# Presentation: 653791

## Brief Overview of Two Tornadic Quasi-Linear Convective System (QLCS) Events over the Mid-Mississippi Valley Region During the Spring 2002 Severe Storm Season

- Ron W. Przybylinski and Gary K. Schmocker
- Weather Forecast Office
- St. Charles MO 63304

## 1. Introduction 

- April 19 – May 12, 2002 was an extremely active severe weather period across the WFO St. Louis County Warning Area (CWA).
- 4 Supercell and 5 squall line (bow echo) events were documented during this period
- 2 tornadoes (F2/F1 damage) was spawned by classic supercells.
- 9 tornadoes (F0-F1) were documented along the leading edge of an HP supercell’s RFD.
- 8 non-supercell tornadoes (F0-F2) occurred with five squall line events either near the intersection of a low-level boundary and convective line, or along the leading edge of a bowing segment.

## This Presentation

- Will focus on two of the five non-supercell tornadic events over the STL CWA.
- May 9, 2002 QLCS event – 1 non-supercell tornado occurred during the early morning (resulting in two fatalities)
- May 12, 2002 QLCS event – 2 non-supercell tornadoes occurred during the early evening.
- Detailed damage surveys were completed for both events.

## QLCS Tornadic Events: 

- _The first case (May 12, 2002)_
- Signals in the vertical wind shear profile, degree of instability, and location of mesoscale boundariesaided forecasters in this event.
- Reflectivity clues assisted forecasters in diagnosing
-     the location of a probable low-level boundary intersecting the larger convective line.
- The incipient tornadic circulation was observed 20
-     minutes preceding the first tornado touchdown.

## QLCS Tornadic Events

- _The second case (May 9, 2002)_
- Was very challenging
- Warning forecasters were not expecting tornadoes with a broad bowing structure within the QLCS.
- Difficulty in observing the tornadic circulation - due to size of the vortex (small-scale vortex)- due to the distance between the small-scale vortexand the WSR-88D.

## Past Studies

- Using WSR-88D radar, Przybylinski et al. 1997, Funk et al. 1997, Schmocker et al. 2000 and Wolf 2000 showed that non-supercell tornadogenesis occurred:- near the intersection of a pre-existing low-level boundary and larger convective line - near or north of the apex of bowing segments within a larger convective lines.
- Przybylinski et al. 2000 revealed four conceptual models of the *‘ Intensifying Stage’ *of MCS evolution based on 24 MCS events.  First non-supercell tornadoes occurred at the low-level boundary – convective line intersection while subsequent tornadoes formed near or north of the apex of the larger bowing segment.
- Recent observational studies on non-supercell tornadogenesis included work completed by Atkins and Przybylinski (2002),  Atkins and Arnott (2002), Ketcham and Przybylinski (2002) and Wolf (2002).

## The First Case (May 12, 2002) 

- Surface Analysis(1800 UTC) 12 May 02

- - Stationary front extended across central Illinois through southwest Missouri. - Narrow ribbon of lower 70 degree dewpoints extended along and just south of the frontal boundary over Missouri.

## Skew-T Log-P KSGF 1200 UTC 12 May 02

-   CAPE  - 1275 J/Kg / 0 – 3 km SR Helicity 276 m2 s-2.-  Bulk Shear  0 – 3 km  14 m s-1;  0 – 5 km   21 m s-1.

## Bloomfield Missouri Profiler (KBLM) 

- Bulk Shear at 1900 UTC;  0 - 3 km  17 m s-1 / 0 – 5 km  20 m s-1(Moderate Shear 0 - 3 km; Strong Shear 0 - 5 km layer)

## KLSX Reflectivity Image 2231 UTC 

- - Archive Level II Imageryfrom KLSX used in all casesstudied, including this case.

-  _Two convective modes __were present. _- _Multicell line_ of stormsembedded within a largerarea of convection extended from STL to just south of Ft Leonardwood (TBN).- Smaller cells extended southeast from the line. - _Supercells_ were observed to the west and north of STL.

## KLSX Reflectivity / SRM (0.5°slice) at 2246 UTC

- - One of the isolated cells south of the line intensified.- Weak small-scale vortex formed along the leading edge of a small bowing segment embedded within the line.

## KLSX  Reflectivity / SRM for 2302 UTC

- - Isolated cells formed along a low-level convergent boundary which appeared to be generated by the strong isolated cell. - The line of storms form into a broad bowing segment.- C1 slightly deepens and intensified within the lowest 3 kmas it remained along the convective line’s leading edge.

## KLSX Reflectivity / SRM for 2312 UTC 

- - The low-level boundary / convergent line continues to    retreat northward. *(Note) *There is no sign of a circulationnear the LL Boundary – convective line intersection.- C1 continued to intensify within the lowest 2 km with Rotational Velocity (Vr) values of 20 m s-1.

## KLSX Reflectivity / SRM for 2317 UTC

- - Additional weak convective elements outline the location of low-level boundary.  - LL boundary approached C1. This vortex further intensified at this time with Vr magnitudes of 21 m s-1 (42 kts) at 1.3 km agl. - Tornado initially touched down 5 mi WSW of Desloge MO.

## KLSX  Reflectivity / SRM for 2322 UTC

- The bow echo accelerated eastward towards the isolated cell.- Low-level boundary / convergent line interacted with C1.- C1 reached it greatest Vr magnitudes (23 m s-1) at 0.5° slice.    Tornado caused F1 damage at 2322 UTC.

## Rotational Velocity (Vr) Trace of Circ 1

- C1 deepened and intensified after 2300 UTC.- 1st brief tornado touchdown occurred approximately 5 minutesafter initial deepening and intensification of C1.- 2nd tornado touchdown occurred after mid-level rotation slightly weakened, although strong low-level rotation was present.

## Overall mapping of C1’s path

- - Black dots represent 5 minute positions of Tornadic Circ 1- Squall line positions are denoted every 30 minutes beginning with 2300 UTC.

## The second case  – May 9, 2002 QLCS 

- Surface Analysis0300 UTC 9 May 02

- - Cold front extended from western Wisconsin through northwest Missouri and southwest into western Oklahoma.Surface dewpoints reached the upper 60s over SE Missouri.

## Skew-T Log-P KSGF 0000 UTC 9 May 02

- CAPE – 2503 J/kg  /  0 – 3 km SR Helicity 287 m2 s-2Bulk Shear  0 – 3 km  19 m s-1;  0 – 5 km 21 m s-1

## Bloomfield Missouri Profiler (KBLM) 

- Bulk Shear at 0600 UTC;  0 – 3 km   16 m s-1;  0 – 5 km  17 m s –1(Moderate Shear at both layers)

## KLSX Reflectivity Image for 0626 UTC (0.5° slice)

## KLSX Reflectivity / SRM for 0626 UTC

- - Reflectivity Image (0.5° slice) showed small convective segmentsembedded within the larger line. - SRM velocity showed two small-scale vortices along the leadingedge of the line (white arrows).

## KLSX Reflectivity for 0646 UTC

- - Convective segment 50 nm east of KLSX began to intensify.

## KLSX Reflectivity / SRM velocity at 0646 UTC

- Northern storm appeared to exhibit supercellular structures.- Small-scale circulations identified along the leading edge revealed short lifetimes (10 minutes or less). Vr magnitudes of C1 was 15 m s-1;  C2  - 31 m s-1.

## KLSX Reflectivity / SRM 0701 UTC

- - Small convective segments embedded within the larger linecontinued to be present. - Small-scale vorticies continued to form along the leading edge   C3 exhibited Vr magnitudes of   8 m s-1.

## KLSX Reflectivity / SRM 0705 UTC

- Small-scale convective segments continued to persistwithin the larger line.

## KLSX Reflectivity / SRM for 0705 UTC(Close-in View)

- - C3 slightly intensified with Vr values of 12 m s-1.Radar sampling (aspect ratio) issues are critical in detecting small-scale circulations at far ranges.

## KLSX Reflectivity / SRM at 0710 UTC

- - Vr magnitudes of C3 at this time were 11 m s-1. Tornado caused F1 damage. (2 fatalities).A very challenging situation for the warning forecaster.

## Conclusions

- - Two of many challenging tornadic cases which occurred during the spring 2002 convective season were shown. - May 12, 2002 case showed that a mesocyclone, located along the leading edge of a bowing segment, intensifiedas a low-level boundary generated by an isolated stormdownwind, moved north and interacted with the circulation.- When the low-level boundary was within 10 km of the vortex, the circulation spawned a tornado (F1 damage).- May 9, 2002 tornadic event over south-central Illinois was extremely difficult.

## Conclusions Cont’

- Several small convective line segments were embeddedwithin the larger QLCS.- Shallow small-scale circulations, exhibiting short lifetimes(10 minutes or less), were identified along the leading edgeof a few of the convective segments. - The small-scale vortex which spawned the tornado nearCentralia Illinois was very weak (Vr magnitudes of 12 m s-1) and exhibited a short lifetime.
- Radar sampling (aspect ratio/ radar horizon) issues playeda significant role towards the detection of the tornadic vortex. - Bow Echo and MCV Experiment (BAMEX) Spring 2003

## Our WEB Address:

*www.crh.noaa.gov/lsx/science*