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2. SYNOPTIC ENVIRONMENT

The 1200 UTC (hereafter all times UTC) 11 February 1999 synoptic environment was characterized by strong dynamic forcing, similar to that described by Johns (1993).  The convective line was oriented parallel and downwind from a strong upper-level jet core at 300 mb exiting the base of a deep level trough which extended from the western Dakotas through eastern New Mexico (not shown).  At the same time, a 25 m s-1 low-level jet transported unseasonably unstable air along and ahead of the convective line with 850 mb dewpoints reaching 12°C across western Missouri.  The 1800 UTC surface analysis showed a strong cold front extending from an area of low pressure over southwest Wisconsin through central Missouri and southwest into eastern Texas.  Surface temperatures fell as much as 15°C immediately behind the cold front.  A weakly unstable and strongly sheared environment existed across the Mid-Mississippi Valley region.  Sounding data at 1200 UTC from Springfield, Missouri (KSGF) just ahead of the convective line showed a lifted index of -2 and a CAPE of 438 J Kg-1 (Fig. 1).  The sounding's vertical wind shear profile revealed bulk shear values of 20 m s-1 within the lowest 0-2 and 0-3 km layer.  Weisman (1993) has shown that high ambient low-level shear maintains long-lived squall lines and embedded bowing segments within the line. 

Fig. 1. Skew-T Log-P sounding from Springfield Missouri (KSGF) at 1200 UTC 11 February 1999.



3. WSR-88D OBSERVATIONS

Recent work completed by Parker and Johnson (2000) surveyed 88 linear MCSs from May 1996 and May 1997 across the central U.S.  They were able to classify this group of MCSs into three categories: a) trailing stratiform (TS), b) leading stratiform, and c) parallel stratiform (PS).  Of their total sample, 58% of the cases fell into the category of leading line - trailing stratiform events (TS), while leading stratiform (LS) and parallel stratiform (PS) cases each composed 19% of the events.  The February 11, 1999 linear MCS can be classified as a leading stratiform (LS) events where the stratiform rain region extended as much as 60 - 80 km downshear (northeast) from the line of intense convective cells (Fig. 2). 

Fig. 2. Feb 11, 2002; 1900 UTC WSR-88D planview reflectivity image (0.5° slice)(left); storm-relative
velocity (right) from KLSX (WFO St. Louis).

From figure 2, reflectivity imagery at 1900 UTC (hereafter all times UTC) from WFO St. Louis (KLSX) suggested the presence of several convective cells embedded within two large storm complexes 110 - 130 km northwest (A), and 90 km west (B) of St. Louis.  A third storm complex (C) was noted between 90 and 130 km southwest of St. Louis.  The overall reflectivity pattern with storm B suggested the presence of a hybrid supercellular structure with a sprial reflectivity structure within the southern flank of this storm.  Weak rotation was identified at this location with magnitudes of rotational velocites (Vr) only reaching 9 m s-1 (1.5° slice).  Rotation identified within the high reflectivity core region of storm A (top white arrow - SRM) was slightly stronger with values reaching 12 m s-1 (0.5° slice).  A third center of weak rotation was detected with the small comma-shaped echo embedded within the southern part of storm A (northeast Audrain county).  Magnitudes of Vr however, were again weak with values only 10 m s-1 (0.5° slice).  Even though, weak magnitudes of Vr were detected at this time, the relatively high helicity values noted within the lowest 0-2 and 0-3 km between 1900 and 1930 UTC from KLSX WSR-88D Velocity Wind Profile (VWP) suggested the potential for strong mesocyclone development (Fig. 6).

Fig. 3. Same as Fig. 2 except for 1916 UTC.

At 1916 UTC both storms A and B continued to be comprised of several convective cells, suggesting multicellular traits with each complex (Fig. 3). Storm C showed a similar type pattern with a small comma-shaped echo at the northern end of the system and a larger bowing complex over the southern part.  Several weak and transcient circulations formed and weakened within storms A and B between 1900 and 1916.  However, the northern most vortex C1 became a persistent feature through this period.  Radial convergent velocity signatures were observed at two areas along the leading edge of Storm B at 80 km (southeast Audrain county) and 76 km (central Montgomery county) from KLSX radar.  These velocity signatures were shallow and only detected at the lowest elevation slice at altitudes of 1.2 km  and 1.0 km respectively.  The character and depth of this zone of radial convergence is in contrast to studies completed by Lemon and Parker (1998) and Schmocker et al. (1996) where a deep convergence zone (DCZ) was documented along the leading edge of supercells or Mid-Altitude Radial Convergence (MARC) observed along the forward side of quasi-linear convective lines respectively.

Fig. 4. Same as Figure 2 except for 1931 UTC.

Approximately 15 minutes later (1931 UTC) the nearly solid convective line, extended from 140 km north to 65 km northwest of KLSX (Fig. 4).  The strongest storms (A; B) continued to remain at the far ends of the convective line while several convective cells were embedded across the central part of the line.   The stratiform rain region significantly increased in areal coverage and intensity downstream from the northern part of the convective line.  However, little change was noted further south.  Circulation C1, located within the comma-head of Storm A  continued to show persistent weak rotation with Vr magnitudes near 8 m s-1.  This vortex was responsible for spawning a brief tornado at 1936 UTC over far southeast Adams county Illinois (30 km southeast of Quincy IL). Transient vortices continued across the central part of the line, exhibiting broad and weak rotation between 1916 and 1931.  No reports of severe weather were received over this part of the line since 1901 UTC.  Storm C showed signs of weakening at 1931 compared to the 1916 reflectivity imagery.  The small comma-shaped echo earlier identified near the northern part of this storm complex evolved through its final stages.  Scattered reports of wind damage were received between 1901 and 1931 along Storm C's path.  The overall reflectivity pattern of Storm B suggested the presence of a High-Precipitation (HP) supercell with a weak reflectivity notch and strong low-level reflectivity gradient along the storm's forward flank.  These reflectivity features revealed the location of the storm's updraft region.  However, Doppler velocity data from KLSX continued to reveal a low-level convergent velocity signature (white arrow) but the absence of a well defined circulation in the vicinity of the storm's forward flank.

Fig 5. WSR-88D Velocity Wind Profile (VWP) (left) from KLSX for the period 1916 to 2002 UTC. Hodograph and Storm-Relative Helicity (SRH) data for 1926 UTC (right) from WATADS.     

Storm B and other convective storms within the quasi-linear convective line across the Mid-Mississippi Valley region maintained their  presence within a strongly sheared environment across the Mid-Mississippi Valley region.  The vertical wind shear profile shown in figure 5 depicts a broad curved hodograph-type structure, similiar to numerical simulations shown by Weisman and Klemp (1986) of a comma-bow echo type pattern (Case F).  Magnitudes of storm-relative helicity (SRH) from KLSX Velocity Wind Profile (VWP) at 1926 within the lowest 0-1 / 0-2 / 0-3 km were 175, 352, and 386 m2 s-2 suggesting strong helical flow and a high potential for tornadic activity over the region (Fig. 5).  These magnitudes varied little during the subsequent fifteen minutes as the convective line moved northeast of KLSX.  



3.1  WSR-88D OBSERVATIONS OF STORM B

One persistent cyclonic circulation was identified with the far northern storm (Storm A), while a total of five cyclonic circulations were
documented with Storm B as it passed north of KLSX .  The tracks of each of these circulations are shown in figure 6.   Circulation (C2) embedded within the northern part of Storm B exhibited the longest lifetime of the group.  Circulations (C3 and C4) formed near the apex of storm B's southern small bowing segment (or leading edge of the HP storm's Rear Flank Downdraft RFD) with Circulation 4 spawning several tornadoes (F0 - F2 damage intensity) along its path.  Two other circulations (C5 and C6) were identified during and after the storm's most intense phase with C5 spawning a weak tornado (F1 intensity).  However their lifetimes were short lived.   Storm B's evolution and associated circulations are discussed in the following sections.

Fig. 6.  Circulation tracks identified with Storm B.  Black symbols along each of the tracks represent 5 min interval circulation positions with the beginning time (in UTC) indicated.


a)  Storm B's early phase 1931 - 1947

The proximity of Storm B to KLSX radar was very ideal to survey the storm's evolutionary reflectivity and Doppler velocity  characteristics during the period of 1931 through 2017.  Between 1931 and 1942, Storm B intensified quickly with mid-level rotation (Circulation 2 - C2) meeting mesocyclone criteria within the storm's high reflectivity core region (Fig. 7a and 7b).  Storm B's reflectivity structure evolved into an 'S" shaped pattern with C2 embedded within a 55-61 dBZ ring echo.  The ring echo signified a Bounded Weak Echo Region (BWER).  C2 evolved from a cyclonic convergent (1931) to a nearly symmetrical vortex structure by 1942 (1.5° slice) at 1.8 km.  A low-level reflectivity notch (0.5° slice) capped by 55 dBZ reflectivities aloft along the storm's forward flank signified the location of the storm's updraft region or (WER).  This low-level notch structure was only apparent at one time (1942).   A small isolated convective cell (IC) 5 km downstream from Storm B moved northeast at nearly the same speed of Storm B.  This cell maintained its identity through 1952.  Strong gate-to-gate shears detected within C2 at 1.8 km (1.5° slice - 1942) suggested an elevated tornadic vortex signature (ETVS).  Delta V values reached  45 m s-1 at this slice.  This intense vortex was short-lived and only detected at 1942.  Further south, a second small-scale vortex (C3) formed at 1931 at the leading edge of   Storm B's southern flank (or leading edge of Storm B's RFD).  At first, C3 exhibited gate-to-gate (delta-V) magnitudes of 11 - 15 m s-1. However this small-scale vortex briefly intensified at 1942 and revealed delta-V magnitudes of 35 m s-1 at the lowest elevation slice before weakening again at 1947.  A tornado briefly touched down at 1942 over north-central Lincoln County Missouri and caused F0 damage. C3 was a shallow vortex throughout its lifetime with an overall depth of less than 2 km.  The location of this vortex and C4 along the leading edge of a small bowing segment is similar to observations shown by Bluestein et al. 2002.       

Fig. 7. Four panel presentation of planview reflectivity / storm-relative velocity for 1931 UTC (left) and 1942 UTC (right) of Storm B. Top (bottom) panels are reflectivity / storm-relative velocity images at 1.5° (0.5°) slices respectively.

b) Mature Phase 1952 - 2007 UTC

Storm B's southern flank began to accelerate and showed a pronounced small bow echo and rear inflow notch (RIN) by 1952 UTC  (Fig 8).  The isolated cell (IC) continued to persist immediately downwind of the bowing segment.  However, the emergence of 35 to 40 dBZ echo between these features suggested the presence of enhanced updrafts.  Multiple reflectivity notches along the storm's northern forward flank identified at this time continued to indicate a persistent inflow region across this area.  C2 intensified and deepened between 1942 and 1952.  Vr magnitudes at 0.5° (1.5°) slice increased from 15 (15) m/s to 16 (18) m/s while the core diameter dropped from 7.6 to 5.5 km at the lowest slice (See Fig. 12).  However, no tornadic activity was reported with this vortex.  A new small-scale vortex (C4) exhibiting a gate-to-gate couplet within the lowest two slices, formed immediately north of the apex of Storm B's  accelerated southern bowing segment in the region of the 35 to 40 dBZ echo.  Magnitudes of delta-V exceeded 28 m/s at the lowest two slices with a core diameter of less than 1 km.  C4 spawned a weak tornado just after 1952 and caused F1 damage over north-central Lincoln county.  

Fig. 8.  February 11 1999; 1952 UTC WSR-88D plan view reflectivity (0.5° slice)( left); storm-relative velocity (right) from KLSX (WFO St. Louis)

C2 further intensified and slightly deepened while the core diameter dropped between 1952 and 1957 UTC (Fig. 9).  The greatest increase in rotation occurred at the 1.5/2.4° (2.3 km / 3.4 km) slices where magnitudes of Vr rose from 18 to 22 / 13 to 18 m/s respectively (see Fig. 12).   The 55 dBZ reflectivity evolved into a 'comma-shaped' like echo pattern during this period adding credence to the intensifying vortex.  The weaker reflectivity identified between IC and the small bowing segment at the 0.5° slice near C4 at 1952 significantly changed at 1957 as a merger between these features occurred resulting in a region of 55 dbZ echo at 0.5°/1.5° slices.  The higher reflectivities reflected a stronger cold pool over this part of the storm.   Numerical simulations by Finley et al. 2001 and observations shown by Lee et al. 1992a showed that the cold pool strengthens after cell mergers and may result in the development of a bow echo.  The depth of C4 dropped from 2.7 to 1.8 km, however a gate-to-gate couplet was still present at the lowest slice with delta-V magnitudes remaining strong (30 m/s) (See Fig. 13).   The sudden drop in vortex depth may have resulted from the rapid increase in mass during merger where the vortex was likely disrupted.  Tornadic damage with C4 intensified from F0/F1 to F2 at 1957 UTC during the period of merger.       

Fig. 9. Same as Fig. 6 except for 1952 (left); 1957 (right).

At 2003 plan view reflectivity images at 0.5° and 1.5° slices showed new convective cells immediately downshear (northeast) of Storm B with the highest reflectivities recorded aloft (1.5° slice) suggesting strong convective updrafts and multicell evolution (Fig. 10).  Storm B's  comma-head broadened and a RIN was evident by 2007 along the trailing flank as the echo pattern became more influenced by the increased core diameter of C2.   C2 reached its strongest rotation at 2003 at 0.5°/1.5° elevation slices with Vr magnitudes of 20 and 22 m s-1 respectively.  A smaller and shallow vortex (C5) formed within C2 and spawned a weak tornado (F1 damage) just after 2003 over northern Calhoun county.  The damage path from this tornado was less than 2 km.  C4 located just north of the expanded region of 50 dBZ echo revealed tornadocyclone characteristics as the vortex deepened and magnitudes of gate-to-gate shears significantly increased to 55 m s-1 (0.5°slice) (see Fig 11).  The early stages of this circulation appeared to show similar characteristics to observations shown by Trapp et al. 1999 where the small but intense circulation showed non-descending traits.   A nearly continuous path of tornadic damage rated F1-F2 intensity occurred just after 2003 across parts of northern Calhoun county.  Between 2003 and 2007, C4 weakened considerably and was absorbed by the larger vortex C2.  At 2007 the small-scale vortex was located located under the southern part of the larger vortex C2.   However, weak tornadoes (F0 damage intensity) continued with this vortex as it moved into southeast Pike county Illinois.   

Fig. 10. Same as Fig. 6 except for 2003 UTC (left) and 2007 UTC (right). 

After 2012 UTC. Storm B exhibited a classic comma-shaped echo (Fig. 11) as it progressed across southeast Pike and Scott counties in west-central Illinois.  C2 revealing a broad but relatively strong core appeared to play a major role in redistributing Storm B's overall precipitation field resulting in comma-shaped pattern.  The strongest rotation was confined to the lowest 3 km (lowest two elevation slices) where Vr magnitudes ranged between 18-19 m s-1 (see Fig. 12).  C4 was located in the vicinity of the tip of the comma-head and within the larger circulation C2.  The strongest rotation was confined to the lowest 1.5 km where Vr magnitudes reached 17 m s-1 at the 0.5° slice (see Fig. 13).  C4 continued to spawn weak torandoes causing F0 - F1 damage over parts of southeast Pike County Illinois.  C4 weakened after 2017 while a new circulation (C6) formed over southern Scott county Illinois and spawned additional weak tornadoes over central parts of this county.  The comma-shaped pattern and location of tornadic circulation C4 is similar to observations documented by Pence et al. 1998 of the Montogomery Alabama tornado and Przybylinski 1988.   A map of the tornadic damage paths associated with Storm B are shown in figure 14.  Many of these damage tracks are coincident with C4's path across Lincoln, Pike counties in Illinois and Calhoun and Pike counties in Illinois.  

Fig. 11. Same as Fig. 8 except for 2012 UTC 11 Feburary 1999.

Fig. 12.  Rotational velocity (Vr) time-height trace for Circulation 2. Magnitudes of Vr are in m s-1.  (  ) represent the identification of gate-to-gate shears.  Time is represented in UTC while height is shown in km.

Fig. 13.  Same as Fig. 12 except for Circulation 4.

Fig. 14.  Map of tornadic damage paths associated with Storm B.


4. SUMMARY
  
A detailed study of the 11 February 1999 tornadic event over eastern Missouri showed that the storms near the ends of a quasi-linear convective system spawned tornadoes and scattered wind damage while there was an absence of severe weather across the central part of the line.  The northern storm (Storm A) spawned one weak tornado while the southern storm (Storm B) spawned a series of tornadoes acoss parts of east-central Missouri through southwest and west-central Illinois.  Velocity Wind Profile (VWP) data from the WSR-88D at KLSX was important diagnostic tool in revealing the magnitude of SRH and potential for tornadogenesis.  An isolated multicell cluster of storms 20 km south of the quasi-linear convective line produced isolated to scattered wind damage.  During the early stages of Storm B's lifecycle, radial convergence was observed along the system's forward flank.  The storm evolved into an poorly defined "S" shaped structure (hybrid HP storm structure) at its midpoint, then gradually became a well-defined comma-shaped reflectivity pattern.  Five cyclonic circulations were identified with Storm B with three of the five circulations becoming tornadic.   Circulation 2 originated from mid-level beginnings persisted near the storm's northern end.  The vortex initially revealed moderate Vr values (broad diameter), quickly strengthen (smaller core diameter), then remained strong but acquired a broad diameter greater than 15 km.  A weak tornado occurred during the period of strongest rotation and smallest core diameter.  Circulation 3 originated near the leading edge of a storm B's southern flank (leading edge of the RFD).  C4 also formed along the leading edge of a small bowing segment (RFD) and in the vicinity of an isolated cell-line merger.  This small-scale vortex briefly weakened during the period of expanded 50-55 dBZ echo (just after merger), then rapidly deepened and intensified suggesting non-descending characteristics.   Tornadic damage, rated F2, occurred just after this period of rapid deepening and intensification.  Overall circulation characteristics of C4 was one simliar to a tornadocyclone where the core diameter was consistently less than 1 km throughout much of its lifetime.  C4 was responsible for producing a series of tornadoes from its very early stages through nearly the end of its lifetime. 

The rapid movement combined with the complicated evolution and interactions Storm B and neighboring cells is a classic case that will continue to challenge forecaster's skills to issue timely and accurate warnings.  Future work will continue on circulation evolution with both warm and cool season MCS events across the Mid-Mississippi Valley region. 

5. ACKNOWLEDGEMENTS

The authors are grateful to Mr. Steven Thomas (MIC) National Weather Service St. Louis for his support in this research.   Additional thanks goes to Arno Perlow for assisting in two figures.  


6. References

Bluestein, H.B., C.C. Weiss, A.L. Pazmany, Wen-Chau Lee and M. Bell: 2002: Tornadogenesis and tornado-vortex structure in a supercell.  Preprints: 21st Conf. on Severe Local Storms, San Antonio, TX. Amer. Meteor. Soc. 461-464. 

Finley, C.A., W.R. Cotton, and R.A. Pielke Sr., 2001: Numerical simulation of tornadogenesis in a High-Precipitation Supercell. Part 1: Storm evolution and transition into a bow echo. J. Atmos. Sci., 58, 1597-1629

Johns, 1993: Meteorological conditions associated with bow echo development in convective storms.  Wea Forecasting, 8, 294-299.

Lee L.G. and W.A. Jones, 1998: Characteristics of WSR-88D velocity and reflectivity patterns associated with a cold season non-supercell tornado in upstate South Carolina.   Preprints: 19th Conf. on Severe Local Storms, Minneapolis, MN.  Amer. Meteor. Soc. 151-154.

Lemon, L.R. and S. Parker, 1996: The Lohoma storm Deep Convergence Zone: Its characteristics and role in storm dynamics and severity.  Preprints: 18th Conf. on Severe Local Storms.  San Francisco, CA. Amer. Meteor. Soc. 70-75.

McAvoy, B.P., W.A. Jones, P.D. Moore:, 2000: Investigation of an unusal storm structure associated with weak to occasionally strong tornadoes over the eastern United States.   Preprints, 20th Conf. on Severe Local Storms,  Orlando, FL.   Amer. Meteor. Soc. 182-185.

Moller, A.R., C.A. Doswell III, and R.W. Przybylinski, 1990:  High-Precipitation Supercells: A conceptual model and documentation. Preprints, 16th Conf. on Severe Local Storms. Kananaskis Park, Canada, Amer. Meteor. Soc. 52-57. 

Pence, K.J., J.T. Bradshaw, and M.W. Rose, 1998: The Central Alabama tornadoes of 6 March 1996.  Preprints, 19th Conf. on Severe Local Storms,  Minneaspolis MN.  Amer. Meteor. Soc.  147-150.

Przybylinski, R.W., 1998: Radar signatures associated with the 10 March 1986 tornado outbreak over central Indiana.  Preprints, 15th Conf. on Severe Local Storms.   Baltimore, MD.  Amer. Meteor. Soc.  253-265.

____________, S. Runnels, P. Spoden and S. Summy, 1990:  The Allendale Illinois Tornado, 7 January 1989: One type of a High-Precipitation Supercell.  Preprints, 16th Conf. on Severe Local Storms.  Kananaskis Park, Alta. Amer. Meteor. Soc. 516-521.

Schmocker, G.K., R.W. Przybylinski, and Y.J. Lin, 1996:  Forecasting the initial onset of damaging downburst winds associated with a mesoscale convective system (MCS) using the Mid-Altitude Radial Convergence (MARC) signature.  Preprints, 15th Conf. on Wea. Analysis and Forecasting.  Norfolk VA. Amer. Meteor. Soc. 306-311.  
     
Sharp, D.W., A.J. Cristaldi, S.M. Spratt, and B.C. Hagermeyer, 1998:  Multifaceted general overview of the East-Central Florida Tornado Outbreak of 22-23 February 1998.   Preprints, 19th Conf. on Severe Local Storms. Minneapolis MN. Amer. Meteor. Soc. 140-145. 

Trapp, R.J., E.D. Mitchell, G.A. Tipton, D.W. Effretz, A.I. Watson, D.L. Andra, and M.A. Magsig, 1999:  Descending and non-descending tornadic vortex signatures detected by WSR-88Ds.  Wea. Forecasting, 14, 625-639.