A Detailed WSR-88D Radar and Damage Survey
of a Severe Bow Echo Event on 14 June 1998
over the Mid-Mississippi Valley Region

Gary K. Schmocker (Forecaster) WFO St. Louis
Ron W. Przybylinski (SOO) WFO St. Louis

1. Introduction

   - During VORTEX-95 P. Markowski, et al. 1998 found that the majority of tornadoes in
        supercell storms occurred near external, pre-existing low level boundaries; usually
        within 30 km of the boundary  -  on the cool side where baroclinic horizontal vorticity
       generation occurs.

     - COMET Cooperative project between WFO St. Louis and St. Louis Univ; in a number
       of our cases, we found that quasi-stationary frontal or pre-existing outflow boundaries
       frequently intersected the northern end of convective line segments.  Short-lived
       tornadoes often formed at the intersection (e.g. July 2, 1992; July 8, 1995).

     - In this presentation we will show:
       - the affects of a pre-existing outflow boundary which intersected a convective line;
       - how it caused a dramatic change in the local low-level wind shear profile
         and appeared to aid in vortex development.

     - Although the radar detected fine line was moving quickly northeastward (about 20
       m/s), the absence of a relatively large and abrupt pressure change on the WFO
       St. Louis barograph trace lead to the conclusion that this was an outflow boundary
       rather than a gravity wave    

 

Wind damage map across east central Missouri & southwest Illinois

     - numerous reports of wind damage (40-50 m/s peak gusts) between 1030 and 1300
       UTC across east central Missouri and southwestern Illinois including the northern
       portion of the St. Louis metro area
     - at least 3 non-supercell tornadoes (F0-F1)
     - detailed storm survey by SOO, WCM and one forecaster revealed classic intense
       microburst damage patterns

2. Synoptic and Mesoscale Features

                                             500 mb 1200 UTC June 14 1998
     - 50 to 60 kt southwesterly wind max from Texas Panhandle northeast into
       central Missouri ahead of shortwave trough extending from eastern Nebraska
       southwest through the Texas Panhandle; this jet streak was advecting dry
       mid-level air into Missouri

 

                                              Surface 1100 UTC June 14 1998
     - warm front stretched from west central Missouri southeast through northeastern
       Arkansas; surface trough just north of warm front from west central through southeast
       Missouri; 64-70°F dewpoints across central and southern Missouri along and south of a
       line from MCI to STL

                                             SGF 1200 UTC Sounding June 14 1998
     - CAPE of 2006 J/Kg
     - SR Helicity (0-2km) of 294 m2/s2
     - pronounced drying between 800 and 600 mb
      - 'Inverted V' structure (700 - 500 mb) signify damaging downburst potential.

 

    

                                            KLSX WSR-88D VWP 1050-1137 UTC
     - strong low-level wind shear (at least 20 m/s in the lowest 3 km) with strengthening and
        increased veering of the winds between 0930 and 1130 UTC in the lowest 2 km
     - rapid increase in the SR helicity (0-3km) between 1000 and 1130 UTC
     - 0-2 / 0-3 km SR helicity increased from 340 (500) m2/s2 at 1025 UTC
       to 670 (993) m2/s2 at 1111 UTC -  near the time of boundary passage at the radar site.

3. Initial Radar Analysis and MARC Signatures

0.5° reflectivity at 1030 UTC

     - two parallel lines of convection, oriented northeast-southwest, rapidly formed 90
       to 120 km northwest of KLSX between 0900 and 1000 UTC
     - the second line merged with the leading line after 1030 UTC resulting in the
       intensification of the leading convective line
     - asymmetric structure of convective line
     

MARC Signatures and Wind Damage Plot

   -2 MARC velocity signatures identified

  MARC #2 (291-301 radial) time-height plot

     - second (and strongest) MARC signature identified along the leading edge of
       convective line at 1040 UTC, 11 minutes prior to time of initial wind damage
       across Montgomery county; MARC increased to a maximum value of 35 m/s
       (3.8 km height) at 1101 UTC during the time of wind damage

     - MARC magnitudes identified in this case were not as strong or long lasting
       as compared to other convective line cases we have studied.
     - a strong - persistent mesoscale RIJ current was not observed in the SRM data; 
     - we have also noted that MARC is often not as easy to identify with nocturnal
       convection).

    4. Circulation evolution near intersection of convective line and
                               pre-existing outflow boundary

      - after 1045 UTC, one convective cell along the southwestern part of the leading
       convective line intersected with a NW-SE outflow boundary produced by an
       earlier (0600-0900) MCS

0.5° reflectivity image at 1056 UTC

- note the weak convective cells oriented northwest-southeast signifying the location of the
  outflow boundary and intersecting the larger convective line to the northwest.

Circulation Track Map

      - three vortices rapidly formed just north of the intersection between the convective
       line and the NW-SE outflow boundary (on the cool side of the boundary)
       with the second circulation (Circ #2) becoming the strongest and longest lived.

 

Circulation #1 time - height trace

     - Circ #1 formed at 1050 UTC in the region of the storm's inflow notch.
     - throughout much of its life cycle this circulation was generally a shallow vortex
       with the strongest cyclonic shears remaining in the lowest 2 km. 

4 Panel 0.5° & 1.5° reflectivity and SRM velocity images at 1101 UTC
depicting circs. #1 & #2

 

       1111 UTC 4 panel 0.5° and 1.5° reflectivity and SRM velocity images       

     - Between 1101 and 1111 UTC,  Circ. #2 intensifies as it moves eastward into Lincoln
       county
     - Circ. #2 remains located in the region of the storm's inflow notch
     - northeastward moving outflow boundary extends from inflow notch in western
       Lincoln county southeast to just east of KLSX at 1111 UTC
     - convective line bowing just south of circ. #2

Circulation #2 time height section

     - Circ. #2 initially formed below 2 km, rapidly intensified within the lowest 2 km and
       gradually deepened to an overall depth of 5 km midway through its life cycle
     - Circ. #2 lasted over one hour and spawned an F(0) tornado
     - Circ. #2 appeared to aid in the acceleration of the bowing segment south of the
        vortex and helped to focus clusters of intense damaging winds across the
        northern parts of the St. Louis metro area
      - strong circulations of this type, as well as other circs. we have studied in past cases
        appear to enhance the downward transport of momentum leading to enhanced wind
        damage south and southeast of the vortex
        

      1131 UTC - Six panel presentation of reflectivity / SRM images for 3.4, 2.4 and 1.5
      degrees (left to right).

 

- the 1131 UTC 6 panel image shows the overall reflectivity / SRM velocity structures.
  Circ 2 is seen within the weak reflectivity notch / along the leading edge of the
  convective line segment.
- Circ 2's strongest cyclonic shears remained within the lowest 2.5 km while overall core
  diameter varied between  4 and 6 km.
- a strong gate-to-gate velocity couplet (delta V ~ 25 m/s) is observed at the lowest
  slice. These very small couplets are frequently observed for only one or two volume scans.
- the location of Circ 2 at and beyond 1131 UTC did not favor an optimum viewing angle,
  thus Vr values were likely underestimated.
- the strongest degree of wind damage occurred just south of Circ 2.

 

5. Summary

a) The June 14th bowing MCS evolved in a dynamic environment characterized by moderate instability and strong wind fields.

b) The MARC velocity signature was not as pronounced in this nocturnal event as compared to other cases documented during the afternoon and early evening hours.

c) An outflow boundary orthogonal to the convective line was identified by small convective towers. 0-3 km SR helicity values significantly increased from 500 to 993 m2/s2 over a 45 minute period as the boundary traveled at 20 m/s over the KLSX radar site.

d)  After 1050 UTC, three circulations formed near and north of the intersection of the external boundary and convective line. Circ #2, (a non-descending vortex) initially formed below 2 km, and appeared to aid in the acceleration of the bowing line segment south of the vortex.  

e)  Circ #2 along with other vortices we have studied in past cases appeared to enhance the downward transport of momentum leading to enhanced wind damage south and southeast of the vortex.

f)  This case has shown that a boundary ('even a rapidly moving one') intersecting a convective line can augment the horizontal vorticity (SR helicity) leading to low-level mesocyclogenesis.

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