Editorial Type:
Article
Article Type:
Research Article

Mesoscale Dynamics in the Palm Sunday Tornado Outbreak

Steven E. Koch Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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David Hamilton Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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Devin Kramer Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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Adam Langmaid Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

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Print Publication:
01 Aug 1998
DOI:
https://doi.org/10.1175/1520-0493(1998)126<2031:MDITPS>2.0.CO;2
Page(s):
2031–2060
Restricted access

Abstract

Radar and satellite imagery suggest that strong mesoscale forcing occurred in the Palm Sunday tornado outbreak on 27 March 1994. Parallel lines of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in Mississippi and Alabama on this date. Analyses of routine meteorological observations, barograph data, and forecasts from the Eta and NGM models and a mesoscale research model (MASS) are used to examine the relative roles of large-scale dynamics and mesoscale processes in triggering and organizing the mesoscale convection.

Quasigeostrophic forcing was absent in the outbreak region. Likewise, a thermally direct circulation system transverse to the upper-level jet that was present to the northwest of the outbreak region was decoupled from the strong low-level ascent occurring in northern Alabama and Mississippi at the time of the outbreak. Strong ageostrophic frontogenesis in the presence of conditional symmetric instability (CSI) was the chief cause for the intense low-level ascent along and behind the front, consistent with the line of severe storms that developed explosively along the front and an observed postfrontal precipitation band. However, the strongest supercells developed in segmented lines 100–200 km ahead of and parallel to the frontal boundary in an atmosphere that the MASS model indicates was inertially unstable due to a mesoscale midlevel jetlet. Analysis suggests that these storms developed in a manner consistent with the predictions of asymmetric inertial instability theory in the presence of convective instability.

Several mesolows were observed to have traveled along the frontal boundary and to have played a key role in focusing the frontogenesis. Similar frontal mesolows were simulated by the MASS model. Strong low-level ascent in the presence of conditional instability helped to deepen the mesolows, but they were strongly modulated by a train of gravity waves propagating on the cold side of the front. A combination of ducting and wave-CISK (conditional instability of the second kind) processes maintained the waves, which remained coupled to the jetlets as they propagated from intense convection in northeastern Texas. A time-to-space conversion objective analysis of bandpass-filtered barograph data reveals that similar waves emanated from this same region.

The lifting patterns produced by the complex interactions between the gravity waves, CSI, asymmetric inertial instability, and frontogenesis satisfactorily explains the development, configuration, spacing, and relative movement of the severe mesoconvective systems on Palm Sunday. All of these mesoscale phenomena were coupled to or strongly influenced by the jetlets, which were produced by strong convection at an earlier time within the region of quasigeostrophic forcing far removed from the tornado outbreak.

Corresponding author address: Dr. Steven Koch, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box 8208, Raleigh, NC 27695-8208.

Email: Steve_Koch@ncsu.edu

Abstract

Radar and satellite imagery suggest that strong mesoscale forcing occurred in the Palm Sunday tornado outbreak on 27 March 1994. Parallel lines of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in Mississippi and Alabama on this date. Analyses of routine meteorological observations, barograph data, and forecasts from the Eta and NGM models and a mesoscale research model (MASS) are used to examine the relative roles of large-scale dynamics and mesoscale processes in triggering and organizing the mesoscale convection.

Quasigeostrophic forcing was absent in the outbreak region. Likewise, a thermally direct circulation system transverse to the upper-level jet that was present to the northwest of the outbreak region was decoupled from the strong low-level ascent occurring in northern Alabama and Mississippi at the time of the outbreak. Strong ageostrophic frontogenesis in the presence of conditional symmetric instability (CSI) was the chief cause for the intense low-level ascent along and behind the front, consistent with the line of severe storms that developed explosively along the front and an observed postfrontal precipitation band. However, the strongest supercells developed in segmented lines 100–200 km ahead of and parallel to the frontal boundary in an atmosphere that the MASS model indicates was inertially unstable due to a mesoscale midlevel jetlet. Analysis suggests that these storms developed in a manner consistent with the predictions of asymmetric inertial instability theory in the presence of convective instability.

Several mesolows were observed to have traveled along the frontal boundary and to have played a key role in focusing the frontogenesis. Similar frontal mesolows were simulated by the MASS model. Strong low-level ascent in the presence of conditional instability helped to deepen the mesolows, but they were strongly modulated by a train of gravity waves propagating on the cold side of the front. A combination of ducting and wave-CISK (conditional instability of the second kind) processes maintained the waves, which remained coupled to the jetlets as they propagated from intense convection in northeastern Texas. A time-to-space conversion objective analysis of bandpass-filtered barograph data reveals that similar waves emanated from this same region.

The lifting patterns produced by the complex interactions between the gravity waves, CSI, asymmetric inertial instability, and frontogenesis satisfactorily explains the development, configuration, spacing, and relative movement of the severe mesoconvective systems on Palm Sunday. All of these mesoscale phenomena were coupled to or strongly influenced by the jetlets, which were produced by strong convection at an earlier time within the region of quasigeostrophic forcing far removed from the tornado outbreak.

Corresponding author address: Dr. Steven Koch, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box 8208, Raleigh, NC 27695-8208.

Email: Steve_Koch@ncsu.edu

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