Event Summary
     National Weather Service, Raleigh NC

May 8, 2008 Severe Weather Event
Updated 2009/06/01





Event Headlines -
...An EF-2 tornado touched down in western Greensboro in Guilford County killing one person and injuring three others...
...An EF-2 tornado moved into Forsyth County, lifted, and then touched down again and strengthened into an EF-3 tornado injuring 2 residents near Clemmons...
...A wind gust of 62 MPH was observed at the Piedmont Triad International Airport in Greensboro damaging buildings and blowing three large aircraft around...
...Nickel to golf ball size hail covering the ground up to 2 inches deep in northern Greensboro...
...The last F3 or EF3 tornado to occur in the RAH CWA was almost 10 years ago to the date of this event in nearly the same location. It occurred on May 7, 1998 in the Waterford subdivision in Clemmons...


Event Overview -
A strong low pressure system approached central North Carolina from the west on the evening of Thursday, May 8, 2008. Ahead of this system, winds at the surface were blowing from the southeast, but just a few thousand feet off the ground, they were blowing from the southwest at speeds of 40 to 50 mph. These strong and shifting winds, combined with an unstable atmosphere characterized by warm and moist air at the surface and cool air aloft, produced ideal conditions for the development of strong to severe thunderstorms across northern North Carolina.


Synoptic Overview -
On the evening of Wednesday, May 7, 2008, a ridge of high pressure was located over the southeastern U.S. with an upper level low over Oklahoma. Further aloft, the exit region of the southern jet stream was located over Texas and Louisiana with the northern jet stream extending across the Great Lakes region. At the surface, a 996 mb low pressure system was located over Oklahoma and northern Texas with a cold front extending south across Texas and another front extending northeastward into the Great Lakes region. A large area of precipitation was falling across the Ohio and Missouri Valleys with a band of convection moving across the ArkLaTex region. Another area of convection was moving east across Tennessee and Kentucky.

By Thursday morning the upper level low had de-amplified and weakened into a trough across Missouri. The surface low had weakened and was now centered in Missouri with the cold front across the Great Lakes dropping southeastward into the Ohio Valley and Northeast. Low level thermal and moisture advection was increasing across the Carolinas. The convection across Tennessee and Kentucky on Wednesday evening survived the trip across the Appalachian Mountains and by Thursday morning it consisted of a band of light rain showers across portions of central North Carolina. While the rainfall amounts with this area of showers was generally light and less than 0.05 inches, the precipitation and cloudiness persisted into the early afternoon, reducing sunshine and surface heating.

By the mid afternoon hours on Thursday, the showers were weakening and they had shifted into the Coastal Plain. The upper trough continued to weaken although the 500 mb flow over central North Carolina had increased 20 to 30 knots to around 50 knots at 19 UTC. Low level moisture at the surface and at 850 mb was increasing while mid level moisture at 700 mb was decreasing as noted on the water vapor imagery.

By the late afternoon and early evening hours, skies cleared across much of central North Carolina with an area of showers and thunderstorms approaching the Appalachian Mountains. The sunshine and increasing southerly flow allowed low level (mixed layer) instability to increase while mid level lapse rates were decreasing. At this time, the first convective elements were developing east of the Appalachians.

Just before sunset, much of the region was under a tornado watch and the anticipated severe weather event began. An area of surface pressure falls across the Northwest Piedmont had persisted for a few hours allowing surface winds to back which increased the low level shear and convergence. In addition, the surface to 1km shear values increased to between 20 and 30 knots. The deep layer shear from the surface to 6km was approaching 50 knots. The mixed layer instability was modest but sufficient for convection. The 00 UTC RAOB from Greensboro shows the CAPE at around 1500 J/Kg with an impressive wind field supportive of organized convection. The low level jet continued to strengthen during the following few hours supporting the development of tornadic supercells.

Toward daybreak, the cold front moved into the Piedmont reducing the near term threat of severe weather but not before two tornadoes would touch down killing one person and inuring several others.


Severe Weather Reports -
Text of severe weather reports across central North Carolina





The Clemmons Tornado

The National Weather Service, in conjunction with local emergency management, found that an EF-2 tornado tracked northeast out of Davie County, crossing the Yadkin River into Forsyth County near the old Clemmons water treatment plant along Idols Dam Road at approximately 1025 PM EDT. The tornado tracked northeast through a heavily wooded area for about one-quarter mile, lifting off the ground before reaching the Woodmont Subdivision.

The tornado then touched down for a second time in a heavily wooded area, just southwest of Hampton Road. At approximately 1028 PM EDT. Three metal barns sustained major damage around the 4800 block on Hampton Road, with minor damage to 2 homes. There were no injuries.

The tornado continued northeast, through wooded farmland, before slamming into the Bridgepoint subdivision. It was in the Bridgepoint subdivision where the damage was most severe, strengthening to an EF-3, with winds around 140 MPH. Three homes were destroyed and approximately 30 homes sustained moderate damage. There were only two minor injuries in the Bridgepoint subdivision.

The tornado continued to track northeast across Frye Bridge Road, through a heavily wooded area, before dissipating near the intersection of Cooper Road and Fraternity Church Road. Homes sustained minor damage, primarily from fallen trees. The extensive hardwood tree damage was consistent with EF-2 rating, as tree trunks were snapped in a 200 to 300 yard wide path.

Tornado: EF-3
Peak Wind: 140 mph
Path Length: 6 miles
Time/Date: 1025 PM - 1030 PM Thursday May 8, 2008
Injuries: 2
Fatalities: None

Special thanks go to the Forsyth County emergency services and Clemmons Fire Department for assisting with the survey and providing Access to damaged areas.


Clemmons Tornado Track -




Clemmons Tornado Damage Photos -

Photos courtesy of Forsyth County emergency management and the National Weather Service.
(Click the image to enlarge)


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The Sandy Ridge-Greensboro Tornado and Straight Line Wind Damage

The tornado which touched down in western Guilford County on the west side of Greensboro was the second of two supercells to move across the area. The first calls of tornado damage in Guilford County were received by the 911 center at 1129 PM EDT and the caller reported vehicles flipped over on Interstate 40. The initial tornado touchdown actually occurred earlier, a few miles southwest of the initial 911 call. The tornado, initially an EF-0, touched down just north of Squire Davis Park near the intersection of Sandy Ridge Road and Johnson Street. The tornado then tracked northeast and intensified to EF-1 intensity as it approached the Farmers Market and Interstate 40.

Crossing Interstate 40, the tornado overturned several cars and tractor trailers. A roof was blown off of an office building just north of the interstate as the tornado continued to intensify. As the tornado moved further northeast into the industrial complex, it strengthened to EF-2 intensity with winds estimated around 130 MPH based on damage to warehouses. Numerous warehouses along little Santee Road, Capital Drive, and West Market Street sustained significant damage. Numerous vehicles and tractor trailers were also overturned in the industrial complex. At its widest point the tornado was just over 200 yards wide. The tornado quickly lifted off the ground after crossing West Market Street near the post office. The tornado path length was around 4 miles.

One fatality occurred along West Market Street next to the lamination service building located at 8717 West Market Street. The fatality occurred as a man slept in the rig of his tractor trailer. Three injuries were also reported in the area. Two injuries occurred in automobiles and the third was in the I. H. Caffey warehouse distribution center. All injuries were not life threatening. Wind damage sustained to the Piedmont Triad Airport was the result of severe straight line winds likely feeding into the tornadic vortex. The tornado itself tracked just to the west of the airport and lifted off the ground as it passed by the airport.

Tornado: EF-2
Peak Wind: 130 mph
Path Length: 4 miles
Time/Date: 1129 PM - 1135 PM Thursday May 8, 2008
Injuries: 3
Fatalities: 1

Special thanks go to the Guilford County emergency services and the Greensboro Police Department for assisting with the survey and providing access to damaged areas.


Sandy Ridge-Greensboro Tornado Track -




Sandy Ridge-Greensboro Tornado Damage Photos -

Photos courtesy of the National Weather Service.
(Click the image to enlarge)


Photo courtesy of the National Weather Service - Click to enlarge           Photo courtesy of the National Weather Service - Click to enlarge           Photo courtesy of the National Weather Service - Click to enlarge           Photo courtesy of the National Weather Service - Click to enlarge           Photo courtesy of the National Weather Service - Click to enlarge

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The Three Body Scatter Spike (TBSS) and Large Hail in Asheboro and Greensboro

The first severe thunderstorm that developed on the evening of May 8, 2008 produced large hail west of Asheboro. The storm initially formed just east of Charlotte and moved to the north into Stanly County, moving through the town of Albemarle as a fairly benign shower. The storm quickly intensified, prompting the National Weather Service in Raleigh to issue a Severe Thunderstorm Warning at 0017 UTC (817pm). Moving northeast at 36 mph, the storm moved into southwestern Randolph County by 0046 UTC, where it intensified further.

Hail cores in thunderstorms can be identified by very high radar reflectivities, especially if those reflectivities are located above the freezing level. One suggested criteria for identifying the presence of severe hail (or severe weather in general) is a 65 dBz core above the freezing level. This criteria is often modified depending on the case and environmental conditions, but when present, it has been shown to produce a 90% chance of severe weather.

Another signal that hail is likely present in a thunderstorm is the mid-level three body scatter spike (TBSS) radar signature. The TBSS is typically a 10-30 km long region of reflectivities aligned radially downrange from a highly reflective (>60 dBZ) echo core. Radar energy strikes a 60 dBZ core or greater and radar energy is scattered back toward the radar and toward the ground. The radar energy from the ground is reflected back to the hail and then back to the radar. The radar algorithm is confused by the delayed return of the echo and believes it is coming from further away and it is then displayed downrange on the radial as a 5 to 20 dBZ spike.

Mid-level three body scatter spike (TBSS) conceptual model - click to enlarge Characteristics of a TBSS:
  • Low Reflectivity values of 5 to 20 dBZ
  • Low radial Velocities
  • High Spectrum Width
  • More elevated hail events produce longer spikes
  • Strength of the hail spike is proportional to hail size, concentration, wetness, and vertical extent of core
  • Position of hail spike along the radial is proportional to core altitude above the surface (why it appears at higher altitudes in the storm)

    TBSS visible in the 0.5, 0.9, 1.3, and 1.8 degree reflectivity slice at 0055 UTC - click to enlarge As the Randolph County storm moved along highway 49 in southwestern Randolph County, 70 dBz values were evident on the 1.8 and 2.4 degree radar scans. The thunderstorm was located around 85 miles from the radar, which meant that these elevation angles scan the atmosphere at between 17,000 and 22,000 feet. The freezing level based on the Greensboro upper-air sounding was around 10,000 feet. The deep 65+ dBz core extended well above the freezing level, and even up to the -20C level. The cell also produced an impressive three body scatter spike (TBSS) that was 4 to 5 scans deep. The spike can be seen at 0.5 degrees, 0.9 degrees, 1.3 degrees, and 1.8 degrees of the radar scan and in the 4 panel composite image to the right.

    Radar reflectivity imagery from the Blacksburg VA radar at 0407 UTC (1207 AM) showing the very high reflectivities with a TBSS visible - click to enlarge A second severe thunderstorm was responsible for dumping large hail north of Greensboro, NC at 0435 UTC (1235 AM). The storm initially formed well to the west of Greensboro in a convective line with the storms that would eventually produce tornadoes over the Triad before midnight. At 0159 UTC (959 PM), the storms were just moving into the WFO Raleigh County Warning Area and they were already severe. Over the next two hours, the storms slowly moved through the Triad, producing tremendous damage (see tornado descriptions above), and by midnight, the trailing cells were moving through northwestern Greensboro. One cell in particular rapidly intensified as it moved through northern Greensboro. At 0407 UTC (1207 AM), very high reflectivities (noted by purple and pink) were noted from the Blacksburg, VA radar. One scan revealed up to 76 dBz, with 70+ dBz reaching above 20,000 ft. A four panel image of the 1.2, 1.7, 2.3, and 3.0 degree elevation angles shown to the right shows the extreme reflectivities residing in multiple pixels with strong vertical continuity. Such reflectivity cores are not nearly as common over the Piedmont of North Carolina as they are over the Midwest and Central Plains. At around 0430 UTC (1230 PM), a spotter in north Greensboro reported 5 minutes of nickel to golf ball size hail that eventually covered the ground up to 2 inches deep.


  • Large Number of Tornadoes across the U.S. in 2008 and the ENSO Cycle

    Daily tornado trend plot from the SPC - click to enlarge The 2008 tornado season has been very active across the country. The initial number of tornadoes reported through June 1, 2008 suggests that 2008 may be a record year. The preliminary number of tornadoes for 2008 is based on Local Storm Reports (LSR’s) transmitted by local NWS Weather Forecast Offices (WFO's) around the country. This preliminary number is likely an overestimate due to duplicate reports that are often received for the same tornado. These numbers will be refined during the next few months and published in the National Weather Service Storm Data publication.

    Through June 1, 2008, there have been 5 confirmed tornado touch downs in central North Carolina (WFO Raleigh's forecast area.) While this number is not remarkable, there have been some significant tornadoes including an EF-3 tornado that touched down near Clemmons on May 8 and an EF-2 tornado that also touched in Greensboro on May 8, 2008. Other significant tornado outbreaks occurred this year in Southeastern Virginia on April 28 and across South Carolina and Georgia on March 15, 2008.

    El Niño-Southern Oscillation (ENSO, commonly referred to as simply El Niño) is a global coupled ocean-atmosphere phenomenon that has important consequences for weather and climate around the globe. A weak La Niña developed during the summer of 2007 and then strengthened in early 2008 before weakening during the spring 2008 (latest ENSO Diagnostic Discussion from the Climate Prediction center).

    The connections between ENSO and tornado frequency and strength over the southeastern U.S. including North Carolina are not clear. However, recent research indicates that tornado occurrences across the U.S. may not favor one ENSO phase over another, but ENSO may drive a shift in the locations of tornadoes.

    Agee and Zurn-Birkhimer (1998) found that tornadoes are more frequent over the lower Midwest, the Ohio and Tennessee valleys, and the mid-Atlantic region during the La Niña phase than any other phase. During cool season La Niña events, the jet stream (or storm track) is typically farther north than usual. This leads to warmer conditions across the southern states which can tighten the thermal gradient between the arctic air to the north and the warmer air to the south. This may enhance the storm track under the jet stream from the Tennessee valley through the Mid-Atlantic States. In addition, the occurrence of EF-2 or greater intensity tornadoes appears to increase during La Niña cool season months (January-March) over the southern U.S.

    On a National level, a study of the top 15 tornado outbreaks (at least 40 tornadoes) showed that only one outbreak occurred during an El Nino event. Six occurred during a La Niña event, while the remaining eight occurred during neutral ENSO years.

    Specifically in central North Carolina, some of the more violent tornadoes have occurred during years with a La Niña in the spring. This includes the May 5, 1989, F3 tornado in Winston-Salem, Forsyth County, and the May 8, 2008, EF-3, Clemmons, Forsyth County tornado. These are the strongest tornadoes recorded in northwestern North Carolina (Forsyth County) since records have been kept (NCDC-Storm Data).

    In addition, the most devastating tornado outbreak in North Carolina history (March 1984), the most violent tornado to hit Raleigh, an F4, (November 1988), and the violent November 1992 tornado outbreak which stretched from Mississippi to North Carolina, all occurred during either La Niña or Neutral ENSO cycles.

    The large number of tornadoes across the Southeast during the sprnig of 2008 and the very active severe weather season in central North Carolina cannot be directly linked to the La Niña pattern present during the spring of 2008. Forecasters have noted that severe weather seasons that are coincident with La Niña are often very active but linking the two directly is premature.


    Regional Radar Loop

    A Java Loop of regional reflectivity imagery from 2058 UTC on May 8 through 1158 UTC on May 9, 2008 is available here. Note - this loop includes 68 frames.

    The regional reflectivity image below is from 0328 UTC on March 5, 2008. At this time there was a tornado on the ground in western Guilford county (local reflectivity imagery from 0331 UTC  |  Tornado Warning issued nearly 20 minutes earlier).

    The loop shows the area of showers that moved across much of central North Carolina earlier in the day was moving offshore during the late afternoon. A couple of discrete cells developed in the Southern Piedmont and moved rapidly northeast just after 00 UTC with one cell splitting and moving rapidly northward. A large area of convection moved across the Northwest Piedmont and Triad area between 02 and 05 UTC producing the tornadoes in Clemmons and Greensboro. A third round of convection, weaker then the previous storms moved across central North Carolina between 06 and 11 UTC.


    Regional reflectivity image - click to load loop



    KRAX Radar Loops

    Overview of the entire event with images from every 15 minutes between 2300 UTC (700 PM EDT) May 8, 2008 through 0858 UTC (458 AM EDT) May 9, 2008.
    Java Loop of KRAX reflectivity imagery every 15 minutes from 2300 UTC May 8 through 0858 UTC May 9, 2008.
    Note - this loop includes 41 frames

    Overview of the entire event with images from every volume scan between 2300 UTC (700 PM EDT) May 8, 2008 through 0858 UTC (458 AM EDT) May 9, 2008.
    Java Loop of KRAX reflectivity imagery from 2300 UTC May 8 through 0858 UTC May 9, 2008.
    Note - this loop includes 131 frames

    The KRAX reflectivity image below is from 0227 UTC or 1027 PM EST on Thursday, May 8, 2008 or just a few minutes after the a tornado moved from Davie County into southwestern Forsyth County near Clemmons.





    Mesoscale Data

    Analyzed surface temperatures (red/purple), dew points (brown/green), and wind barbs from SPC at 01 UTC on Friday, May 9, 2008
    Surface winds across the Northwest Piedmont have backed slightly to the southeast and dew point temperatures are in the lower to mid 60s. Surface temperatures range in the lower to mid 70s.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    850 MB heights, temperatures (red/blue), dew points (green), and wind barbs (black) from SPC at 01 UTC on Friday, May 9, 2008
    A broad trough of low pressure can be seen in the 850 mb pattern across the Ohio and Tennessee valleys. A moderate southerly flow at around 40 knots is present across Georgia and South Carolina. Temperatures up to 18 degrees C can be seen across South Carolina with the southerly flow producing good warm advection and lift across central and western North Carolina.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    Analyzed low level lapse rates in the 0-3 km layer (blue, green, and orange) from SPC at 01 UTC on Friday, May 9, 2008
    A lapse rate is the rate of temperature change with height and the image below is for the layer from the surface to around 10,000 feet. Note the surface based, low level lapse rates shown below range in the 6.5 to 7.0 deg C/km across include much of central North Carolina. Values less than 6 degrees C/km represent "stable" conditions, while values near 9 degrees C/km are considered "absolutely unstable."

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    Analyzed surface based convective available potential energy (SBCAPE) (red) and surface based convective inhibition (blue lines - shaded) from SPC at 01 UTC on Friday, May 9, 2008
    SBCAPE values ranged between 500 and around 1000 J/kg across the Northwest Piedmont and Triad area. There was around 50 to 100 J/kg of convective inhibition (CIN) in this region as well suggesting that the convection was not entirely surface based initially.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    Analyzed most unstable convective available potential energy (MUCAPE) (red) and lifted parcel level (dashed black lines and shaded in yellow and green) from SPC at 01 UTC on Friday, May 9, 2008
    MUCAPE values ranged between 1000 and around 1500 J/kg across the Northwest Piedmont and Triad area. The shading area across western portions of the Northwest Piedmont suggests that the greatest instability was not surface based and that parcels were lifted at around 500 meters above the surface.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    0-3 Km Storm Relative Helicity (blue) and storm motion (brown) from SPC at 01 UTC on Friday, May 9, 2008
    Note the 0-3 Km SRH values in excess of 250 units and approaching 300 units in the Western Piedmont and Triad area.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    Analyzed Lifting Condensation Level (red, blue, and green) from SPC at 01 UTC on Friday, May 9, 2008
    The LCL height is the height at which a parcel becomes saturated when lifted dry adiabatically. The importance of LCL height is thought to relate to sub-cloud evaporation and the potential for outflow dominance. Low LCL heights imply less evaporational cooling from precipitation and less potential for a strong outflow that would likely inhibit low-level mesocyclone development. Thunderstorms that produce tornadoes generally have a lower LCL height with LCL heights less than 1,000 meters typically favorable for tornado development. The LCL values during this event were around or just less than 1000 meters across the Northwest Piedmont.

    SPC Analysis at 01 UTC on Friday, May 9, 2008



    NWS Composite Reflectivity Imagery from 0130 UTC on Thursday, May 8, 2008 (930 PM EST).
    The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.

    Composite Reflectivity Imagery from 0130 UTC on Thursday, May 8, 2008



    Archived Text Data from the Severe Weather Event

    Select the desired product along with the date and click "Get Archive Data."
    Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EDT time + 4 hours).


    Product ID information for the most frequently used products...

    RDUAFDRAH - Area Forecast Discussion
    RDUZFPRAH - Zone Forecast Products
    RDUAFMRAH - Area Forecast Matrices
    RDUPFMRAH - Point Forecast Matrices
    RDUHWORAH - Hazardous Weather Outlook
    RDUNOWRAH - Short Term Forecast
    RDUSPSRAH - Special Weather Statement
    RDULSRRAH - Local Storm Reports (reports of severe weather)
    RDUSVRRAH - Severe Thunderstorm Warning
    RDUSVSRAH - Severe Weather Statement
    RDUTORRAH - Tornado Warning


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    Warning Strategies

    Forecasters used surface and meso-analysis throughout the warning process. The SPC meso-analysis page was invaluable and it was used frequently to find the locations that had the greatest tornado and severe thunderstorms threat.

    Just before the outbreak of severe weather, forecasters analyzed the short term mesoscale trends using a variety of tools including the SPC meso-analysis, VAD wind profiles, AMDAR aircraft soundings, along with radar/satellite analysis and provided an excellent summary of the expected evolution of the event in the AFD.

    Much of the severe weather occurred in the Northwest Piedmont region of North Carolina in an area more than 60 miles from the WFO Raleigh Doppler Radar located in Clayton, North Carolina. Forecasters used radar data from WFO Blacksburg, Virginia radar and the Terminal Doppler Weather Radar in Charlotte, North Carolina during the warning decision making process.

    At times the feed of radar data from WFO Blacksburg, VA was slow or unavailable in AWIPS. Forecasters were able to access Level II data outside of AWIPS and view the data on PC based radar visualization software packages. The WFO Blacksburg, VA radar data was invaluable during the warning decision process and this supplemental method of viewing radar data was critical in effectively issuing timely warnings.

    After the Tornado Warning was issued for portions of Davidson, Forsyth, and Guilford Counties, forecasters made several calls to law enforcement and emergency managers in the area to warn them of the extreme threat.

    Forecasters used various situational awareness tools to stay abreast of the events unfolding in the Triad. The exchange of information (damage reports, tornado touchdowns, etc.) from emergency managers in the Triad area was supplemented by viewing local news media reports on a television monitor in the operations area. The Triad television stations cannot be viewed in Raleigh "over the air", so the satellite link was critical in understanding the scope of the event.



    References

    Sankovitch, V., J.T. Schaefer and J.J. Levit, 2004: A Comparison of Rawinsonde Data from the Southeastern United States during El Niño, La Niña, and Neutral Winters. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA.

    Agee, E., S. Zurn-Birkhimer, 1998: Variations in USA tornado occurrences during El Nino and La Nina. Preprints, 19th Conference on Severe Local Storms, Minneapolis, MN., Amer. Meteor. Soc., 287-290.


    Acknowledgements

    Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. The upper air analysis images were obtained from the University of Wyoming. GOES satellite data was obtained from National Environmental Satellite, Data, and Information Service. SPC meso-analysis graphics provided by the Storm Prediction Center. Base maps for the tornado tracks were provided by Google Maps and Google Earth - Google Earth map imagery used under license. Many of the Clemmons tornado damage photos provided by Forsyth County emergency management. Historical tornado data provided by the National Climatic Data Center. Tornado statistics provided by the Storm Prediction Center.


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