| description abstract | A diagonostic analysis is made of a midwinter mesoscale vortex that developed over the Mississippi Valley and produced moderate to heavy snow with gale force winds (>18 m s?1), lightning, and thunder along a narrow track approximately 1500 km in length. The mesoscale vortex resembled the so-called ?polar lows? that form over the subpolar seas. The similarities include development on the cyclonic-shear side of a long-wave trough, strong positive vorticity advection associated with a 500 mb short-wave trough, upstream tilt of the geopotential heights, conditional instability in the lower troposphere, a southeastward track, small (?200 km) diameter, and a 9 mb deepening of the surface mesolow in 12 h. The most intriguing features of the present analysis are the extremely large potential vorticities and horizontal temperature gradients in the midtroposphere, indicating an extrusion of stratospheric air down to levels below 700 mb. | |
