# Presentation: 457607

## OBSERVATIONS OF THE INTERACTION BETWEEN A   SUPERCELL AND A MESOSCALE BOUNDARY

- OBSERVATIONS OF THE INTERACTION BETWEEN A   SUPERCELL AND A MESOSCALE BOUNDARY

*A study that compares 5-minute surface data from the West Texas Mesonet to the evolution of the storm as depicted by the Lubbock WSR-88D  *

- Mark Conder1,2, Steve Cobb2, John Schroeder1, and Gary Skwira2
- 1*Texas Tech University **	**	*2*National Weather Service, Lubbock TX*

- 2158 UTC

- 2209 UTC

- 2219 UTC

- 2229 UTC

- 2239 UTC

- 2249 UTC

- 2300 UTC

- 2310 UTC

- 2320 UTC

- 2330 UTC

- 2341 UTC

- 2351 UTC

- An overview of the evolution of the supercell with respect to the outflow boundary. 0.5 degree reflectivity values of 35, 50 and 65 dBZ are shown to highlight the storm structure. The boundary is identified using a reflectivity contour of 15-20 dBZ. Plots of wind direction (not speed) are also included from the West Texas Mesonet stations at Tulia (Swisher County) and Silverton.

**A****BSTRACT/INTRODUCTION:**

**S****YNOPTIC OVERVIEW**

**A****NALYSES DERIVED FROM THE 20 UTC REESE CENTER SOUNDING**

- West of boundary

- East of boundary

| Parameter | West | East |
| --- | --- | --- |
| LCL | 1364 m | 611 m |
| LFC | 1878 m | 1964 m |
| s-rH* 0-1 km | 46 m2 s-2 | 200 m2 s-2 |
| s-rH* 0-2 km | 67 m2 s-2 | 219 m2 s-2 |
| s-rH* 0-3 km | 92 m2 s-2 | 245 m2 s-2 |
| Total CAPE | 1551 J kg-1 | 1319 J kg-1 |
| Vorticity generation parameter (VGP) 0-4 km | .273 m s-2 | .365 m s-2 |
| BRN shear | 7.4 m s-1 | 10.2 m s-1 |
| CIN | -25 J kg-1 | -79 J kg-1 |

- Panoramic picture from video stills of the storm at 0005 UTC. Location is near the town of Lockney, in northwestern Floyd County. Courtesy Mike Umscheid

**S****UMMARY AND CONCLUSIONS**

- Visible satellite imagery from 2205 UTC with surface observations, MSAS surface temperature advection, and boundary locations overlaid. Storm initiation point in SW Randall county is at the intersection of two wind shift/convergent boundaries (yellow, brown) and local maximum of positive *T**ADV**. *Outflow boundary (blue) is slowly pushing west across the eastern Panhandle and South Plains. Stationary boundary lies across the Permian Basin into Central Texas. The star indicates the location of the sounding from Reese Center.

**Objectively analyzed constant pressure maps for 5 October 2003 12 GMT**
-    @ 500 hPa, CAA and PVA greatest across the northern TX Panhandle
-    @ 850 HPa, deep, rich moist tongue across West-central TX
-          Modest SW wind with WAA – θe advection

- @ 250 hPa, ~60 kt jetmax over NE New Mexico – southern       Panhandle in RR quadrant by afternoon
- @ 500 HPa, fairly constant 20-30 kt core oriented NW-SE     across region of storm initiation

- 0-6 km mean wind: 277o @ 20 kts * Using a storm motion vector of 320/18 kts (Bunkers method)

**R****EESE CENTER SOUNDING DERIVED PARAMETERS**

-      On 5 October 2003, a nascent supercell interacted with an outflow boundary, resulting in a rapid strengthening of the storm’s mesocyclone and the subsequent development of a tornado that lasted 15 minutes and caused F1 damage.  The pre-storm synoptic environment appeared somewhat favorable for supercell development, but unfavorable for tornadoes. Five minute observations from stations of the West Texas Mesonet were used to track the evolution of the storm and the boundary; investigating the low-level processes that may have led to the storm’s intensification.

- Surface parcel lifted from modified sounding *west* of the outflow boundary

- Surface parcel lifted from modified sounding *east* of the outflow boundary

**H****ODOGRAPHS**

- The fourth point in the hodographs roughly corresponds to one kilometer AGL. The hodograph on the left shows a northwest flow-type environment with a low storm relative helicity.  When the storm motion vector (320 @ 18 kts) is plotted on the hodograph east of the boundary, it is apparent how the storm relative flow increases greatly through the first kilometer. In fact, as the storm encounters the boundary, its motion vector is closer to 330o, but 320o is used as an average.

**T****IME SERIES OF RADAR REFLECTIVITY**

-      Reflectivity and SR-velocity images (2255 UTC) show the initial intensification of the low-level mesocyclone as the supercell crosses and interacts with the outflow boundary.  The graph of rotational velocity (*V**r*) shows this intensification through the lowest three elevation slices 5-10 minutes before the first report of tornado touchdown. The strong rotation was maintained for 15 minutes before it relaxed.  Although the 1.5 and 2.4 degrees tilts periodically showed strengthening and reduction cycles for the remainder of the supercell’s lifespan, the rotational velocities at the lowest tilt remained relatively weak.  As the storm moved further away from the outflow boundary, the storm apparently lost the influx of low-level horizontal streamwise vorticity required to maintain the low-level mesocyclone.

**S****UPERCELL EVOLUTION AND DAMAGE TRACK**

-      After the storm formed in Randall County, there are indications that it may never have become tornadic with out the aid of the outflow boundary moving westward. A few of the storms that formed further southwest along the other boundaries in the western South Plains showed supercell structure and produced large hail - but no reported tornadoes.  The air behind the outflow boundary consisted of relatively cooler and more moist air that originated from overnight convection in western Oklahoma and the Red River Valley. The baroclinicity generated along the boundary was reinforced by differential heating during the morning and early afternoon as extensive low clouds and fog inhibited heating east of the boundary. Cloud streets in the stable boundary layer can be see in the satellite imagery to the left.

-      The image above shows storm reports along the supercell’s path from Randall to Floyd County.

**How do we explain the tornado development in a high CIN (79 J/Kg) / mediocre CAPE (~1400) / marginally high LFC  (~2000 m) environment?**
-   In this environment, although there was sufficient 0-1 km SRH, stretching of low-level horizontal vorticity becomes disconnected from region of ascent thus reducing the chances of tornado development (Davies, 2004)
- Possible Answers:
- 1. Application of virtual temperature calculation (Doswell and Rasmussen, 1994). The general effect raises CAPE, lowers CIN and LFC height on      profiles that have a deep moist layer.
- 2. Davies suggests that strong vertical shear may overcome large CIN by inducing strong vertical pressure gradients that enhance upward      accelerations. Could this play a small role, despite shear not being large?
- 3. Another mechanism might be an *advancing* warm front, with strong low-level WAA into region near/below the cap – some weak WAA appeared      to be present in this case
- 4. Mesoscale lift was certainly created and enhanced along boundary – slope determines how far east lift will be present.
- Best guess: Combination of all these factors mentioned above proved sufficient to produce the F1 tornado in this case. The negative factors most likely contributed to the short life span of the tornadic circulation(s).
-   In this case the West Texas Mesonet observations provided excellent utility in diagnosing boundary motion and surface thermodynamic and       kinematic fields in vicinity of the boundary,
-   The outflow boundary caused changes in the storm environment that altered the storm type. Tracking the WTXM observations may allow the       forecaster to anticipate these changes.

-      The image below at 2235 UTC shows the brief second (F0) tornado. Courtesy Mike Umscheid

**S****TORM STRUCTURE AND THE STORM ENVIRONMENT**

-      The two figures on the left are analyses of equivalent potential temperature and highlight the disposition of the air masses around the storm. The storm forms in a well-defined north-south θe axis (2200 UTC). 30 minutes later, the axis remains distinct while slowly shifting westward following the movement of the outflow boundary.

-      The figures on the left are objective analyses of the divergence of mixing ratio. They show that around the time of storm initiation (2200 UTC), the storm forms on the south side of a convergence maximum whose axis runs SE along the outflow boundary. BY 2300 UTC, the convergence is maximized around the storm and southeast along the boundary.

-      An analysis of the absolute surface vorticity field at 2200 UTC shows that it is heavily influenced by the wind shift associated with the boundaries.  By 2300 UTC the storm has modified its own environment through adjustment of the wind field by the RFD.  Coupled with the orientation of the storm to the outflow boundary, it is surmised that strong low-level horizontal streamwise vorticity was ingested by the storm’s updraft.