METEOROLOGICAL OVERVIEW OF THE DEVASTATING 27 APRIL 2011 TORNADO OUTBREAK BY KEVIN R. KNUPP, TODD A. MURPHY, TIMOTHY A. COLEMAN, RYAN A. WADE, STEPHANIE A. MULLINS, CHRISTOPHER J. SCHULTZ, ELISE V. SCHULTZ, LAWRENCE CAREY, ADAM SHERRER, EUGENE W. MCCAUL JR., BRIAN CARCIONE, STEPHEN LATIMER, ANDY KULA, KEVIN LAWS, PATRIck T. MARSH, AND KIM KLOckOW The outbreak of 199 tornadoes on 27 April 2011, the most significant since the dawn of reliable records, was generated by parent storm systems ranging from quasi-linear convective systems to long-lived discrete supercell storms. large number of tornadoes were recorded during 170 tornadoes, mostly EF-0 to EF-2 on the enhanced the spring 2011 season, particularly a record Fujita (EF) scale over primarily three regions: A number (around 758) during the month of April Oklahoma (OK)–Arkansas (AR), southern Mississippi (NOAA 2011; NOAA 2012; Simmons and Sutter (MS)/Alabama (AL), and the mid-Atlantic] and 25–28 2012a,b). A few tornado outbreaks accounted for the April from Texas (TX) to eastern Virginia (VA) (~350 majority of the most damaging and lethal tornadoes, tornadoes, 321 fatalities) and 1-day outbreaks on 22 including extended outbreaks on 14–16 April [about May [~48 tornadoes from OK to Wisconsin (WI), including the lethal EF-5 tornado in Joplin, Missouri AFFILIATIONS: KNUPP, MURPHY, COLEMAN, WADE, MULLINS, C. J. (MO)] and 24 May [~47 tornadoes in Kansas (KS), SCHULTZ, E. V. SCHULTZ, CAREY, AND SHERRER—University of Alabama OK, AR, and TX, including one EF-5 in OK]. in Huntsville, Huntsville, Alabama; MCCAUL—Universities Space In this overview paper, we summarize the tornado Research Association, Columbia, Maryland; CARCIONE, LATIMER, super outbreak of 27 April 2011, defined herein as the AND KULA—National Weather Service, Huntsville, Alabama; 24-h period midnight–midnight central daylight time Laws—National Weather Service, Birmingham, Alabama; (CDT) (0500 UTC). By many metrics this disastrous MARSH—Cooperative Institute for Mesoscale Meteorological outbreak exceeds the super outbreak of 3–4 April Studies/National Severe Storms Laboratory, Norman, Oklahoma; 1974 (Fujita 1974; Corfidi et al. 2010). The number KLOckOW—University of Oklahoma, Norman, Oklahoma CORRESPONDING AUTHOR: Dr. Kevin Knupp, University of of tornadoes over a 24-h period for the 27 April 2011 Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35805 outbreak was 199 (Fig. 1); tornado fatalities were E-mail: [email protected] 316 (the most in a 24-h period since 21 March 1932; The abstract for this article can be found in this issue, following the NOAA 2011), and injuries exceeded 2,700. For the table of contents. month of April, insurable losses from tornadoes sur- DOI:10.1175 / BAMS - D -11- 0 0229.1 passed $11 billion (U.S. dollars), while the total loss is estimated at around $15.5 billion (Simmons et al. A supplement to this article is available online (10.1175 / BAMS - D -11- 0 0229.2 ) 2012). The super outbreak on 27 April accounted for In final form 21 August 2013 the majority of these losses. The amount of debris ©2014 American Meteorological Society produced by the 27 April outbreak was staggering, AMERICAN METEOROLOGICAL SOCIETY JULY 2014 | 1041 Unauthenticated | Downloaded 10/10/21 07:11 AM UTC this paper (available online at http://dx.doi.org/10.1175 / BAM S - D -11- 0 0229.2). 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 (0500 UTC 27 April to 0500 UTC 28 April): 1) a mesoscale convective system (MCS) that evolved to an intense quasi-linear convective system (QLCS) during the early morn- ing hours, 2) a smaller QLCS over northern AL during the midday hours, and 3) widespread discrete FIG. 1. Map of tornado paths on calendar day 27 Apr 2011. Tornado EF scale supercell storms during is color coded according to the scale given in the bottom right. Fatalities are the afternoon–evening given for each county according to the color scale in the bottom (center). time period. We present a Tornadoes continued over the Carolinas, northern Virginia, Indiana, Ohio, Pennsylvania, Maryland, and New York on 28 Apr. look at the environment(s) that supported these three rounds of severe weather, with media-reported estimates of 10 million cubic discuss some details and early analyses of each yards (Birmingham News 2011).1 sequence, and mention projects related to the event, The 27 April tornado outbreak was the major including some investigations that are not purely component of an extended 4-day episode of torna- meteorological. Characteristics of these episodes are does starting on 25 April and ending on 28 April. On documented by Weather Surveillance Radar-1988 25 April, around 64 tornadoes (only four were EF-2 Dopplers (WSR-88Ds), a mix of surface measure- or greater intensity) were spawned between north- ments, ground surveys, and instruments operated east TX and western Tennessee (TN) and Kentucky by the University of Alabama in Huntsville (UAH), (KY). On 26 April, approximately 50 tornadoes were including the Advanced Radar for Meteorologi- documented from eastern TX to the western portions cal and Operational Research (ARMOR) C-band of TN and KY. After the catastrophic outbreak on dual-polarization radar, the Mobile Alabama X-band 27 April, the residual synoptic-scale system produced (MAX) dual-polarization radar, and the Mobile 43 additional tornadoes (four EF-2 and one deadly Integrated Profiling System (MIPS). EF-3) from Georgia (GA) to New York (NY). A variety of initial analyses have been conducted A primary motivation for this paper is to docu- and brief summaries of the more interesting aspects ment the mesoscale and storm-scale characteristics of are reported herein. Some of the more intriguing this outbreak, describe unique datasets, and present aspects of this outbreak include the following: some initial analyses that are deemed worthy of more detailed investigations. Because numerous special • Multiple modes of convective organization, datasets were acquired near the geographical core including QLCSs, mesovortices embedded within of this outbreak, we are compelled to describe some the QLCSs, and discrete supercell storms (some initial results without showing many details due to associated with a thermal boundary) occurred the space limitations of this article. Additional details during the outbreak. and figures, including radar animations and sound- • An impressive mesoscale vortex spawned within ings, are provided in the supplemental information of the early morning QLCS was associated with the 1 Since trees are numerous in the southeast, tornadoes in this outbreak destroyed perhaps millions of trees. 1042 | JULY 2014 Unauthenticated | Downloaded 10/10/21 07:11 AM UTC rapid development of 16 tornadoes over north- deep, negatively tilted upper-level trough was located central to northeast AL. over AR and Louisiana (LA). At 300 hPa, a 50 m s–1 • The parent storms, including supercell storms and jet extended around the base of the trough, from the strong convective elements within QLCSs, were Rockies into western AL (Fig. 2a). Positive differen- efficient in producing tornadoes. For example, tial vorticity advection at 500 hPa (DPVA; Fig. 2b) about 90% of the supercell storms within the was present from AR into northern AL as air parcels outbreak region produced at least one tornado. exited the sharply curved upper trough. Paired with • Many tornadoes were long tracked, wide, and negligible thermal advection, the resulting net mass intense. divergence within the column was associated with • Analysis of polarimetric variables, combined with rapid surface pressure falls and associated isallobaric dual-Doppler analyses of a supercell storm, graphi- flow into a surface cyclone (995-hPa central pressure) cally illustrate that debris was effectively lofted to centered in western KY at 2200 UTC (Figs. 2c,d), con- relatively high altitudes. sistent with patterns documented in previous signifi- • f Many o the violent tornadoes exhibited horizontal cant tornado outbreaks (Mercer et al. 2012). A strong vortices along their periphery. surface cold front extended from the cyclone center in • External influences, including a thermal bound- western KY into central LA at 2200 UTC. The rapid ary, possible gravity waves, and topography, low-level height falls east of the trough axis in western appeared to play a role in tornadogenesis and KY contributed to a 30 m s–1 south-southwesterly low- tornado intensity change. level jet at 850 hPa over AL (Fig. 2c). The rapid height • Despite timely and accurate warnings by the and surface pressure falls promoted ageostrophic National Weather Service (NWS) forecast offices, flow within the lower boundary layer (Fig. 2d), where the number of fatalities (316) was high. friction played an additional role in backing surface wind vectors at angles approximately 60° to the The environment described in the following sec- isobars, maintaining SRH values > 500 m2 s–2 over tion was extremely conducive to the development of the outbreak region. strong, long-track tornadoes. Although this danger- The upper-level divergence and associated vertical ous situation was anticipated exceptionally well by the motion helped promote steep lapse rates of 7.5°C km–1 National Oceanic and Atmospheric Administration between 700 and 300 hPa, according to the 2100 UTC (NOAA) Storm Prediction Center (SPC), this paper North American Mesoscale Model (NAM) sounding does not focus on the forecasting aspects of this event, at Cullman (north central), AL (Fig. 2h). The south- which are expected to be documented elsewhere. erly flow at low levels transported warm and moist air northward from the Gulf of Mexico into central THE SYNOPTIC AND MESOSCALE AL, where surface temperatures and dewpoints ENVIRONMENT OF THE OUTBREAK. This reached 25°–27°C and 19°–22°C, respectively, by section presents an analysis of the environmental 2200 UTC.