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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.