Case #2: May 24, 2004  High Precipitation Supercell to tornadic QLCS case from Northern Missouri through West-Central Illinois Tornadic QLCS event.   WES case event.  Updated  07/25/2012.

This case was presented at the 30th Annual National Weather Association (NWA) Meetings held in St. Louis Missouri during the week of October 18, 2005.   (Last update May 22,  2008)

1. Introduction

A broken line of thunderstorms rapidly developed over parts of northeast Missouri from two High-Precipitation supercells which traversed over northwest and parts of north-central Missouri during the evening hours of 24 May 2004. The broken line of storms formed into an organized bow echo which produced clusters of downbursts and several QLCS tornadoes. There was one fatality and several injuries as a QLCS tornado moved through parts of the town of Winchester (Scott County Illinois) in west-central Illinois. Damage to trees, house trailers and homes also occurred in the north part of the town in Summer Hill (Pike County Illinois- MV 3) and the southern part of New Hartford (Pike county Illinois MV3).  The The following is a summary of the 24 May 2004 event. We will first begin with the environmental setting followed by the storm evolution as seen from the KLSX WSR-88D at Weldon Springs MO. Vr traces will also be shown to reveal the evolution of the tornadic mesovortices. 

2.  Storm Prediction Center (SPC Outlook for 2133 UTC 

Below is the 2133 UTC Storm Prediction Center Outlook for the Midwest.

Fig. 1a.  SPC's severe weather outlook for 2133 UTC (left);  tornadic probability outlook (center); and wind outlook (right). 
(Click on image for a larger image).  



Fig 1b.  SPC Mesoscale Convective Discussions 0874 (2226 UTC - left) and 0880 (0115 UTC - right)  and Tornado Watch 0281 issued at 500 PM CDT. Tornado Watch 0281 is valid from 5:45 PM - 10:45 PM CDT.

3. Pre and Near-storm environment

The 300 mb analysis at 0000 UTC 25 May 2004 showed strong west-southwest flow with a 75 kt jet core approaching eastern Kansas and western Missouri.  Southern Iowa - northern Missouri was in the vicinity of the left-front exit region of the upper level jet.  Note the diffluent flow over this area.  Several weak shortwaves over the Central Plans were riding up the west-southwest flow at 500 mb.  The 5H thermal field revealed a series of weak thermal troughs embedded with the mean flow.  The 700 mb flow was similar to the 5H flow showing a west-southwest flow over the region.   7H warm advection was observed over the Mid-Mississippi Valley region.  It is interesting to note that steep mid-level lapse rates were present over western Missouri through Kansas and Oklahoma at 0000 UTC with 5H-7H delta T's of 19 to 23C.  The 850 mb pattern showed a warm and moist southwest flow of air over the region. . An axis of 8H dewpoints equal or exceeding 15C extended from the Missouri - Kansas border southward into northern Texas.  This axis of relatively high dewpoints was advecting northeast into northern and central Missouri.  (Click on the graphics below in Fig. 1 to show UA analysis for 0000 UTC.    



Fig 2. Upper air analysis for 0000 UTC 25 May 2004 from Storm Prediction Center (SPC).  Far left (300 mb), near left (500 mb), near right (700 mb), and far right (850 mb).  (Click on image for a larger image).

On the surface at 0000 UTC, a warm frontal boundary extended from an area of low pressure near St. Joseph Missouri to near St. Louis and then southeast into southern Illinois.  A surface trough stretched southwest from the surface low through south-central Kansas. One axis of 70 degree + dewpoints stretched from southeast Kansas through the Kansas City metro area. while a secondary area of 70 degree + dewpoints covered parts of southeast Missouri, southern Illinois and across western Kentucky and Tennessee. A thermal gradient marked the location of the warm frontal boundary from north-central Missouri through south-central Illinois as surface temperatures ranged from the mid and upper 70s north of the front to the low and mid 80s south of the front. RUC surface-based CAPE values over central Missouri exceeded 4000 J/kg suggesting the presence of an highly unstable atmosphere.  At 0000 UTC the Pleasant Hill MO WSR-88D (EAX)showed a large supercell over north-central Missouri (40 km southwest of Kirksville MO (IRK)) while a second smaller supercell was observed just south of the Missouri - Iowa border or approximately 45 km northwest of IRK (See Fig. 2). 


Fig. 3. AWIPs surface analysis for 0000 UTC, 25 May 2004 with surface-based CAPE overlaid
in the background (left side) . WSR-88D reflectivity data at 0000 UTC from Pleasant
Hill MO (KEAX - right side).  (Click on image for a larger image).

Fig 4. Skew-t Log-P diagram for 0000 UTC 25 May 2004 from WFO Springfield (KSGF) (left);
and WFO Lincoln (KILX) (right). (Click on image for a larger image). 

The Skew-T Log-P sounding for Springfield Missouri (KSGF) at 0000 UTC, 25 May 2004 is shown in Figure 3. The sounding showed a elevated mixed layer between 780 - 580 mb with the lapse rate nearly dry adiabatic. A shallow moist layer was confined between 920 and 860 mb. Surface-based CAPE (CIN) values were 3411 (159) J/kg. Bulk shear (0 - 3 / 0 - 5 km) magnitudes were 17 (21) 17 (21) m s-1 respectively. The overall sounding suggested a strong potential for damaging winds.

The Lincoln Illinois (KILX) sounding at 0000 UTC, 25 May 2004 was characterized by a nearly dry-adiabatic lapse rate from the surface to 850 mb while a deep dry layer of air was noted above the subsidence inversion through 300 mb. The overall sounding suggested a strong potential for damaging winds. Bulk Shear (0 - 3 / 0 - 5 km) magnitudes ranged from 10 to 17 m s-1 respectively. The sounding was taken well north of the warm frontal boundary further revealed by surface-based CAPE(CIN)magnitudes of 426 (232) J Kg-1 respectively. 

Storm Prediction Center Mesoscale Analysis Fields for 0000 UTC 25 May 2004 

Fig. 5a Mesoscale analysis products from Storm Prediction Center (SPC) at 0000 UTC 25 May 2004.  (Far left) 0-6 km Shear Vector (kts); (Near left) 0-1 km Shear (kts); (Near right) 0-1 km Storm Relative Helicity  m2/s2 (SRH); (Far right) 0-3 km m2/s2 SRH. (Click on image for a larger image).




Fig. 5b. Mesoscale analysis products from Storm Prediction Center (SPC) at 0000 UTC 25 May 2004.  (Far left) ML CAPE / CIN (J/Kg);  (Near left) (SB CAPE / CIN (J/Kg); (Near right) MUCAPE (J/Kg); (Far right) 100 mb Mean Parcel LCL Height(m AGL). (Click on image for a larger image).

Deep Layer shear (0-6 km Shear Vector (kts) (Fig 5a) was present over much of Missouri with the highest magnitudes of 50 kts (25 m s-1) or greater from southeast Kansas through east-central Missoui suggesting the presence of a strong shear environment and supportive of supercell development.  0-1 km shear was also quite impressive and supportive for tornado occurrenceover mch of the northwest two-thirds of Missouri as values ranged from 25 to 30 kts. The axis of highest 0-1 km storm-relative helicity at 0000 UTC extended from central Iowa through east-central Kansas with magnitudes exceeding 250 m2 s-2 along this axis.  A similar pattern was also shown for the 0-3 km layer SRH with the axis extending from central Iowa through west-central Missouri and east-central Kansas. Magnitudes of 400 m2 s-2 and greater were common from south-central Iowa into west-central Illinois. 

To further show the degree of instability across the Mid-Mississippi Valley region Storm Prediction Center (SPC) objective mesoscale analyses output was utilized.  Fig 5b show the Mixed layer and Surface-based Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN).  The axis of greatest instability at 0000 UTC extended from near Kansas City through southern Illinois with the highest values exceeding 3000 J/Kg for ML CAPE and 4500 J/Kg for Surface-based CAPE.  Magnitudes of ML CIN were quite impressive with values increasing from 100 J/Kg north of St. Louis to 250 J/Kg over northeast Missouri through west central-Illinois including the Quincy Illinois area. Similar the Surface-based CIN values ranged from near zero over the St. Louis area to 150 J/Kg over the Quincy - Pittsfield Illinois over West-central Illinois. One could vision a deepening stable layer as one moves further north into far northeast Missouri through west-central Illinois.   On the far right in figure 4a, Mean Parcel LCL heights revealed heights well above  1100 m over much of northeast and east-central Missouri through west-central and southwest Illinois. Lower LCL height were noted in the vicinity of the supercell storms over northwest Missouri at this time.

Fig. 6. Wind profile from the Winchester Illinois (WCN) profiler between 1600 - 0400 UTC 24- 25 May 2004. 

Between 2300 and 0200 UTC, the Winchester profiler site (WCN) showed strong turning of the vertical wind shear within the lowest 2 km suggesting increasing potential for supercell growth and development. The magnitudes of Bulk Shear (0 -3 km) between 0000 to 0200 UTC, increased from 14 to 22 m s-1 while the value of 0 - 6 km shear remained nearly steady (25 / 24 m s-1) during the two hour period. The 0000 UTC magnitudes of (0 - 3 / 0 -5 km) suggested (moderate / strong) shear values respectively and strong shear within both layers at 0200 UTC. The potential for both supercell and severe convective line formation was high. Previous cases show that Moderate to High magnitudes of 0-3 / 0-5 km shear is a good predictor for severe squall line convection.



Fig. 7. Same as Fig. 1 except for 0200 UTC and WSR-88D data from KLSX (St. Louis) .  

The 0200 UTC surface analysis showed the surface low over northwest Missouri as the warm frontal boundary moved slowly northward into parts of northeast Missouri through south-central Illinois. The warm front was just south of the Quincy - Hannibal area at this time. The area of lower 70 degree dewpoints over southeast Missouri moved northward into parts of central and east-central Missouri and southwest Illinois at this time. RUC surface-based CAPEs continued to exceed 4000 J/Kg from central through parts of east-central Missouri at this time. WSR-88D reflectivity data from WFO St. Louis (KLSX) revealed two supercells aligned nearly east-west across Macon and Shelby counties in northeast Missouri (70 to 100 km west of Quincy UIN).  The eastern most supercell spawned a tornado just east of the town of Macon in Macon County at approximately 0145 - 0150 UTC.   

Mesoscale Analysis Fields for 0200 UTC 25 May 2004.

Fig. 8a: Same as Fig. 5a except for 0200 UTC. (Click on Image for a larger image). 


Fig. 8b. Same as Fig. 5b except for 0200 UTC. (Click on image for a larger image).

Axis of highest 0-1 km storm-relative helicity shifted further east and extended from south-central Iowa into north-central and northeast Missouri with magnitudes exceeding 600 m2 s-2 (Fig. 8a). A secondary axis developed over parts of northeast Missouri through south-central Illinois at this time in the vicinity of the warm frontal boundary.  This axis was not observed on the 0000 UTC field. Values exceedinig 200 m2 s-2 were now common over much of west-central Illinois. The deeper layer  (0 - 3 km) of  SR helicity showed higher magnitudes compared to the 0 - 1 km field where values exceeded 300 m2 s-2 over much of west-central Illinois and over 450 m2 s-2 over northeast Missouri. The supercell complex was located in the vicinity of the northwest - southeast axis of higher SR Helicity for both the  0 - 1 km / 0 - 3 km layers at 0200 UTC. On the far right the 100 mb Mean Parcel LCL heights at 0200 UTC showed that the heights dropped at or below 1000 m over all of northeast through parts of east-central Missouri. Heights dropped 250 or greater meters over the past two hours over this region. 100 mb MP LCL heights also fell over much of west-central Illinois between 1000 - 1250 m at 0200 UTC from heights greater than 1500 meters at 0000 UTC.  

Similar to the 0000 UTC SPC mesoanalysis output, the degree of Surface-based and ML CAPE at 0200 UTC continued to exceed 3000 J/Kg along an axis from east-central Kansas through east-central Missouri to the Indiana-Kentucky stateline (Fig. 8b).  Surface-based and ML CIN values across northern Missouri through west-central Illinois further increased while only a small area of CIN below 100 J/Kg extended from the St. Louis area and areas to the southeast. Over parts of northeast Missouri through west-central Illinois Surface-based CIN values ranged from 250 to 350 J/Kg were common over parts of northeast Missouri and west-central Illinois. The stable layer continued to deepened as one moved further north over this area. One of the key questions with this case focused on the degree of stability as storms moved across west-central Illinois. Could convective-scale downdrafts be able to penetrate the increasing low-level stable layer over parts of west-central Illinois? MUCAPE showed that the Most-Unstable parcels were slightly elevated over nearly all of northeast Missouri and west-central Illinois at or above 750 m while areas over the southern third of Missouri was nearly surface-based. The far right graphic in Fig. 7b shows the 100 mb Mean Parcel LCL height at or below 1000 m from north-central through east-central Missouri. It is interesting to note how the 100 mb Mean Parcel LCL height rose to over 1000 m over parts of central and east-central Illinois.  


Fig 9.  LAPS sounding for 0300 UTC 25 May 2004 at Pittsfield IL (PPQ - Left) and Beardstown (Right) .  (Click on image for a larger image).

At 0300 UTC the LAPS sounding at PPQ showed an increase in instability and modest increase in the magnitude of Bulk Shear compared to the KILX sounding at 0000 UTC. Magnitude of surface-based CAPE at this time was 1419 J Kg-1 (CIN  165 J/Kg) while 0-3 / 0-5 km Bulk Shear magnitudes were 17 and 25 m s-1 respectively. 

4. Storm evolution over northeast Missouri through west-central Illinois starting at 0259 UTC.

The following is a sequence of reflectivity and Doppler velocity images from KLSX


Fig 10. Planview reflectivity (left); storm-relative velocity (right) from KLSX (0.5° slice) at 0259 UTC

Plan-view reflectivity image at 0259 UTC shows a bow echo over Marion County Missouri entering western Adams County Illinois and the city of Qunicy Illinois (Fig. 10).  The two supercells identified earlier at  0200 UTC became part of a bowing structure.  The western most supercell of the supercell complex maintained it identity and was located near the southwest flank of this bow (similar to Type 3 cases - Przybylinski and DeCaire 1985).  This supercell broke away from the bow echo and moved north-northeast and eventually merged near the northern end of the upwind developing convective line segment over Shelby county after 0315 UTC. This convective line segment rapidly intensified between 0240 and 0255 UTC after small isolated cells merged with the segment.  The storm-relative velocity presentation at 0259 UTC showed several weak and shallow mesovortices along the leading edge of the leading bow while a larger mesovortex was observed near the northern end of the bow.  Damaging winds occurred over parts of southern Lewis, eastern half of Marion counties in Missouri and western Adams county (including the city of Quincy) in Illinois with the leading bow.  It is interesting to note that there was an absence of wind damage east of UIN as the first bow echo moved eastward across west-central Illinois.  Was the absence of wind damage due to higher magnitudes of CIN?  Remember CIN values east and north of UIN increased to 300 J/kg.  

MARC velocity signature is noted along the leading edge of the convective line segment over Shelby County at 0259 UTC. Delta-V magnitudes of MARC over southeastern Shelby county were 29 m s-1 (or 4 m s-1 above the 25 m s-1 threshold). The convective line segment and adjacent storms to the south evolved into a larger bowing line segment during the following 45 minutes. The MARC velocity signature continued to be identified along the leading edge of this line segment through 0407 UTC. The overall time-height trace of the MARC velocity signature is shown in Fig. 6. Some of the first wind damage reports were received over parts of northeast -central Monroe and Ralls counties in northeast Missouri after 0320 UTC. The trace showed that magnitudes of MARC were steadily increasing from 0259 through 0320 UTC preceding the first reports of wind damage. The viewing angle appeared to partially prohibit us from viewing the entire ribbon of MARC along the leading edge of the convective line segment. Viewing angle considerations should always be part of the equation when viewing line segments nearly normal to the radar beam. Additionally, warning forecasters should also be alert for the potential of damaging winds when HP storms evolve to convective line segments and bowing structures.

Fig. 11. Plot and time-height cross-section of the magnitudes of MARC associated with the
second HP storm - convective line segment. Time on the MARC trace begins at 0253 UTC
and ends at 0403 UTC. Magnitudes of MARC are in m s-1. (Click on image for a larger image).

Fig 12. Planview reflectivity (left) (0.5°) and storm-relative velocity (right) (1.5° slice)
at 0330 UTC from KLSX. (Click on image for a larger image).

Approximately thirty minutes later (0330 UTC) , the leading bow echo further bowed over eastern Adams County.  However, there were no reports of wind damage east of Quincy across central and eastern sections of Adams County Illinois.  Higher magnitudes of  Surface-based CIN over areas east and north of Quincy may have played a role in preventing convective-scale downdrafts from reaching the surface. The leading bow echo also appeared to lay a surface boundary which extended from southeast  Adams County through extreme northeast Ralls County Illinois.  The supercell originally located near the southwest of the first bow became detached between 0315 and 0330 UTC and eventually merged near the northern flank of the second larger bowing QLCS between 0336 and 0341 UTC.  The question raised here is what role did this supercell if any playif any after it merged with the northern end of the larger QLCS to the west of Hannibal MissouriMesovortex 2 (C2) appeared to form just after 0336 UTC over southeast Marion...far northeast Ralls county Missouri and northwest Pike county Illinois and revealed the characteristics of a large 'Line-end Vortex.'  The broad-scale vortex rapidly evolved after supercell - convective line merger (between 0336 and 0341 UTC) and caused wind damage along the southern periphery of the circulation from far southeast Marion County Missouri and northwest through north-central Pike County Illinois.  C2 traversed east-northeast over northern Pike...southern Adams and Brown counties in Illinois during the next 30 minutes (see Fig. 12).  Note how the strongest Rotational Velocity (Vr) values was identified from 0341 through 0407 UTC with the strongest rotation noted at or below 3 km from 0351 through 0407 UTC.  This is a common evolution found in Line-end Vortices were the strongest rotation often descending to the lower part of the circulation. Based on previous studies of Line-end vorticies, some of the strongest wind damage will occur when the strongest rotation descends to the lowest levels of the large mesovortex. 

The first of several mesovortices associated with the second bowing segment (mesovortex C1) was also noted at 0330 UTC near the intersection of the broken convection extending eastward and the developing bow.  C1 briefly showed non-descending characteristics at 0325 and 0330 UTC, however vortex depth was relatively deep extending to a height approaching 9.0 km.  Magnitudes of Vr were quite strong during its brief lifespan with values equalling or slightly exceeding 20 m s-1 throughout the entire depth of the mesovortex at 0330 UTC. No wind damage was reported with C1.  The following sequence of images shown below from 0351 through 0428 UTC (every ten minutes) reveals the evolution of the second bow echo and growth of mesovortices across eastern Ralls County Missouri through Pike and Scott Counties in Illinois.


Fig. 13. Plan-view reflectivity (left)( 0.5°slice) and storm-relative velocity (right)(0.5° slice)
at 0351 UTC from KLSX.
(Click on image for a larger image).

At 0351 UTC (Fig. 13) the leading bow echo became further diffused as it moved east-northeast into Schuyler and Brown Counties in west-central Illinois.  However, another supercell was identified along the southern end of the weakening bowing segment.  The mesovortex with this supercell briefly intensified for one volume scan at 0351 UTC over northeast Pike County Illinois then weakened.  There were no reports of severe weather with this storm.  A new mesovortex (Mesovortex 3 - C3)  formed at 0341 UTC north of the apex of the upwind bowing segment and near the intersection of the surface boundary in extreme northern Pike County Missouri.  C3 is located in the vicinity of the weak echo notch north of the surging 45-50 dBZ echo in northern Pike county. This mesovortex moved over the small community of New London MO and caused wind damage in town and a brief tornado touchdown east of New London near the Pike - Ralls county line. Burlington Northern Railroad employees reported a tornado crossing the tracks 5 miles southeast of the town of Saverton MO in extreme eastern Ralls county. Several trees were downed by the tornado.

Fig. 14. Plan view reflectivity (left) 0.5°  and storm-relative velocity  (right) (0.5° slice)
for 0402 UTC from KLSX (Click on image for a larger image)

After 0355 UTC the bow echo moved into western Pike county Illinois and central Pike county Missouri. The supercell over eastern Brown county Illinois appeared to continue to lay a surface outflow boundary extending southwest and intersecting the northern part of the larger mature bow over west-central Pike county Illinois.  At 0402 UTC (Fig. 14), the leading edge of the low-level reflectivity pattern surged eastward (downshear) from the taller convective towers associated with the bowing convective line. One would expect a large and continuous swath of damaging winds in the vicinity of the surge. However, such was not the case. Mesovortex #3 and a new mesovortex (C4 located south of C3) traversed eastward between Canton and Atlas Illinois laying two discrete swaths of tree damage (See Fig. 19).  Within the northern damage swath (C3's path), some of the tree damage was caused by a weak tornado (F0 intensity) crossing Illinois Highway 94 around 0400 UTC.  A second swath of large tree limbs and tree damage blocked Highway 94 and occurred over the northern part of Rockport IIllnois. Some homes were damaged by the fallen trees and large limbs.  This swath of tree damage was associated with C4's path.   Between the two damage swaths, no tree or limb damage was identified during the damage assessment.  Damage widths varied from 50 to 150 yards. These observations are similar to the findings of a damage assessment completed by Atkins and Przybylinski of the June 10, 2003 St. Louis bow echo event where discrete swaths of tree damage with embedded tornadic damage was uncovered over parts of southwest Illinois (northern St. Clair County Illinois). These discrete swaths were associated with the tracks of the individual mesovortices. (See the June 10 2003 Bow Echo webpage or the publication page - Extended AMS abstracts 22nd Conference on Severe Local Storms - June 10, 2003 St. Louis Bow Echo storm). At 0357 UTC,  aliased velocities observed at the 0.5 and 1.5 degree slices prohibited the warning forecaster from observing the location of C3 and C4.  Additionally, due to the small core diameter of C4 and the distance between this vortex and the WSR-88D at WFO St. Louis, the warning forecaster experienced difficulty in identifying C4 during its early stages (beam width sampling / aspect ratio issues).

The first of three tornadoes associated with C4 occurred between 0402 and 0407 UTC.  The first tornado touchdown occurred approximately 0.50 mile south of New Hartford Illinois and crossed U.S Highway 54.  Numerous trees were snapped quarter to half way up while other large limbs were tossed several hundred yards downwind to the east.  The damage with this tornado was rated F(1) on the traditional Fujita-scale. Other large limbs and some trees were down on a few homes and house trailers in the New Hartford area.  C3's damage path ended northwest of Pittsfield IL while C4's damage path continued through the southwest and southern parts of Pittsfield Illinois.  Several large trees and numerous large limbs were down across the southern part of town damaging a number of homes.  The width of the damage path varied between 50 to 100 yards.  Surprising, tree damage was absent from the southern part of C4's damage path to 10 - 12 miles south of Pittsfield Illinois (southern Pike County Illinois).  A new mesovortex (C5) formed just south of C4's location at 0412 UTC near the apex of the bow. This vortex rapidly intensified during the subsequent volume scans. 


Fig. 15. Plan view reflectivity (left - 0.5°) and storm-relative velocity (right - 0.5° slice)
at 0412 UTC from KLSX. (Click on image for a larger image)

The second supercell northeast of the bow echo appeared to lay a surface boundary intersecting the northern part of the larger bow near the vicinity of C3 and C4 at 0412 UTC (Fig. 15).  The leading edge of the bow echo's outflow showed slight weakening in the reflectivity field (reflectivity surge) while the tall convective towers with the larger bowing convective line also weakened over central Pike County IL. Concurrently, two Rear Inflow Notches (RINS) were identified along the convective line's trailing flank. The further weakening of the reflectivity field along the leading edge and demise of the tall convective towers signified that the system cold pool overwhelmed the ambient shear.   Also note the strong low-level reflectivity gradient at 0412 UTC (15 to 50 dBZ change) near the apex of the bow.  Past observational studies and numerical simulations has shown that such reflectivity gradients are suggestive of strong gradients of potential temperatures, vertical velocities, leading edge of the cold pool and mesovortex growth.  Atkins and Arnott (2004) have shown that a mesovortex can rapidly intensify just behind the leading edge of the cold pool within strong gradients of potential temperature and enhanced vertical velocities.


Fig. 16. Plan-view reflectivity (left - 0.5°); storm-relative velocity (right 0.5°)
at 0423 UTC from KLSX (Click on image for a larger image).

At 0423 UTC the accelerated bowing line nearly caught up with the downshear supercell (SC) over Cass County Illinois. New convective elements developed a second time (multicell evolution) along the leading edge of the accelerated part of the bowing segment over extreme eastern Pike and western Scott Counties in Illinois. However, these new cells were short-lived and did not become mature convective towers due to the strength of the system cold pool.  The comma-head reflectivity structure over the northern part of the bow reflects the location of the weakening C2 line-end vortex.   The surface boundary near the southwest flank of the supercell (SC) appeared to intersect the weak reflectivity notch over west-central Scott County Illinois. Just south of the notch strong low-level reflectivity gradients along the leading edge of the bowing segment suggested a ribbon of very intense updrafts.  Taller convective towers south of the apex along the southern part of the larger bow further  weakened at this time from 0412 UTC and traversed rearward suggesting a strong cold pool overwhelming the ambient shear and continued potential for damaging winds along this part of the line. 

SRM velocity data continued to show C4 (in the vicinity of the notch) and C5 (just north of the apex of the bowing segment) over western Scott County Illinois.  C4 continued to lay a continuous discrete path of tree and structural damage 100 to 200 yards wide, east to slightly northeast of Pittsfield IL along Illinois Highway 100/106 through the small community of Detroit Il.  The second tornado (F1 damage) caused tree and structural damage 5 to 6 miles east-northeast of Pittsfield or 1.0 to 0.5 miles northwest of Detroit IL.  The tornado snapped or twisted several large trees and threw large branches 1/4 to 1/2 mile to the east in an open field (north of Detroit). Striking, the wind damage area ended abruptly 40 yards south of Detroit IL.  In the small community of Detroit, one garage was damaged while several small sheds were destroyed.  Further south in extreme eastern Pike...southwest Scott county, C5 further deepened, intensified and showed classic non-descending mesovortex characteristics (See Fig. 17 ). At 0417 UTC aliased velocities were noted at the 0.5 degree elevation slice near the town of Milton in extreme eastern Pike county Illinois. C5 produced a swath of wind and tornadic damage from extreme eastern Pike through central Scott County. The tornadic damage (F1) track extended from  6 miles WSW of Winchester through the town of Winchester. Two miles southwest of Winchester, the tornado destroyed a mobile home killing one person and injuring a second. Several homes in town sustained varying degrees of roof damage and a number of sheds were destroyed. Several large trees were also damaged by the tornado.  



Fig. 17. Composite of mesovortex tracks and squall line positions every 30 minutes between 0300 - 0500 UTC.
(Click on image for a larger image) 


Fig.18. Time-height Rotational Velocity (Vr) traces of mesovortices C2 Line-end vortex (far left); C3 (left center); C4 (right center) and C5 (far right).  Magnitude of Vr are in m s-1.  Height (y axis) in km, time in UTC (x axis).  (Click on image for a larger image).

Fig. 19. Time-height traces of the mesovortex core diameters for Line-End Vortex C2 (far left); C3 (left center); C4 (right center); and C5 (far right). Mesovortex core diameters are in km (y axis), time in UTC (x axis). Each contour represents a elevation slice. (Click on image for a larger image).  

Mesovortex 2 - is characterized as a strong mesovortex having a relatively large core diameter. The strongest rotation remains in the lowest 3 km throughout much of the circulation's lifetime with the strongest magnitudes at 0351 UTC (26 m s-1). Wind damage occurred along the southern periphery of C2 over extreme northwest Pike County Illinois (in the town of Hull, Illinois - northwest Pike County Illinois) around 0355 UTC. This large mesovortex weakened after 0415 UTC.

Mesovortex 3 - This mesovortex rapidly developed after 0340 UTC and formed near the surface boundary - convective line intersection (northeast Ralls into west-central Pike County Illinois. Vr magnitudes of 20 m s-1 or greater was noted through 4 km during the very early stages of this circulation lifetime. C3 deepened and slightly intensified at 0351 UTC with Vr magnitudes of 20 m s-1 at 3.5 km.  A weak tornado crossed the Burlington Northern railroad line 5 miles southeast of Saverton Missouri (far northeast Ralls County) at approximately 0353 UTC.  Aliased velocities within the lowest two elevations slices (0.5° and 1.5°) prohibited velocity data analysis at 0357 UTC. However, on the following volume scan (0402 UTC) C3 continued to show a relatively deep mesovortex (5 km deep) with the strongest rotation (Vr - 20 m s-1) detected at the lowest elevation slice. A second weak tornado occurred at this time crossing Illinois Highway 96 - four miles southeast of Canton Illinois. Numerous trees were snapped half way up or uprooted within a 75 yard wide area. The corn fields west of Highway 96 also showed a convergent damage pattern. Due to the mesovortex distance from the WSR-88D and core diameter of C3, beam width sampling -aspect ratio problems were observed at times with C3.       


Mesovortex 4 - This mesovortex formed over extreme northern Pike County Missouri near or just north of the apex of the accelerated bowing segment embedded within the larger convective line.  C4 formed at 0351 UTC south of C3) between the 2 and 4 km layer and rapidly deepened by the next volume scan (0357 UTC) (third elevation slice - 20 m s-1). Aliased velocities obscured the lowest two elevations slices at 0357 UTC. The first of three tornadoes associated with C4 occurred between 0402 - 0407 UTC approximately 1/2 mile south of New Hartford Illinois. The location and growth of this mesovortex is similar to numerical simulation results shown by Atkins and St. Laurent Part 1 (2009 MWR)  where the vortex lines are oriented to the south.  Localized updrafts tilt the initially horizontal cold-pool vortex lines upward  creating an arches of vortex lines centered on the updraft. Thus cyclonic (anticyclonic) vertical vorticity was produced on the northern (southern) sides of the updraft respectively.  Remember, the mesovortex forms just behind the leading gust front in the region of strong theta-gradients. Doppler velocity data shows that C4 formed just north of the apex of the bow echo. C4 slightly weakened and became shallow by 0407 UTC.  At 0412 UTC C4 deepened and intensified a second time with the strongest low-level rotation noted within the lowest 2 km (Vr magnitude of 21 m s-1). The TVS signature associated with C4 tripped at 0412 UTC. The strength of rotation is also observed with NSSL's Warning Decision Support System (WDSSii) product of Azmuithal Shear (See Fig. 20).  A second tornado touched down 4-5 miles east-northeast of Pittsfield Illinois or 1.0 - 0.2 miles northwest of Detroit Illinois between 0412 and 0417 UTC. C4 deepened a third time to a height of nearly 8km at 0428 UTC with the strongest rotation still within the lowest 2.5 km. Similar to C3, beam width sampling - aspect ratio issues were also noted with C4 throughout much of its lifetime mainly due to C4's small core diameter. Such sampling problems will result in a degradation of peak radial velocity measurements (Burgess 1986). C4 continued to remain a relatively deep mesovortex through 0438 UTC with the strongest Vr magnitudes noted within the lowest 4 km. A third tornado touched down over southwest parts of Jacksonville Illinois and caused structural and tree damage ranging from F1 - F2 intensity.    

Mesovortex 5also formed near or just north of the apex of the bowing segment over central Pike county Illinois at 0412 UTC. C5 formed within the lowest 3 km revealing the strongest rotation noted within the lowest 1 km of the mesovortex (Vr  at 0.5° slice of 22 m s-1). The location and growth of C5 is also similar to numerical simulation results shown by Trapp and Weisman (2003). This mesovortex rapidly intensified and slightly deepened during the subsequent two volume scans. The strongest rotation was confined to the lowest 2 km though 0428 UTC. Delta-V magnitudes equalled or exceeded 38 m s-1 at 0423 and 0428 UTC while the mesovortex deepened to a height of 5 km.  C5 spawned a tornado at 0423 UTC and caused F1 damage from 6 miles WSW through the town of Winchester Illinois. Note that vortex deepening and low-level intensification occurred prior to tornado touchdown. However tornado touchdown occurred 10 minutes after initial mesovortex identification suggesting a relatively short lead time. This is very challenging to the warning forecaster. We found similar limited lead times with the June 10, 2003 tornadic bow echo case during the BAMEX project.   


Fig. 20. (a) left - Overall damage map across northeast - east central Missouri through parts of west-central and southwest Illinois. (b) right - detail mapping of the downburst winds swaths and tornado damage tracks over Pike County Illinois (from damage assessment).    


The overall and detailed damage maps show that much of the tree and structural damage occurred near and north of the apex of the bow echo mainly along the tracks of each of the mesovortices from eastern Ralls County Missouri through central Pike County Illinois. The width of the tree and structural damage varied from as little as 75 yards to as large as 1/4 mile wide in some areas. Wind damage was nearly absent south of the mesovortex tracks across much of the southern half of Pike County Illinois. Except for the damage along the mesovortex tracks, the stable layer (ML CIN) over parts Pike County Illinois appeared to prevent the stronger convective downdrafts from reaching the surface over this area.  Wind damage over the Hannibal Missouri area - northwest part of Pike County Illinois appeared to be associated with the larger Bookend Vortex (C2).  Isolated to scattered wind damage reports occurred south of this area with the larger bow structure eastern Missouri and parts of southwest Illinois. 








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