A significant, historic serial derecho raked a large swath from Arkansas to Washington D.C. to New York state April 9-10, 1991 with 24 tornadoes & +300 reports severe gusts & wind damage.
Dry air impinging from the Plains at mid-levels (enhancing downdrafts), +100 mph jet streak from the west & thus rapidly strengthening low with strong surface cold front allowed this derecho to reach historic status. Also, a rapid pressure fall & rise was evident in the passage & exit of the derecho as meso-low formed with the derecho & lobe of strong surface high wedged in behind it.
The tornadoes seemed to be correspondent with the more directional shear of the derecho’s self-made meso-low near & just south of the “Bookend Vortex” (or comma head) & not the intersection of any warm front or triple point (except possibly some of the tornadoes in southern New York state). All tornadoes were of F0-F2 rating.
Wind gusts of up to 100 mph occurred with +60 mph gusts in the southeastern areas of the Tri-State. 5 people alone were injured from falling trees & limbs in Owensboro. Wind damage was reported in Muhlenberg, Daviess (KY) & Perry counties. Hopkinsville measured a gust of 74 mph, while Nashville recorded an official gust of 67 mph.
Some isolated large hail occurred with the t’storm line north of the derecho. +100 people were injured in the derecho overall & 5 people were killed. It has often been called the “1991 West Virginia Derecho”.
It was the “Bookend Vortex” or comma head of the derecho that rakes our southeastern counties with severe wind gusts.
Traced images of actual radar at the time are below (from J. W. Duke and J. A. Rogash paper Multi-Scale Review of the Development and Early Evolution of the 9 April 1991 Derecho in 1992’s Issue 7 of Weather Forecasting (pages 623-634):
Storm reports map courtesy of SPC:
Surface maps (courtesy of NOAA):