Why does one supercell thunderstorm produce a tornado and another nearby storm does not? What are some of the causes of winds moving at different speeds or directions that create the rotation? What are other circulation sources for tornadoes? What is the role of downdrafts (a sinking current of air) and the distribution of temperature and moisture (both horizontally and vertically) in tornadogenesis? Scientists hope to learn more about the processes that create wind shear and rotation, tilt it vertically, and concentrate the rotation into a tornado when they participate in a large field experiment in 2007.
And, since not all tornadoes come from supercells, what about tornadogenesis in non-supercell thunderstorms?
NON-SUPERCELL TORNADOGENESIS
A non-supercell tornado does not form from organized storm-scale rotation. These tornadoes form from a vertically spinning parcel of air already occurring near the ground, about 1-10 km in diameter, that is caused by wind shear from a warm, cold, or sea breeze front, or a dryline. When an updraft moves over the spinning, and stretches it, a tornado can form. Eastern Colorado experiences non-supercell tornadoes when cool air rushes down off the Rocky Mountains and collides with the hot dry air of the plains. Since these types of tornadoes happen mostly over scarcely populated land, scientists are not sure how strong they are, but they tend to be small. Waterspouts and gustnadoes are formed in this way too.
HOW DOES NSSL (NATIONAL SEVERE STORMS LABORATORY) CONTRIBUTE?
Tornadogenesis
NSSL scientists are working to understand the origins of tornadoes in thunderstorms through the use of theory. The research focuses on understanding how the local environment around the storm and internal processes within the storm produce tornadoes. New insight into these processes can then be incorporated into NSSL's warning decision technologies to help improve warnings for tornadoes and severe storms.
Field Observations
NSSL scientists go into the field to look for specific processes in storms, like observing how tornadoes form, and to test theories and hypotheses. The recent VORTEX2 field experiment is the largest and most ambitious field experiment in history to explore the origin, structure and evolution of tornadoes.
Storm modeling
Analysis of detailed data sets collected during storm intercept projects provides invaluable knowledge of storm dynamics. This information helps provide a coherent picture of storm structure that can be used in modeling tornadoes and tornado features. The COMMAS model, co-developed by an NSSL researcher can simulate a three-dimensional supercell thunderstorm.
Tornado Climatology
Where and when do tornadoes occur?
Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year.
Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. Both have similar topography with mountains helping catch low-level moisture from over Brazil (Argentina) or from the Indian Ocean (Bangladesh).
About 1,000 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades.
Understanding the Threat Posed by Tornadoes
Residents of Norman, OK experience a distinct tornado season, beginning late February and peaking late May. Even though we are in the heart of tornado alley and can expect one- to one-and one-half tornado days per year, our chances on any particular day peak at only about two percent.
Tornado season usually refers to the time of year where the U.S. sees the most tornadoes. The peak “tornado season” for the southern plains -- often referred to as Tornado Alley -- is during May into early June. On the Gulf coast, it is earlier during the spring. In the northern plains and upper Midwest, tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day, but most tornadoes occur between 4-9 p.m.
TORNADIC VORTEX SIGNATURE (TVS) - Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.
INFLOW BAND - (or Feeder Bands) - Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.
BEAVER'S TAIL - [Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes.
WALLCLOUD - A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion.
RAIN-FREE BASE - A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area.
REAR FLANK DOWNDRAFT (or RFD) - A region of dry air subsiding on the back side of, and wrapping around, a mesocyclone . It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant ; thus the presence of a hook or pendant may indicate the presence of an RFD.
CONDENSATION FUNNEL - A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.)
ALGORITHM - A computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, vertically integrated liquid water, accumulated rainfall, and several other parameters.
Radar Evidence
Mesocyclone
The large rotating updraft that occurs within a supercell is called a mesocyclone when identified by Doppler radar. The WSR-88D Mesocyclone Detection Algorithm analyzes radar data and looks for a rotation pattern meeting specific criteria for size, strength, vertical depth, and duration. The mesocyclone is usually 2-6 miles in diameter, and is much larger than the tornado that may develop within it.
Tornadic Vortex Signature
The Tornadic Vortex Signature, or TVS, is a Doppler radar velocity pattern that indicates a region of intense, concentrated rotation. The TVS appears on radar several kilometers above the ground at least ten minutes before a tornado touches ground. It has smaller, tighter rotation than a mesocyclone. While the existence of a TVS does not guarantee a tornado, it does strongly increase the probability of a tornado occurring.
Hook Echo
Hook echo is a term used to describe a pattern in radar reflectivity images that looks like a hook extending from the radar echo, usually in the right-rear part of the storm (relative to the motion of the storm). A hook is often associated with a mesocyclone and indicates favorable conditions for tornado formation The hook is caused by the rear flank downdraft and is the result of precipitation wrapping around the back side of the updraft.
Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year. The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado. The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."
The dryline is a boundary separating hot, dry air to the west from warm, moist air to the east. You can see it on a weather map by looking for sharp changes in dew point temperatures. Between adjacent weather stations the differences in dew point can vary by as much as 40 degrees or more. The dryline is usually found along the western high plains. Air moving down the eastern slopes of the Rockies warms and dries as it sinks onto the plains, creating a hot, dry, cloud-free zone. During the day, it moves eastward mixing up the warm moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form - because the dryline is the "push" the air needs to start moving up! During the evening, the dryline "retreats" and drifts back to the west. The next day the cycle can start all over again, until a larger weather system pushes through and washes it away.
Tornadoes kill an average of 60 people per year, mostly from flying or falling debris.
The Tri-State Tornado of March 18, 1925 was the deadliest tornado in history, killing 695 people. It is also the longest tornado track ever known - 219 miles - across parts of Missouri, Illinois and Indiana.
Codell, KS was struck by a tornado on May 20 three years in a row: 1916, 1917, and 1918.
Understanding the threat posed by tornadoes in the United States - particularly the threat of strong and violent tornadoes - is valuable knowledge to everyone, but especially to weather forecasters and emergency management people. Knowledge about long-term patterns helps us be better prepared for natural disasters and could also help scientists detect shifting patterns in severe weather events caused by climate change.
HOW DOES NSSL CONTRIBUTE?
Dryline Studies
NSSL researchers combine data collected during a 1991 field experiment with current model data to study factors influencing the location and timing of storm initiation.
Severe Thunderstorm Climatology
An NSSL scientist compiled data from past weather events into a graphical probabilistic data base showing the annual threat of severe thunderstorms occurring in the US. For this aignificant accomplishment Harold Brooks received a Department of Commerce Silver Medal award.
Detecting Tornadoes
What does a tornadic storm look like?
Forecasters and storm spotters have learned to recognize certain thunderstorm features and structure that make tornado formation more likely. Some of these are visual cues, like the rear-flank downdraft, and others are particular patterns in radar images, like the tornadic vortex signature (TVS).
VISUAL EVIDENCE
The most reliable evidence of a tornado is for someone who has been trained to recognize tornadoes to actually see one. Storm spotters report what they see to the National Weather Service and provide important information to warning forecasters who must make critical warning decisions. Storm spotters can be emergency managers or even local people with a keen interest in severe weather who have undergone formal storm spotter training in their community. Some of the features storm spotters and forecasters look for in tornadic storms include:
Inflow Bands
Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south. The presence of inflow bands suggests that the storm is gathering low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.
Beaver's tail
The beaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. It also suggests the presence of rotation.
Wall Cloud
A wall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is usually to the rear of the visible precipitation area. Wall clouds are about two miles in diameter and mark the area of strongest updraft in the storm.
As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses below the rain-free base to form the wall cloud.
A wall cloud that may produce a tornado usually exists for 10-20 minutes before a tornado appears. A wall cloud may also persistently rotate (often visibly), have strong surface winds flowing into it, and may have rapid vertical motion indicated by small cloud elements quickly rising into the rain-free base.
Rear Flank Downdraft
The rear flank downdraft is a downward rush of air on the back side of the storm that descends along with the tornado. The RFD looks like a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. It can also look like curtains of rain wrapping around the cloud base circulation. Eventually, the tornado and RFD will reach the ground within a few minutes of each other. The RFD causes gusty surface winds that occasionally have embedded downbursts The rear flank downdraft is the motion in the storm that causes the hook echo feature on radar.
Condensation funnel
A condensation funnel is made up of water droplets and extends downward from the base of the thunderstorm. If it is In contact with the ground it is a tornado; otherwise it is a funnel cloud. Dust and debris beneath the condensation funnel confirm a tornado's presence.
RADAR EVIDENCE
A tornadic storm observed by radar also has certain distinguishing features. Computer programs, called algorithms, analyze Doppler radar data and display it in ways that make it easier to identify dangerous weather. Forecasters are trained to recognize the precise radar signatures produced automatically by these sophisticated computer applications. Today's weather radars typically provide on average 11 minutes lead-time, and can pinpoint locations directly in harm's way with a high degree of accuracy. Some of the radar features forecasters look for Include: Mesocyclone; Tornadic Vortex Signature; Hook Echo.
The next generation of weather radars is now being developed. Scientists are adapting phased array technology, currently used on Navy ships, for use in weather forecasting. Phased array technology is expected to lengthen the average lead time for tornado warnings from 12 minutes to 20 minutes.
Other computer programs, like the Advanced Weather Interactive Processing System (AWIPS) used in all forecast offices, identify different kinds of severe weather, including tornadoes, using the latest observational data from Doppler radars, surface and upper air observing systems, mesoscale numerical models, satellites, and the National Lightning Detection Network. These computer applications incorporate image recognition, artificial intelligence, data mining, and statistical methods and couple them with state-of-the-art display platforms.
Radar Development
NSSL built the first real-time displays of Doppler velocity data. This lead to an NSSL scientist's discovery of the Tornadic Vortex Signature in radar velocity data in the 1970's. These developments helped spur deployment of the WSR-88D NEXRAD radar network. The Department of Commerce recognized NSSL's contribution to the NEXRAD program and to our Nation by awarding a Gold Medal to NSSL.
NSSL made the first observations of a tornadic storm with two Doppler radars (called dual-Doppler). The radars were located about 40 miles from each other and were able to record data on the same storm but from two different perspectives. The data was used to map the structure of a tornadic storm at several altitudes.
Airborne Doppler
NSSL continues to refine the use of airborne Doppler radar (installed on NOAA's P-3 research aircraft) to study storms. The first direct measurements of a tornado recorded with an airborne Doppler radar were made by NSSL. New concepts of making dual-Doppler measurements using the WSR-88D with the airborne Doppler were first tested in 1989 and are now used routinely.
Mobile Doppler Radars
NSSL collaborated with the University of Oklahoma, Texas Tech and Texas A&M University to build the Shared Mobile Atmospheric Research and Teaching-Radars, two 5-cm wavelength mobile Doppler radars than can scan and penetrate through an entire tornado or hurricane. The SMART-Radars will be used to study convective and mesoscale atmospheric processes to help improve forecasts of significant weather events.
Dual-polarization Offsite link warning
Engineers at NSSL developed polarimetric technology for the NEXRAD Doppler radar network. A polarized radar has the ability to measure reflected power returned to the radar from both horizontal and vertical pulses. Scientists recently discovered polarimetric signatures aloft that might be related to a developing tornado.
WDSS-II
NSSL's second generation Warning Decision Support System, WDSS-II, is an advanced algorithm development and visualization platform that accepts data from multiple sources and organizes it in ways that convey critical severe weather information to warning meteorologists.
NSSL developed the Tornado Detection Algorithm now used by the National Weather Service in their forecasting operations.
Phased Array
The National Weather Radar Testbed at NSSL, constructed in 2003, is where the next generation of weather radars is being developed. NSSL engineers and scientists are adapting phased array technology, currently used on Navy ships, for use in weather forecasting. Phased array technology is expected to lengthen the average lead time for tornado warnings from 11 minutes to 20 minutes.
OK-WARN
The Oklahoma Weather Alert Remote Notification program provides deaf and hard-of-hearing Oklahoman's access to emergency severe weather information via alphanumeric pagers and/or E-mail addresses. NSSL scientist Vincent Wood received the Department of Commerce Gold Medal award for his part in developing this hazardous weather pager program.
Forecasting Tornadoes
Can tornadoes be forecast or predicted?
FORECAST MODELS
Forecasters often rely on massive computer programs called numerical weather prediction models to help them decide if atmospheric conditions will be right to support an environment in which tornadic storms might form. The models start with current weather observations and attempt to predict future weather using physics and dynamics to describe mathematically the atmosphere's behavior.
WATCHES AND WARNINGS
Forecasters at the National Weather Service Storm Prediction Center (SPC) issue daily forecasts, or convective outlooks, for organized severe thunderstorms over the U.S. based on current weather observations and forecast models. They also closely monitor areas they think are at a higher risk for tornadoes.
If conditions develop that are favorable for tornadoes, SPC forecasters issue a severe thunderstorm or tornado watch that typically lasts four to six hours. Local forecast offices, emergency managers, storm spotters and the general public are alerted to the possibility of severe weather.
Tornado warnings are issued by the local National Weather Service Forecast Office when a tornado has been sighted or indicated by weather radar. People in the warning area should seek appropriate shelter immediately.
NSSL is actively involved in refining and building new conceptual models of severe storms, supercells structures and mesoscale convective complexes and systems. These conceptual models have improved our understanding of environments that are favorable for the formation of thunderstorms and tornadoes.
NSSL works with mesoscale models that help determine whether tornadic storms may or may not develop. They then test to see if these models are actually helpful in an operational forecasting situation.
NSSL scientists are studying the technique of "ensemble forecasting." Ensemble forecasting involves running a large number of forecasts with different initial conditions or a large number of different models together. This appears to provide more accurate forecasts than a single model by itself. NSSL scientists have also experimented with using direct forecaster input to identify regions and conditions that seem to be at a higher risk of severe weather. This way the forecaster could use his judgment and experience and input it into the model before the computer begins its computations.
NSSL actively works with the Storm Prediction Center each spring to evaluate the usefulness of new developments in computer models in daily forecasting.
Hazardous Weather Testbed
Inspired by the mutual interests of SPC forecasters and NSSL researchers, the Hazardous Weather Testbed encompasses projects of many shapes, sizes and composition. Interactions range from daily map discussions involving imminent severe weather to loosely related research projects involving 2-3 collaborators to annual intensive collaboration periods such as the SPC/NSSL Spring Program. This relationship provides forecasting support for field research, helps evaluate operational and experimental numerical models and other emerging operational forecast tools, and transitions promising new meteorological insights and technologies into advances in forecasting hazardous mesoscale weather.
Understanding Damage and Impacts
What kinds of damage can tornadoes do?
The Fujita Scale
The F-scale, or Fujita Scale, is a damage scale developed by T.Theodore Fujita to relate the degree of damage to the intensity of the wind. It is not an absolute scale. Many factors need to be taken into consideration including wind direction, wind duration, flying debris, and the strength of the structure.
Weak tornadoes may break branches or damage signs. Damage to buildings primarily affects roofs and windows, and may include loss of the entire roof or just part of the roof covering and sheathing. Windows are usually broken from windborne debris.
In a strong tornado, some buildings may be destroyed but most suffer damage like loss of exterior walls or roof or both; interior walls usually survive.
Violent tornadoes cause severe to incredible damage, including heavy cars lifted off the ground and thrown and strong frame houses leveled off foundations and swept away; trees are uprooted, debarked and splintered.
Weak tornadoes make up 74% of all tornadoes, while 67% of all tornado deaths come from violent tornadoes.
What are some of the economic impacts from tornadoes?
As an example, in 1999, a total of 74 tornadoes touched down across Oklahoma and Kansas in less than 21 hours on May 3 and 4. The strongest tornado, rated a maximum F-5 on the Fujita Tornado Scale, tracked for 38 miles along a path from Chickasha through the south Oklahoma City suburbs of Bridge Creek, Newcastle, Moore, Midwest City and Del City. When it was over, the two states counted 46 dead and 800 injured, more than 8,000 homes damaged or destroyed, and total property damage of nearly $1.5 billion.
For the entire year 1999 Oklahoma tallied:
171 tornadoes, ranked F-0 to F-5 * 42 deaths * 686 injuries * $1.110 billion in property damage
A dollar amount for damage is just one indication of impact. Loss or injury of family members (sometimes the sole breadwinner), loss of businesses (with all their employees), costs to communities for emergency personnel and shelter operation -- there are many intangible and long-lasting costs associated with tornadoes.
A long-term goal at NSSL is to develop statistical models of severe weather threat. One project estimates the daily probability of a tornado occurring in the U.S. Another study looks at tornado reports by damage class, and another looks at the probability of a particular path length or width.
NSSL participates in tornado damage surveys to help correlate radar data to actual damage paths.
An NSSL scientist was a member of the FEMA Building Performance Assessment Team that made observations, recommendations, and provided technical guidance following the Midwest Tornadoes of May 3, 1999.
About Tornadoes... Where do they come from?
Tornadoes come from the energy released in a thunderstorm. As powerful as they are, tornadoes account for only a tiny fraction of the energy in a thunderstorm. What makes them dangerous is that their energy is concentrated in a small area, perhaps only a hundred yards across. Not all tornadoes are the same, of course, and science does not yet completely understand how part of a thunderstorm's energy sometimes gets focused into something as small as a tornado.
Where do they occur?
Whenever and wherever conditions are right, tornadoes are possible, but they are most common in the central plains of North America, east of the Rocky Mountains and west of the Appalachian Mountains. They occur mostly during the spring and summer; the tornado season comes early in the south and later in the north because spring comes later in the year as one moves northward. They usually occur during the late afternoon and early evening. However, they have been known to occur in every state in the United States, on any day of the year, and at any hour. They also occur in many other parts of the world, including Australia, Europe, Africa, Asia, and South America.
If you'd like to plot tornado tracks, download Severe Plot and the associated data from the NOAA Storm Prediction Center.
What type of damage can they do?
The damage from tornadoes comes from the strong winds they contain. It is generally believed that tornadic wind speeds can be as high as 300 mph in the most violent tornadoes. Wind speeds that high can cause automobiles to become airborne, rip ordinary homes to shreds, and turn broken glass and other debris into lethal missiles. The biggest threat to living creatures (including humans) from tornadoes is from flying debris and from being tossed about in the wind. It used to be believed that the low pressure in a tornado contributed to the damage by making buildings "explode" but this is no longer believed to be true.
How are tornadoes detected?
Today, the development of Doppler radar has made it possible, under certain circumstances, to detect a tornado's winds with a radar. However, human beings remain an important part of the system to detect tornadoes, because not all tornadoes occur in situations where the radar can "see" them. Ordinary citizen volunteers make up what is called the SKYWARN network of storm spotters, who work with their local communities to watch for approaching tornadoes, so those communities can take appropriate action in the event of a tornado. Spotter information is relayed to the National Weather Service, which operates the national Doppler radar network and which issues warnings to the public by radio, TV, and NOAA Weather Radio, using information obtained from weather maps, weather radars, and local storm spotters.
Can tornadoes be predicted?
Yes, but only to a limited extent. Although the process by which tornadoes form is not completely understood, scientific research has revealed that tornadoes usually form under certain types of atmospheric conditions. When forecasters see those conditions, they can predict that tornadoes are likely to occur. However, it is not yet possible to predict in advance exactly when and where they will develop, how strong they will be, or precisely what path they will follow. There are some "surprises" every year, when tornadoes form in situations that do not look like the right conditions in advance, but these are becoming less frequent. Once a tornado is formed and has been detected, warnings can be issued based on the path of the storm producing the tornado, but even these cannot be perfectly precise about who will or will not be struck.
How fast can a tornado go?
We're not really sure what the highest wind speed might be inside a tornado. Since strong and violent tornadoes destroy weather instruments. we really only have measurements of the winds inside weaker tornadoes. Mobile Doppler radars can measure wind speeds in a tornado above ground level -- and the strongest was 318 mph measured on May 3, 1999 near Bridge Creek/Moore, Oklahoma.
What are the people called who study tornadoes?
People who study tornadoes are just research meteorologists. You may have heard another term - storm chaser - but that really refers to people who chase tornadoes for a hobby. Research meteorologists do science. They have to come up with questions they think they can answer by taking certain measurements.
How fast do tornadoes move?
We don't have detailed statistics about this. Movement can range from almost stationary to more than 60 mph. A typical tornado travels at around 10-20 miles per hour.
How long is a tornado usually on the ground?
Detailed statistics about the time a tornado is on the ground are not available. This time can range from an instant to several hours. The average is about five minutes.
Does NSSL do things like they showed in the movie "Twister"?
The movie Twister was based upon work NSSL did in the mid-1980s using a 55-gallon drum outfitted with various meteorological sensors. It was called TOTO (TOtable Tornado Observatory). NSSL tried for several years to put it in the path of an oncoming tornado, but had minimal success. It did not have the sensors that fly up into the tornado, like in the movie. However, that is not a bad idea and with all the advances being made in computer technology, we might be able to do that someday.
Has every state had a tornado?
Yes, although some states have many more tornadoes than others.
Are there tornadoes in the Arctic Circle?
We are not aware of any tornadoes occurring in the Arctic Circle. Tornadoes need moisture and warm air to form, which is unusual at that lattitude. Plus tornadoes or their evidence have to be observed by someone, and the Arctic Circle has few residents!
Do tornadoes really stay away from gullies, rivers and mountains?
A gully could actually make a tornado more intense, just as an ice skater spins faster when he or she stands up tall and stretches their arms up straight over their heads. Every major river east of the Rockies has been crossed by a significant tornado, and high elevations in the Appalachians, Rockies, and Sierra Nevada have all experienced tornadoes. A violent tornado crossed the Continental Divide in Yellowstone National Park.
Do tornadoes always come from a wall cloud?
A wall cloud is not always present. It is possible, though, that you cannot see a wall cloud because of your viewing angle.
What does a tornado sound like?
People who have been in a tornado say it sounds like a jet engine or a freight train and is very loud. They said it hurt their ears but they were more worried about what might happen to them than they were about the pain in their ears.
Can tornadoes be stopped?
You have to consider that the tornado is part of something bigger - the supercell thunderstorm. Unless you disrupt the supercell thunderstorm itself, you would likely have another tornado, even if you were able to destroy the first. The thunderstorm's energy is much greater than the tornado. No one has tried to disrupt the tornado because the methods to do so could likely cause even more damage than the tornado. Detonating a hydrogen bomb, for example, to disrupt a tornado 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.
How much advance warning can forecasters give us before a tornado strikes?
The current average lead-time for tornado warnings is 11 minutes. NSSL is working to increase tornado warning lead-times to 20 minutes.
What is the difference between a tornado watch and a tornado warning?
A tornado watch defines a box-shaped area where tornadoes and other kinds of severe weather are possible in the next several hours. It means that you need to be alert, and be prepared to go to safe shelter if tornadoes happen or a warning is issued. If you have a NOAA Weather Radio and have it set up correctly it will alert you to the watch. Tune in to local TV or radio for more information. A tornado warning means that a tornado has been spotted, or that Doppler radar shows a thunderstorm circulation which can spawn a tornado. When a tornado warning is issued for your area, seek safe shelter immediately. The Storm Prediction Center issues tornado and severe thunderstorm watches. Your local National Weather Service offices issue tornado warnings.
Can a tornado dig up the ground?
There have been reports of tornadoes blowing dirt and creating a trench 3 feet deep, but it is very uncommon. Tornadoes have been known to strip asphalt pavement.
What is the worst tornado that you know of?
The deadliest US tornadoes [more than one?] were in 1925 on March 18. It is called the "Tri-state" tornado and killed 695 people along its 219 mile track through Missouri, Illinois and Indiana.
Do tornadoes have a positive effect on the environment?
Not that we know of yet...but the thunderstorms that produce tornadoes release energy that builds up in the atmosphere.
How many people are killed by tornadoes each year?
On the average tornadoes kill about 60 people each year, mostly from flying or falling debris.
What would it be like to be in the eye of a tornado?
The very center of the tornado is probably almost calm, but may have some downward motion in it. There have not been any direct measurements of the winds because the instruments used to measure wind speed can't survive long enough to measure the eye.
How many tornadoes hit the US each year?
About 1000
How often do F4/F5 tornadoes occur?
Of the 1000 tornadoes that occur each year, about 2% of them are rated F4 or F5. That means that there are as many as 20 devastating tornadoes each year. It is possible that meteorologists have underestimated the number of violent tornadoes that occur each year. Tornadoes are rated only by damage they do to man-made structures. Therefore, if a tornado doesn't hit a structure of some kind, we cannot estimate its strength. Also, a tornado varies in strength during its lifetime and could be its strongest while between areas of houses or other buildings.
Could there be an F-6 tornado?
Dr. Ted Fujita, the inventor of the "F-scale", did plot hypothetical winds at an "F6" level. But, it was untested theory and therefore he was making educated guesses. The F-scale is based on damage to man-made structures, not precise wind speeds. Scientists have very few measurements of tornado winds. F5 is the most intense possible damage level - total destruction. Fujita said that although he speculated that a tornado could be stronger than an F5, even a structural engineer could not distinguish F6 damage from F5. F5 is the highest scientifically accepted rating for tornadoes. The highest recorded windspeed in a tornado was 318 mph, still within the bounds of the F5 description. These precise wind speed numbers are actually educated guesses and have never been scientifically verified. As we obtain more measurements on tornadoes, we may actually learn that the wind estimates on the Fujita scale are wrong! Damage can vary from place to place or even building to building based on construction, even if the wind speeds were the same.
What effect do El Niño/La Niña have on tornadoes?
There have only been two strong La Niña events. Although scientists have looked for a correlation between La Niña and tornadoes, there just isn't enough data to make any conclusions.
I have a theory about tornadoes, who do I talk to?
We receive literally hundreds of ideas for observing, controlling, or stopping destructive storms. Our scientists are likely to look at ideas that are investigated by a researcher who publishes the results in a peer-reviewed journal. In this way they can review, and if necessary, replicate the results, which then will suggest the next step to move the science forward.
How do I become a storm spotter?
Visit www.skywarn.org. SKYWARN is a cooperative effort between the National Weather Service and communities to organize spotters. On that site there is a link to local SKYWARN groups. If your area is not listed, contact your local National Weather Service Office.
I would like to volunteer to help NSSL during a tornado intercept field project.
Unfortunately, government regulations make it impossible to accept offers from the public to do volunteer field work for any tornado intercept programs. Legal liability questions prevent NSSL from accepting volunteers, even at their own risk.
I want to chase tornadoes on my vacation – what time of year and what locations are best?
Check the Severe Weather Climatology page at www.nssl.noaa.gov/hazard/
Which is stronger, a hurricane or tornado?
The winds from a strong tornado (F4 or F5 - 207 mph or higher) are significantly stronger than the highest category of hurricane (Saffir-Simpson Scale Category 4 or 5 - 131 mph and higher). However, hurricanes tend to cause much more destruction than tornadoes because they cover a much larger area, last longer, and have a variety of destructive forces (the eyewall, storm surge, flooding, and sustained strong winds). Tornadoes, in contrast, tend to be a mile or smaller in diameter, last for minutes and primarily cause damage from their extreme winds.
Where is the most common birthplace for a violent tornado?
According to research done by Dr. Harold Brooks here at NSSL, the most common birthplace for a violent tornado (F4 or greater) is in south-central Oklahoma.
How is the strength of a tornado determined?
The most common and practical way to determine the strength of a tornado is to look at the damage it caused. From the damage, we can estimate the wind speeds. An “Enhanced Fujita Scale” was implemented by the National Weather Service in 2007 to rate tornadoes in a more consistent and accurate manner. The EF-Scale takes into account more variables than the original Fujita Scale (F-Scale) when assigning a wind speed rating to a tornado, incorporating 28 damage indicators such as building type, structures and trees. For each damage indicator, there are 8 degrees of damage ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F scale did not take these details into account. The original F Scale historical data base will not change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may have been somewhat less than previously estimated. A correlation between the original F Scale and the EF Scale has been developed. This makes it possible to express ratings in terms of one scale to the other, preserving the historical database. Go to: www.spc.noaa.gov/faq/tornado/ef-scale.html
Do tornadoes target mobile home parks?
While it may appear tornadoes target mobile home parks, they actually do not. An F1 tornado might do significant damage to a mobile home, and cause minor damage to a site built home -- looking like the tornado "skipped" the house. Mobile homes are, in general, much easier for a tornado to damage and destroy than well-built houses and office buildings. A mobile home, or manufactured home, by definition, is built at a factory and taken to the place they will occupy--so they are much more affordable than a house built on-site. Also, they are often built with lighter-weight materials, which do not hold up well in tornadic winds.
Straight-line winds can also destroy a mobile home as easily as a tornado, especially one that is not anchored. Any wind gust that is sustained for 3 seconds over 50mph can cause damage to mobile homes.
These websites may be of interest to you:
http://www.spc.noaa.gov/faq/tornado/mhdeadly.htm
http://www.ametsoc.org/POLICY/statement 2004 mobilehomes.html
http://sciencepolicy.colorado.edu/zine/archives/1-20/29/guest.html
Some states are beginning to require storm shelters for their residents. The statistics definitely support this --7% of our population lives in mobile homes, and almost half of tornado fatalities in the U.S. occur in mobile homes. The problem of warning and sheltering mobile home residents has become the biggest obstacle to continuing to reduce death tolls from tornadoes.
Do wider tornadoes cause more damage?
There is a statistical trend toward wide tornadoes having higher F-scale damage. This can be because of more strength or because of greater opportunity for targets to damage - or a combination of both. However, the size or shape of any particular tornado does not say anything conclusive about its strength. Some small tornadoes can still do very violent damage of F4 or F5. And, some very large tornadoes over a quarter-mile wide have produced only weak damage.
What is the difference between a tornado and a cyclone?
A tornado is a small-scale cyclonic circulation, and in the past, has been referred to as a cyclone.
The term cyclone was used to describe anything that rotated counterclockwise, so often tornado (a small-scale cyclonic circulation) and cyclone were interchangeable. However, modern meteorology now restricts the use of the term to the larger-scale circulations -- usually also accompanied by low pressure and bad weather. But, people still use it both ways.
Are there electromagnetic or magnetohydrodynamic explanations for the development of tornadoes?
As far as scientists understand, tornadoes are formed and sustained by a purely thermodynamic process. As a result, their research efforts are towards that end. They have spent a lot of time modeling the formation of a tornado and measuring many parameters in and around a tornado when it is forming and going through its life cycle. They have not seen any evidence to support magnetism or electricity playing a role.
Can my TV signal detect tornadoes?
You may have read about a technique called "the Weller Method" of tornado detection. The idea was to be able to use your TV as a lightning detector to detect the radio waves emitted by a lightning flash, with the assumption that tornadic thunderstorms were very active lightning producers. But, not all tornadic storms produce large amounts of lightning. And, TV's are all different and have different sensitivities, and some are even made to filter out lightning signals. Plus, if you are connected to cable, it won't work. The method was found to be completely unreliable and it has mostly been abandoned.
Do tornadoes occur when it is cold?
There is no particular temperature at which tornadoes form. It is more about what the surface temperature is in relation to the temperature higher up in the atmosphere. Even if it is cold near the surface, as long as it is colder higher up, the winds are right to set up low-level wind shear, along with other necessary ingredients, a tornado is possible.
What direction do tornadoes spin?
Most tornadoes (but not all) in the Northern Hemisphere spin counterclockwise.
Do rocks, hills, or trees increase or decrease the wind speeds in a tornado?
Unfortunately, there is no clear answer. Both observations (of real tornadoes), computer simulations, and laboratory studies (in tornado vortex chambers) have shown that the "surface roughness", i.e., the measure of how disrupted the wind near the ground is by objects such as dirt, rocks, hills, trees, and even houses, can either increase or decrease the wind speeds in a tornado. How can trees increase the wind speeds? Well, the strongest winds in a tornado occur when air from outside the tornado can flow closest to the center of the vortex. The conservation of angular momentum, e.g., the rotation in the air, requires that as the air flows toward the center of the tornado (as it spirals in) its rotation must increase. Depending on the configuration of the airflow outside of the tornado, sometimes there is not ENOUGH "inflow" toward the center, and so blobs of air outside the tornado do not get very close to the center of rotation before they are lifted upward off the ground. In this case, INCREASING the surface roughness helps get these blobs of air closer to the center of the tornado, where they rotate even faster than before. So occasionally we see in tornado videos the vortex increasing in intensity when it travels from one type of ground surface (say a field) into a grove of trees or a housing subdivision. It does not always happen, but often enough that we are aware of it. This is a case where "friction", which people normally think of slowing things down, actually speeds them up!
Can you recommend a good storm shelter manufacturer?
We do not endorse any particular company or type of storm shelter. Consider checking out the Federal Emergency Management Agency website at fema.gov . They work with communities on tornado preparedness.
Where is tornado alley?
"Tornado Alley" is a just a nickname made up by the media for an area of relatively high tornado occurrence - it is not a clearly defined area. Is tornado alley the area with the most violent tornadoes, or is it the area with the most tornado related deaths, or the highest frequency or tornadoes? It depends on what kind of information you want!