Central Florida Tornado Outbreak of April 25, 1991
A Response to a Challenge
Charles H. Paxton
NWFO Tampa Bay, Florida
In their concluding remarks on Central Florida tornado outbreaks, Hagemeyer and Matney (1994) issued a challenge to "the most perceptive of analysts to diagnose the case of April 25, 1991-perhaps the most unusual of all tornado outbreaks." The typical Florida tornado outbreak occurs when convection is strongly baroclinically forced and pre-existing shear is evident. This tornado outbreak was atypical: it was spawned by a mesoscale convective complex (MCC). The MCC developed over the middle Gulf states during the evening of April 24, 1991. Between 1200 and 1800 UTC April 25, 1991, the MCC split, with part remaining stationary and part moving southeast, producing a squall line. By 1800 UTC, the north-south, 50 nm long, arced line of strong convection was about 50 nm offshore from Florida's west coast. WSR-57 radar reflectivity indicated intense convection; 50 dBZ or greater. The convective band produced widespread wind gusts to 70 kt, hail, and several tornadoes as it moved onshore over Pasco and Pinellas counties and inland at 50 kt.
MCC DESCRIPTION AND 1200 UTC ANALYSIS
At 1200 UTC April 25, 1991, the MCC was centered along the Gulf Coast near Mobile, Alabama, and the circular cloud shield covered most of Mississippi, Alabama, Georgia, and the northern half of Florida. Constant pressure plots and analyses at 1200 UTC (not shown) indicated features consistent with an MCC in the mature and decaying stages.
Maddox (1980) described MCCs as large, organized, long lived, and often nocturnal areas of convection. In the upper levels, Fritsch and Maddox (1981) and Maddox, et al. (1981) found anticyclonic outflow and a nearly circular, cold cloud shield associated with MCCs. In further investigation, Maddox (1983) provided a detailed MCC analysis. Mesoscale convergence, forced by low-level warm advection, interacts with a weak mid-level short wave trough. In the Florida case, 850 mb and 700 mb analyses projected a weak ridge across Louisiana and the southeast United States, with southerly wind flow, low level warm advection (Fig. 1.) and convergence (Fig. 2.) occurring at those levels. Thus, the lower half of the MCC was characterized by a significant net upward mass flux and widespread precipitation. With a large scale ridge in place, broad anticyclonic flow occurred above 700 mb over the region. A short wave embedded in this broad ridge extended across the central U.S. into the MCC. A strong vorticity maximum was associated with the 500 mb short wave over the southern part of the MCC.
As the developing MCC moved east ahead of the mid-level trough, diabatic heating produced a warm core system in the mid-levels and a cold core in the upper levels. Thickness values from 500 mb to 300 mb (Fig. 3) indicated a warm anomaly with this MCC while temperatures in the upper levels were cold. The 150 mb temperature at Slidell, Louisiana was -73 C. The MCC definition includes a persistent area of cloud tops colder than -32 C and larger than 53,000 nm2 . This MCC had cold cloud tops extending over an area roughly double the requirement. The resulting increase in thickness produced anomalously high heights in the upper levels, thus producing an intense outflow. The 300 mb isotach analysis (Fig.4) is consistent with prior MCC descriptions, showing a minimum of 40 kt near the MCC core and a jet maximum of 90 kt north of the system. Ageostrophic wind analysis at 300 mb (Fig. 5) indicated anticyclonic outflow originating from the center of the system.
In further investigation, Cotton, et al. (1989) studied 134 cases and divided MCC evolution into seven categories for analysis, including MCC -12 hr, Pre-MCC, Initial, Growth, Mature, Decay, Dissipation, and Post-MCC stages. The mature stage, when the MCC is largest in size, generally occurred at night between 0700 and 0800 UTC. Such was the case in this study, where the decay stage is characterized by little low-level moisture support, but strong upward vertical motion near 400 mb, continuation of strong anticyclonic shear near 200 mb, and increasing cyclonic shear near 500 mb. These mechanisms may focus convection over a smaller area.
DISSIPATION AND EVOLUTION OF THE MCC INTO A SQUALL LINE
Dissipation occurs as the system moves east of the primary forcing mechanisms, conditional instability, and low-level warm advection. As dissipation occurs, residual temperature perturbations produce an atmospheric response resulting in intensification of the precursor short wave trough (Maddox 1983). This affects the MCC in one of a number of ways. Watson, et al. (1984) noted squall line development in the dissipation stage of an MCC. Smull and Augustine (1993) noted the open wave precipitation pattern of a PRE-STORM MCC over Kansas and Oklahoma. Toward the end of the MCC life cycle, Menard and Fritsch (1989) observed a splitting MCC and then development of a deep vortex extending from the surface to 350 mb, with a maximum between 750 and 650 mb.
In the Florida case, the MCC split in two. This was associated with decay of the system and a sharpening of the associated mid-level short wave. The eastern part of the MCC moved rapidly southeast across the Gulf of Mexico toward the west coast of Florida, and the western portion of the MCC remained stationary. Since radar data are not available over the Gulf of Mexico, lightning data indirectly show evolution of the intense convection associated with development of the squall line and subsequent severe weather. Three-hourly depictions beginning at 1245 UTC (Fig. 6) show the lightning strike density increasing around 1545 UTC (Fig. 7) and moving on to the west coast of Florida by 1845 UTC (Fig. 8). Satellite images (Figs. 9-12) show the system splitting, with the eastern part moving rapidly southeastward across the Gulf of Mexico toward the west coast of Florida and the western portion of the MCC remaining stationary.
In developmental and mature stages, MCCs are responsible for a considerable percentage of the rain which falls over the U.S. In dissipation stages, MCCs often produce a fast-moving squall line. Once an MCC develops, identification is relatively easy. Figure 13 (Bader, et al. 1995) summarizes characteristics common to MCCs in developing and mature stages. Common features include: a broad upper level ridge, a jet entrance region to the north, a cold core with anticyclonic outflow at upper levels, warm core with a small long-lived deep cyclone in the mid levels, a closed low around 800 mb, and a shallow anticyclone and cold pool near the surface. In brief, a dissipating MCC was responsible for what Hagemeyer and Matney (1994) termed the "most unusual of all tornado outbreaks" across Central Florida in April 1991.
Many thanks to Annegret Cornell for her assistance in proofreading and editing, and Andrew Nash for his editorial comments, both at NWSO Tampa Bay Area, and to Dolores Kiessling of COMET for data retrieval. Data and training material developed by COMET also stimulated completion of this study, particularly the COMET MCC symposium attended by the author and other SOOs in February 1998. At that symposium, Prof. James Moore (St. Louis University) supplied the reference to Bader's (1995) helpful figure.
Bader, M. J., G. S. Forbes, J. R. Grant, R. B. E. Lilley, and A. J. Waters, 1995: Images in weather forecasting. p 413.
Cotton, W. R., M. S. Lin, R. L. McAnelly, and C. J. Tremback, 1989: A composite model of mesoscale convective complexes. Mon. Wea. Rev., 117, 765-783.
Fritsch, J. M., and R. A. Maddox, 1981: Convectively driven mesoscale weather systems aloft. Part I: Observations. J. App.l Meteor., 20, 9-19.
Hagemeyer, B. C. and D. A. Matney, 1994: Peninsula Florida tornado outbreaks. NOAA Tech. Memo. NWS SR-151. Fort Worth, TX, 108 pp.
Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61, 1374-1387.
Maddox, R. A., D. J. Perkey and J. M. Fritsch, 1981: Evolution of upper tropospheric features during the development of a mesoscale convective complex. J. Atmos. Sci., 38, 1664-1674.
Maddox, R. A., 1983: Large scale meteorological conditions associated with mid-latitude, mesoscale convective complexes. Mon. Wea. Rev., 111, 1475-1493.
Menard, R. D., and J. M. Fritsch, 1989: A mesoscale convective complex-generated inertially stable warm core vortex. Mon. Wea. Rev., 117, 1237-1261.
Smull, B. F., and J. A. Augustine, 1993: Multiscale analysis of a mature mesoscale convective complex. Mon. Wea. Rev., 121, 103-132.
Watson, A. I., J. G. Meitin, and J. B. Klemp, 1984: The evolution of the kinematic structure and precipitation characteristics of a mesoscale convective system on 20 May 1979. Mon. Wea. Rev., 116, 1555-1567.