|By Klockow, Kim|
The outbreak of 199 tornadoes on
Alarge number of tornadoes were recorded during the spring 2011 season, particularly a record number (around 758) during the month of April (NOAA 2011; NOAA 2012; Simmons and Sutter 2012a,b). A few tornado outbreaks accounted for the majority of the most damaging and lethal tornadoes, including extended outbreaks on 14-16 April [about 170 tornadoes, mostly EF-0 to EF-2 on the enhanced
In this overview paper, we summarize the tornado super outbreak of
The 27 April tornado outbreak was the major component of an extended 4-day episode of torna- does starting on 25 April and ending on 28 April. On 25 April, around 64 tornadoes (only four were EF-2 or greater intensity) were spawned between north- east TX and western
A primary motivation for this paper is to docu- ment the mesoscale and storm-scale characteristics of this outbreak, describe unique datasets, and present some initial analyses that are deemed worthy of more detailed investigations. Because numerous special datasets were acquired near the geographical core of this outbreak, we are compelled to describe some initial results without showing many details due to the space limitations of this article. Additional details and figures, including radar animations and sound- ings, are provided in the supplemental information of this paper (available online at http://dx.doi.org/10.1175 /BAMS-D-11-00229.2).
This paper includes a meteorological overview of this outbreak and illus- trates some unique features of three distinct episodes of tornadoes over this 24-h period (
A variety of initial analyses have been conducted and brief summaries of the more interesting aspects are reported herein. Some of the more intriguing aspects of this outbreak include the following:
* Multiple modes of convective organization, including QLCSs, mesovortices embedded within the QLCSs, and discrete supercell storms (some associated with a thermal boundary) occurred during the outbreak.
* An impressive mesoscale vortex spawned within the early morning QLCS was associated with the rapid development of 16 tornadoes over north- central to northeast AL.
* The parent storms, including supercell storms and strong convective elements within QLCSs, were efficient in producing tornadoes. For example, about 90% of the supercell storms within the outbreak region produced at least one tornado.
* Many tornadoes were long tracked, wide, and intense.
* Analysis of polarimetric variables, combined with dual-Doppler analyses of a supercell storm, graphi- cally illustrate that debris was effectively lofted to relatively high altitudes.
* Many of the violent tornadoes exhibited horizontal vortices along their periphery.
* External inf luences, including a thermal bound- ary, possible gravity waves, and topography, appeared to play a role in tornadogenesis and tornado intensity change.
* Despite timely and accurate warnings by the
The environment described in the following sec- tion was extremely conducive to the development of strong, long-track tornadoes. Although this danger- ous situation was anticipated exceptionally well by the
THE SYNOPTIC AND MESOSCALE ENVIRONMENT OF THE OUTBREAK. This section presents an analysis of the environmental parameters highly conducive for tornadoes through- out the day across much of the region affected. We begin with the early component of the event. The envi- ronment during the early morning (
In contrast, the environment over the region during the afternoon supercell outbreak was one of the most conducive to violent tornadoes ever docu- mented in the literature. At
The upper-level divergence and associated vertical motion helped promote steep lapse rates of 7.5°C km-1 between 700 and 300 hPa, according to the
The strong upper-level trough and associated low- level height falls aided the production of a strong low- level wind field with extreme vertical wind shear as indicated by 0-1-km SRH values between 600 and 900 m2 s-2 over the north- ern half of AL (Fig. 2f ). In addition, an east-to-west- oriented thermal boundary developed during the late morning and early after- noon hours over north- ern AL owing to the cold outflow air produced by a midday QLCS, followed by persistent clouds and rain showers that main- tained this cool air mass over northern AL, while relatively high insolation heated much of central AL. This boundary intensi- fied by midafternoon and likely produced a low-level thermally direct circulation that enhanced the vertical wind shear along it (e.g., increased southerly flow above the cool air and di- minished southerly flow near the surface within the cool air).
The significant tornado parameter (STP), which includes CAPE, SRH, and the effect of a low LCL (T07), attained values > 5 over a relatively large re- gion including northeast- ern MS, central to northern AL, and areas to the north- east. Such a large area of high values serves as a good discriminator for tornado outbreaks (Shafer et al. 2012). A maximum STP value > 10 was diagnosed in eastern MS and western AL at
OVERVIEW OF THE TORNADO OUTBREAK. Tracks of all tornadoes for the 24-h period
Table 1 presents other statistics obtained from the SPC database, including the destruction potential index (DPI; Thompson and Vescio 1998; Doswell et al. 2006), the N2 parameter2 introduced by Doswell et al. (2006) and further expanded by Shafer and Doswell (2010), and tornado fatalities for the top five outbreaks in each category. The
Figure 3 depicts a time series of tornadogenesis events per 30-min time interval over the outbreak area shown in Fig. 1. Tornadogenesis events were absent in only six 30-min time blocks over this 24-h period. Three episodes of tornado activity are apparent. The first 76 tornadoes occurred during the 0500-1500 UTC time frame and were associated with a strong MCS that intensified over MS and evolved into a QLCS during the early morning hours over AL. The peak near
A second, but shorter QLCS developed over north- ern AL around
Finally, supercell storms formed over eastern MS and western AL during the afternoon hours, account- ing for 29 of 34 tornadoes of EF-3 intensity or greater. The afternoon hours were very active, with an aver- age of six tornadogenesis events per 30 min for the 7.5-h period between 1900 and 0230. The four EF-5 and 11 EF-4 tornadoes were initiated between 1930 and
The distributions of the EF scale, pathlength, and path width, all obtained from the SPC database, are plotted in Fig. 4. A modest correlation (R 2 = 0.50) between pathlength and path width (and tornado intensity) is suggested. There were 23 tornadoes that developed damage swaths greater than or equal to about 1 km, and 18 tornadoes had pathlengths exceeding 50 km (six exceeded a 100-km path- length). The
Primarily EF-3 and stronger tornadoes exhibited pathlengths exceeding 60 km, consistent with Brooks (2004). A broader range of tornado intensities (from EF-2 to EF-5) exhibited path widths exceeding 1 km. According to Brooks (2004, his Fig. 4) EF-2, EF-3, EF-4, and EF-5 tornadoes that achieve path widths greater than 1 km fall in the respective cumulative distribution function (CDF) values of approximately 99%, 95%, 90%, and 88%. Mean width values for EF-2 to EF-5 tornadoes (in the Brooks study) are 126, 264, 460, and 555 m, respectively.
OUTBREAK COMPONENTS AND CHARACTERISTICS. The following sections summarize the mesoscale and storm-scale features of three contrasting modes of mesoscale organization of deep convection that produced tornadoes during this outbreak.
Early morning QLCS. A large MCS formed over the LA-AR region and organized over MS around mid- night CDT (
An image from the
Component 1 rapidly expanded to form a bow echo after
A second significant MV (MV2) formed within this QLCS by
Since MV2 was located within a prime area of the dual-Doppler lobe formed by the KHTX and ARMOR radars (68-km baseline), a preliminary dual-Doppler analysis was conducted using standard techniques outlined in the online supplement. Figure 6 indicates that the horizontal flow relative to the translating MV is filled with positive vertical vorticity peaking at about 12 × 10-3 s-1 at the lowest analysis level of 1 km AGL. Based on initial results, we note the following apparent aspects of MV2: 1) it contained multiple vorticity centers at some analysis times; 2) cores of vertical vorticity from the northernmost convective cells along the attendant QLCS to the south advanced toward MV2 and merged with the primary vorticity area; 3) updrafts were shallow and relatively weak (<10 m s-1) and their magnitudes peaked below 6-km height on the northern to western sides of the vortex; and 4) the storm-relative horizontal wind maxi- mum migrated counterclockwise with time. These inceptive findings highlight the utility of a complete investigation of this MV and the potential mecha- nisms (e.g., Trapp and Weisman 2003; Weisman and Trapp 2003;
The vertical extent of significant precipitation (40-dBZ echo) within MV2 was mainly confined below 4-5 km, and the absence of lightning, as indicated by the
Midday QLCS over northern AL. A second, smaller (about 100 km in the north-south direction) QLCS formed as an intense convective line over northern MS and rotat- ed from an initial east-west orientation at
Other special observations include a sounding just ahead of the QLCS at
A second trailing line of intense deep convection developed around
It is noteworthy that the early morning and midday QLCS events produced significant local power out- ages from central to northern AL that significantly reduced warning dis- semination (owing to power outages, disabled sirens, and reduced weather radio transmissions) to these areas during the more significant afternoon supercell-spawned tornadoes (TRAC 2012). Additionally, these systems laid the basis for an influential thermal boundary that appears to have aided in the strength of the northernmost afternoon supercells.
Characteristics of the thermal boundary. The thermal boundary produced by the midday QLCS and rein- forced by subsequent overcast and scattered showers within the northern portion of the outbreak area was a prominent mesoscale feature. The potential importance of thermal boundaries has been in- vestigated in previous studies (Davies et al. 1994; Markowski et al. 1998; Langmaid and Riordan 1998; Koch et al. 1998; Rasmussen et al. 2000; Thompson and Edwards 2000; Rogash and Smith 2000). Surface data from a variety of sources were used to estimate the time-resolved boundary location from the time of formation in the wake of the midday QLCS at 1500-1700 UTC through the afternoon hours. Potential temperature plotted in Fig. 8 shows the boundary extending from northeastern MS to north- ern AL at
The boundary may have played a key role in convective initiation, and it interacted with several storms (discussed in the following section), including the
Supercell storms during the afternoon and evening. Discrete supercell storms formed near
Figures 5e and 5f show the field of supercell echoes sampled by the
1) A supercell that produced an EF-5 tornado was approaching
3) South of the KGWX radar, the
7) The seventh supercell in the lower-left corner of Fig. 5e produced an EF-3 tornado near Hubbertville, AL, about 90 min after the time of this radar image.
Figure 5f shows four significant tornadic supercell storms at
1) The highly publicized
2) The Cordova EF-4 long-track tornado was also in progress.
3) The Marion-Winston EF-3 tornado (in progress, parent storm is also annotated in Fig. 5e) devel- oped a track length and width of 52 and 1.2 km, respectively.
4) The Green-
The high efficiency of tornadogenesis is corrobo- rated in Figs. 5e and 5f; with the exception of the
Table 2 presents statistics on three of the most proficient supercell storms, where proficiency is de- fined herein by the accumulative tornado pathlength divided by the total distance traveled by the parent storm. The
A closer examination of three of the aforemen- tioned significant storms and associated tornadoes further illustrates some important aspects of the structure of supercell storms and tornadoes within this outbreak. Figure 10 shows a dual-Doppler analy- sis of the
Figure 11 presents a visual image and radar im- ages of the
The Tuscaloosa EF-4 tornado was well docu- mented visually and on radar. Figure 12 presents an image of the tornado around
Elsewhere, TN and GA experienced 71 and 13 tor- nadoes, respectively, some of which were produced by the
EXAMPLES OF EXTREME DAMAGE FROM VIOLENT TORNADOES. In view of the large, long-tracked, and violent tornadoes that developed on
* The EF-5
* The first EF-5 tornado near
* The Tuscaloosa-Birmingham EF-4 tornado produced phenomenal damage northeast of
* The Rainsville EF-5 tornado nearly completely eroded a mounded aboveground earthen storm shelter, where residents inside narrowly escaped severe injury or death.
POTENTIAL IMPORTANCE OF EXTERNAL INFLUENCES. This extensive outbreak presents an opportunity to further document external inf lu- ences on tornadoes, such as gravity waves, thermal boundaries, and topography. The interaction between the thermal boundary and the
Gravity waves were sampled on the cool (stable) side of the thermal boundary (Fig. 9), where surface temperatures of ~17°C were maintained after the midday QLCS by persistent rain showers and cloud cover. Between 1855 and
In several instances, tornadogenesis corresponded to the merger of a RS with an existing mature or intensifying storm.9 Figure 14 shows Ze and
We have noted that topographic variations fre- quently correlate with variations in tornado intensity, whereby tornado-scale vortices weaken while ascend- ing a slope, and rapidly strengthen on the downslope side. This phenomenon was briefly discussed by
DISCUSSION AND CONCLUSIONS. The tornado outbreak of
Three rounds of contrasting mesoscale organiza- tion of deep convection occurred on 27 April, ranging from QLCSs to isolated supercell storms. In contrast, the
Following a midday QLCS that produced seven EF-0 to EF-1 tornadoes over northern AL, supercell storms developed during the afternoon hours in an extremely favorable environment, producing the majority of EF-3 to EF-5 tornadoes and associated large numbers of fatalities (316), injuries (> 2700), and damage (
One fundamental question posed by both physical and social scientists concerns the large number of fatalities (316 total; 238 in AL). A portion of the work summarized in the sidebar on "Initial investigation of societal aspects" highlighted inconsistencies and improvements needed in the warning dissemination process and progressed the interaction among north AL weather researchers, forecasters, social scientists, EMA personnel, and broadcasters with the goal of ensuring efficient and consistent weather threat com- munication to the public in the future.
Special datasets acquired within the outbreak region, and northern AL in particular, will afford opportunities to expand our understanding of physi- cal processes of tornadogenesis and tornado mainte- nance produced within QLCSs and supercell storms. Specific research prospects include the following:
* A more detailed investigation of the kinematics of reflectivity segments, their relative frequency, and their potential importance in tornadogenesis. This work will provide further details on the physics of the interaction between RSs and supercell storms or QLCSs (Coleman and Knupp 2008;
* Examination of the time-evolving kinematics (vertical vorticity in particular) and tornadogen- esis within a mature MV.
* Analysis of the rapid formation of cyclonic circula- tions along the leading edge of the midday QLCS.
* A close examination of debris transport and the relation to 3D f lows within a long-lived supercell storm.
* A detailed analysis of the evolution of a baroclinic boundary and its impact on supercell storms, two of which spawned EF-5 tornadoes.
* The influence of topography on tornado-scale vortices, including apparent tornadogenesis, varia- tions in intensity, and inferences on variations in the surface wind as inferred from tree fall patterns.
ACKNOWLEDGMENTS. A National Science Foundation Rapid Response Research Grant, AGS-1140387, provided funding for this initial research. Dr.
INITIAL INVESTIGATION OF SOCIETAL ASPECTS
To supplement ongoing work on the
The intensity, duration, and scope of the outbreak created exceptional challenges for EMA offices, specifically widespread and long-lasting (> 5 days in many areas) power outages, which left residents with few if any sources of weather threat information or communication services and sharing of emergency response. Resource allocation as part of the
In addition to stresses on warning dissemination and response, perhaps another component of the tragically high death tolls involved the survivability of the event in the shelter that was available. The large number of violent torna- does and relative lack of basements or below-ground shelters in most AL homes may have led to fatalities for individuals that sought shelter, but in a structure unable to survive the severity of violent tornadoes. Simmons and Sutter (2012b) evaluate this problem with statistical methods and show the high death tolls of 2011 (not just the 27 April event) were likely more due to the extreme nature of the 2011 outbreaks rather than only residents' high climatological and/or social tornado vulnerabilities. Still, they contend that this result does not indicate that social vulnerabilities had no effect.
Significant tornado events are often followed by an "enhanced interest" in obtaining storm shelters and /or safe rooms (Simmons and Sutter 2012a), and in fact, over the last two years, shelter installation within both existing and newly constructed single family dwellings in north AL has risen dramatically and has been frequently used as an incentive in new home sales.
1 Since trees are numerous in the southeast, tornadoes in this outbreak destroyed perhaps millions of trees.
2 The N2 index was selected since it is based on 13 tornado parameters, with the highest weights given to total number of tornadoes, number of significant tornadoes, number of violent tornadoes, destructive potential index, and pathlength of all tornadoes. Refer to Shafer et al. (2012) for details.
3 After 1994, the width in the SPC database was changed from mean to maximum width along the tornado damage path (Brooks 2004). Thus, the numbers that are based on the width, such as DPI, will have a high bias. Given the change in tornado width recording methodology, it should also be noted that the sum of F-scale times pathlength was slightly larger on
4 A reflectivity segment is a quasi-linear feature in reflectivity factor that appears to play a role in tornadogenesis. The RS kine- matic and dynamic features have not been defined previously, but we have observed a correlation between intersection of an RS with an existing supercell storm, and a subsequent increase in circulation, in other tornadogenesis events over northern
5 To calculate CAPE, the missing values above 500 mb of the sounding in Fig. 9 were filled in with values from the
6 The Trapp et al. (2005a) study examined the likelihood of tornadic mesocyclones as a function of the lowest detectable height.
7 The term "specific DPI" is defined as the destructive potential index for a specific tornado.
8 For comparison, Lemon and Umscheid (2008) determined a ?
9 Scientific interest in mergers has increased in recent years (e.g., Lee et al. 2006). Several studies investigating merger events associated with tornadic storms were presented at the recent 26th Conference on Severe Local Storms in
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AFFILIATIONS: Knupp, MuRphy, ColeMAn, WAde, MullinS, C. J. SChulTz, e. v. SChulTz, CARey, And SheRReR-University of
CORRESPONDING AUTHOR: Dr.
The abstract for this ar ticle can be found in this issue, following the table of contents.
A supplement to this article is available online (10.1175/BAMS-D-11-00229.2)
In final form
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