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The Origin of a Severe Thunderstorm in Kansas on 10 May 1985

Frederick SandersMarblehead, Massachusetts

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David O. BlanchardNOAA/NSSL/Mesoscale Research Division, Boulder, Colorado

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Abstract

This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.

Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.

A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.

Abstract

This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.

Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.

A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.

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