Link to NWS Louisville Science and Technology Homepage WSR-88D Images of the May 18, 1995 Squall Line and Bow Echo Event
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During the evening of May 18, 1995, an intense squall line/bow echo raced across south-central Kentucky causing numerous swaths of straight-line wind damage, hail, and several tornadoes. Bow echoes or bowing line segments within squall lines are characterized by a downwind bulge or arc in a line of thunderstorms due to the presence of a convectively-induced channeled stream of very strong winds, called the "rear inflow jet" (RIJ), along and just behind the line of storms. Bow echoes are notorious for producing severe straight-line wind damage. The strength of the winds and associated damage sometimes can be mistaken for a tornado.  While tornadoes can occur along and just north of the bow apex within organized bowing segments, these tornadoes tend to be shorter-lived and less intense than those associated with mature supercell thunderstorms. Nevertheless, bow echo tornadoes do produce damage, although their damage widths are often small and embedded within or near significant straight-line wind damage. The following WSR-88D images show how the May 18 squall line/bow echo evolved across southern Kentucky.
WSR-88D Image 1: May 18, 1995 Severe Weather Event Base reflectivity image (lowest radar elevation angle) of a squall line over south-central KY during the early evening of May 18, 1995. The line is about 60 miles south of the NWS Louisville (KLVX) WSR-88D radar. The line of convection over Green, Metcalfe, and Barren counties was producing heavy rain and strong, gusty winds. This represented the early stages of the bow echo event.
WSR-88D Image 2: May 18, 1995 Severe Weather Event Fifteen minutes later, the line of thunderstorms had intensified rapidly producing a well-defined severe bow echo across southern Taylor and western Adair counties (note the "S" shape in the red colors). The reflectivity gradient along the leading edge of the convection also tightened considerably. These signatures suggested that wind damage had increased significantly along the bow apex in western Adair county. Just behind the intense convection, a "weak echo channel" of lighter rain existed, signifying the presence of a channeled locally-enhanced rear inflow jet that promoted the wind damage.
WSR-88D Image 3: May 18, 1995 Severe Weather Event This image is storm-relative velocity (SRM) data that accompanies the reflectivity data above. Green (red) colors represent storm-relative radial winds directed toward (away) from the radar (located to the northwest); bright/light red and green colors represent stronger winds than darker shades. The image shows a tight cyclonic circulation (mesocyclone) along the Taylor/Adair/Green county border. The mature circulation was located near the apex of the bow echo on reflectivity data, and was producing a tornado at this time. Straight-line wind damage was occurring along the nose of the rear inflow jet (i.e., stronger outbound bright red colors) over western Adair county coincident with the leading edge of the bow apex.
WSR-88D Image 4: May 18, 1995 Severe Weather Event Reflectivity data ten minutes later showed that the bowing line segment had moved east across eastern Taylor and Adair counties. The apex of the bow and a second cyclonic circulation (not shown) were located across northeastern Adair county where wind damage and two funnel clouds were reported. The original strong circulation was weakening over far northern Adair county, and its associated tornado had dissipated.
WSR-88D Image 5: May 18, 1995 Severe Weather Event The line continued to sweep eastward across Casey, eastern Adair, and northern Russell counties. At this time, the bowing signature appeared less pronounced, although multicellular growth of convection still continued along the leading edge of the line, i.e., along the downdraft/updraft interface (gust front). A rear inflow jet still was present across southern Casey county (not shown) which contributed to strong surface winds along the leading edge gust front.
WSR-88D Image 6: May 18, 1995 Severe Weather Event Fifteen minutes later, a resurgence occurred in the bowing line segment across western Pulaski counties. Significant bowing again existed which caused significant straight-line wind damage to trees and property. The narrow ribbon of lighter rain (i.e., weak echo channel) over southeastern Casey and west-central Pulaski counties just behind the bow apex signified the presence of an intense rear inflow jet. Funnel clouds also were reported near and just north of the bow apex within the comma-head type pattern. Farther southwest along the line and away from the main rear inflow, wind damage also occurred over Russell county but was less intense than along the bow apex. However, large hail (up to 1.75 inches in diameter) was reported as the line swept across Russell county.
WSR-88D Image 7: May 18, 1995 Severe Weather Event As the bow echo moved across northern Pulaski county, it continued to intensify with deep convection along and north of the apex. A well-defined comma head signature now existed near the border of Pulaski, Lincoln, and Rockcastle counties. This signature was embedded within a strong shear zone between the rear inflow jet across northwestern Pulaski county (i.e., rear-to-front storm-relative flow) and environmental winds feeding into the convection from ahead of the line (i.e., front-to-rear storm-relative flow). Along the comma tail at this time, a rapidly intensifying new cyclonic circulation (not shown) existed near the bow apex which produced a tornado and F1-F2 damage across northeastern Pulaski county. The bow echo began to exhibit high precipitation (HP) supercell characteristics at this time.
WSR-88D Image 8: May 18, 1995 Severe Weather Event Ten minutes later, HP supercell characteristics became more apparent along the bow apex. A subtle "bounded weak echo region" (BWER) existed along the southern Rockcastle/northeastern Pulaski county border (i.e., the small lighter red area surrounded by darker reds/heavier precipitation). A BWER usually identifies the location of a strongly rotating updraft (i.e., mesocyclone) in HP storms (see below). Meanwhile, thunderstorms were weakening over central Pulaski county southwest of the bowing/HP segment.
WSR-88D Image 9: May 18, 1995 Severe Weather Event The accompanying storm-relative velocity image showed a strong mesocyclone (i.e., green inbound next to red outbound winds) along the Rockcastle/Pulaski border (identified by the arrow). The vortex represented a strongly rotating updraft which was coincident with the BWER in reflectivity data (see above). A tornado was still in progress at this time associated with the mesocyclone. The circulation then began weakening and the tornado dissipated several minutes later. This image also revealed bright red colors, i.e., strong rear-to-front flow, just behind the mesocyclone, which indicated that the locally enhanced rear inflow jet was still present.
WSR-88D Image 10: May 18, 1995 Severe Weather Event Ten minutes later the bow echo/HP supercell storm complex moved into Laurel county in south-central Kentucky. A BWER was no longer clearly evident, although this may have been due to radar sampling issues. Nevertheless, very high reflectivity values existed which signified the presence of hail within the storm complex. At this time, yet another cyclonic circulation (mesocyclone) had developed (not shown), which produced a separate F2 tornado in Laurel county. The tornado and nearby severe straight-line winds damaged or destroyed some buildings and knocked down trees.
WSR-88D Image 11: May 18, 1995 Severe Weather Event A short time later, the bow echo/HP supercell complex pushed into Clay county in southeastern Kentucky, about 115 miles southeast of the radar location. High reflectivity values persisted along the bow apex, while a weak echo channel was still evident behind the intense convection. Thus, severe straight-line wind damage continued. In addition, another cyclonic circulation developed by this time within the HP supercell complex, resulting in another separate tornado in Clay county. During the bow echo's evolution to an HP supercell structure, multicellular convective development continued within the bowing segment. Thus, this appeared to be a "multicell-HP supercell hybrid" type system. After this time, the bowing segment eventually weakened across southeastern Kentucky.

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