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A Preliminary Investigation of Derecho

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A Preliminary Investigation of Derecho P 5.2 THE SUPER OUTBREAK: OUTBREAK OF THE CENTURY Stephen F. Corfidi,* Jason J. Levit and Steven J. Weiss NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK 1. INTRODUCTION The Super Outbreak of tornadoes of 3-4 April 1974 remains the most outstanding severe convective weather episode of record in the continental United States (Figure 1). By nearly every metric imaginable, the outbreak far surpassed previous and succeeding events in severity, longevity and extent. A sampling of statistics only partially conveys its enormity: 148 TORNADOES …95 F2s or stronger …30 F4s or F5s 48 KILLER STORMS …335 dead …More than 6000 injured PATH LENGTHS UP TO 90 miles (145 km) …Total path length > 2500 mi (4000 km) F2s OR GREATER PRESENT FOR EACH THREE HOUR PERIOD BETWEEN 12 UTC / 3rd AND 15 UTC / 4th AT ONE POINT, 15 TORNADOES IN PROGRESS AT SAME TIME TEN STATES DECLARED Figure 1. Tracks of the 148 Super Outbreak FEDERAL DISASTER AREAS tornadoes documented by Fujita (1975). Further appreciation for the phenomenal nature of the Super Outbreak may be gleaned from Figure 2, which depicts the maximum, week-long running total of F2 or greater tornadoes since 1875. Entire years noted 120 for their prominent tornado counts (e.g. 1953 and 2003) pale in comparison to the 18-hour period that 100 began around midday on 3 April 1974. Twenty-five 80 F3 or greater long track (>25 miles (40 km)) tornadoes occurred during the same period, more 60 than triple the annual average of such events since 1880 (C. Broyles, personal communication). 40 Thirty years have passed since the Super Outbreak 20 inflicted its unprecedented human and physical toll on 0 the Midwest and the Ohio and Tennessee Valleys. 1875 1895 1915 1935 1955 1975 1995 Despite its breadth and intensity, comparatively little has been written regarding the event’s synoptic and Grazulis (1875 - 1989) mesoscale meteorological evolution. In particular, NWS (1950 - present) only limited attention has been given to understanding __________________________________________ Figure 2. Maximum, week-long running total of F2 or * Corresponding author address: Stephen F. Corfidi, stronger tornadoes per year since 1875. Data in red NOAA/NWS/NCEP/Storm Prediction Center, 1313 from Grazulis (1993); dotted black, NWS/SPC Halley Ave, Norman, OK 73069; email: database (Courtesy of R. Schneider) [email protected] why so many significant, long-lasting tornadic storms low. This activity soon organized into a forward occurred. propagating MCS that continued east into Arkansas early Wednesday. In the present paper, the surface and upper air analyses contained in Hoxit and Chappell (1975), and The thermodynamic environment across the south additional data are used to present an updated central United States late Tuesday was characterized synoptic and subsynoptic overview of the 1974 Super by a broad swath of steep low to mid tropospheric Outbreak. Emphasis will be placed on (1) identifying lapse rates that extended from the southern and the major factors that contributed to the development central Rockies east to the Mississippi Valley (Figure of the three main convective bands associated with 4a). Lapse rates were close to dry adiabatic over the event, and (2) identifying the conditions which much of western Texas and Oklahoma. While plumes may have contributed to the outstanding number of of mixed layer air originating over the high plateau of intense and long-lasting tornadoes. In addition, the southwestern United States and Mexico are output from a 29 km, 50 layer version of the Eta commonly observed in soundings taken over the forecast model using data from the NCEP/NCAR Plains and Mississippi Valley prior to spring tornado reanalysis grids (Kalnay et al. 1996) will be presented outbreaks, the extent of the potential instability in this to assess the model’s skill in depicting synoptic and case is notable. Nevertheless, because of the mesoscale aspects of the outbreak. absence of substantial boundary layer moisture, significant surface-based convective available potential energy (CAPE) was confined to areas along 2. SYNOPTIC ENVIRONMENT the front near the Gulf Coast (Figure 5a). 2.1. Tue 2 April 2.2 Early morning, Wed 3 April The morning of Tuesday 2 April 1974 was characterized by a broad, low amplitude large scale The combination of potent dynamic forcing and trough over the continental United States (Figure 3a). favorably timed diurnal factors promoted substantial Embedded in the trough were two moderately strong strengthening and broadening of the south shortwave disturbances, one of which was located southwesterly low level jet over the lower Mississippi over the Ohio Valley, and the other just entering the Valley early Wednesday. During the pre-dawn hours, Great Basin. The latter impulse was associated with 850 mb wind speeds increased to more than 50 kts 500 mb wind speeds in excess of 100 kts (50 ms-1) (25 ms-1) across Louisiana, Mississippi and Alabama across southern California, while downstream speeds as the Kansas low deepened to 980 mb. The were less than 50 kts (25 ms-1) over the southern associated northward transport of moisture resulted in Plains. The eastern shortwave was deamplifying at rapid warm sector destabilization from the Gulf Coast this time and was associated with a weakening to the Tennessee Valley. By 1200 UTC, CAPE of surface low over the eastern Great Lakes. 1000 J/kg was present as far north as the Ohio River (Figure 5b) as the Gulf Coast warm front redeveloped By late in the day, the Great Lakes low was over northward (Figure 3b). However, because of the Quebec, with a cold front that curved from off the New presence of the elevated mixed layer plume (Figure England coast across southern Virginia into the 4b), most of the CAPE was “capped” to unassisted northwest Gulf of Mexico (not shown). Surface deep surface-based convection. dewpoints south of the boundary were moist for the time of the year, ranging from the low 60s (16 C) in While the elevated mixed layer plume prohibited the north Florida to the low 70s (21C) over coastal development of deep convection over much of the Louisiana. The western part of the front began to warm sector, the favorable thermodynamic return northward as a warm front Tuesday evening as environment for cold downdraft production, in increasing westerly mid level flow and the eastward conjunction with increasing moisture inflow, fostered advance of the Great Basin impulse induced strong continued intensification of the Arkansas MCS. By lee cyclogenesis east of the central Rockies. This dawn, the convective system extended in a broken cyclogenesis was also fostered by the presence of band from southern Illinois across far western strong, along-stream variation of the upper level flow Kentucky into central Arkansas. The line of storms in the exit region of the western states speed produced severe weather at several locations, and maximium (Uccellini and Johnson 1979); 300 mb was the first of the three, supercell-containing speeds at this time ranged from nearly 140 kts (70 convective bands that would affect the east central ms-1) in southern Nevada to less than 80 kts (40 ms-1) states that day.1 in Oklahoma and north Texas. The jet maximum associated with the Great Basin shortwave trough continued to propagate east southeast Tuesday evening as a weak disturbance in the flow reached central Oklahoma (not shown). At the same time, the lee cyclone deepened to less than 984 mb and edged east into Kansas. As warm advection increased over the southern Plains, 1 In this paper, the term “convective band” will be scattered high-based thunderstorms developed favored over “squall line” to emphasize that the across north Texas and eastern Oklahoma ahead of predominant convective mode remained quasi-cellular the Pacific cold front trailing south from the Kansas rather than linear throughout the event (a) (b) (c) Figure 3. 1200 UTC Surface (left) and 500 mb (right) plots, with English units. 500 mb height contours in Daily Weather Map analyses from (a) 2 April, (b) 3 feet; contour interval 200 ft. Wind speed in knots. April and (c) 4 April 1974. Surface pressure in mb; Half barb = 5 kts (2.5 ms-1), full barb = 10 kts (5 ms-1), contour interval 4 mb. Standard abbreviated station and flag = 50 kts (25 ms-1). Because a thermal gradient was present at 700 mb shows, by 1800 UTC surface-based CAPE in excess west of the Arkansas MCS at 1200 UTC (not shown), of 2500 J/kg was present from the lower Ohio and one might conclude that the system was associated Mississippi River Valleys south and east to the Gulf with an elevated frontal zone or “cold front aloft” as Coast.; values were greater than 3500 J/kg over parts described by Locatelli et al. (1989). However, the of Alabama, Mississippi, Tennessee and Kentucky. In thermal gradient was not well-defined at the time of addition, because of the seasonably rich nature of the storm initiation over the southern Plains, and temporal boundary layer moisture inflow (dewpoints in the mid and spatial continuity of the feature as revealed by the to upper 60s), values of mean mixed layer CAPE radiosonde data is not sufficient to confirm the were nearly as great (not shown). presence of an elevated front. Similar thermal gradients have been attributed to elevated fronts in The Illinois/Kentucky/Arkansas convective band the vicinity of other forward-propagating convective (hereafter referred to as Convective Band One) systems when, in fact, they actually reflected localized continued east across the Tennessee and Ohio corridors of evaporatively-cooled air in the wake of the Valleys at an average speed of 60 kts (30 ms-1) rear-sloping MCSs (e.g., Rose et al.
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