The role of jet couplet dynamics in the lifecycle of a short-duration, severe mid-latitude squall line within an elevated, marginally unstable environment

Shortly after midnight Monday, January 24, 2000, an area of showers and thunderstorms developed north of a warm front draped over southwest Alabama and the western Florida Panhandle. By 3 AM EST, this area of convection rapidly intensified and formed a squall line as it swept eastward toward the eastern Florida Panhandle. During its mature stage, several bow echo line segments evolved. The KTLH WSR-88D detected numerous mesocyclonic signatures embedded within the squall line during its transit from the Florida Panhandle into the Florida Big Bend and extreme southwest Georgia. WFO Tallahassee issued nine severe thunderstorm warnings during a three-hour period (0425-0711 AM EST). A preliminary local storm report tallied eight wind damage reports and one large hail report between 420 AM and 730 AM EST. Localized downbursts or straight-line winds were blamed for downed trees and power lines, and structural damage. The most significant damage occurred at a middle school in Thomasville, Georgia when high winds lifted its roof and deposited it several hundred yards away.

Up to 24 hours prior to the event, the NCEP short range models were forecasting a significant 500-mb shortwave trough to swing southeastward from the Midwest into the Carolinas by 0000 UTC/24 January. The 300-mb wind fields for 1200 UTC/24 January depicted a coupled jet structure, with the left front quadrant of the southern branch over the Florida Panhandle juxtaposed with the right rear quadrant of the northern branch over Georgia. The 1200 UTC/24 January 700-mb omega fields placed a bullseye of maximum upward vertical velocities, in the -15 to -20 microbar/sec range, over the Florida Big Bend and southwest Georgia. NCEP model time-height profiles and soundings portrayed an elevated, marginally unstable airmass, and a unidirectionally wind-sheared environment. The 1200 UTC/24 January NCEP model cycle indicated rapid surface cyclogenesis off the Carolina coast, with a deeepening, negatively tilted 500-mb shortwave trough lifting northeastward into New England during the next 48 hours. A major winter storm ensued, crippling much of the Mid-Atlantic and Northeast U.S. with up to two feet of snow.

This paper will assess the synoptic environment in which this squall line evolved, as well as the mesoscale features of the bow echoes, including the KTLH WSR-88D reflectivity and velocity data. It is the intention of the authors to gain a better understanding of the dynamical structure of jet streak couplets and their contributions to mesoscale convective systems.