SR/SSD 97-9 2-15-97

Technical Attachment


Corey M. Mead and Thomas W. Earle
NWSO Midland, Texas

1. Introduction

On the evening of September 24, 1995, an elevated severe thunderstorm complex containing at least one supercell developed over eastern New Mexico and tracked southeastward across the Permian Basin of Texas, causing over $2.5 million in hail damage to the Midland area. Several reports of hail up to 4.5 cm (1.75 in) were received along with wind gusts in excess of 30 ms-1 (60 kt) over the course of the storm's five-hour lifetime (Fig. 1).

Although a slight chance of thunderstorms was in the forecast, area forecasters were less than optimistic about any organized boundary layer or elevated convective development later on that afternoon or evening. Furthermore, analyzed data suggest that lower tropospheric processes were largely responsible for the destabilization of the local environment, as well as the initiation and maintenance of this particular convective complex. There was little evidence that any upper-level dynamic forcing mechanisms were present at the time of this convective event.

In Section 2 we describe the incipient synoptic-scale conditions at the time of the convective event. Section 3 utilizes 0-hr gridded data sets and radar data to infer what multi-scale processes may have helped initiate and maintain the convective complex.

2. Synoptic Environment

The 1800 UTC surface analysis on September 24 indicated a southward-moving cold front that extended from the eastern New Mexico plains, southeastward across the South Plains of Texas, and then back northeastward into central Oklahoma (Fig. 2). With low clouds to the north of this boundary suppressing diurnal heating, and temperatures over the Permian Basin warming into the 80s, a significant temperature gradient developed between Midland and Lubbock. This highly baroclinic frontal zone exhibited atypical characteristics as it continued to surge south into the Permian Basin during the time of maximum heating. By 0000 UTC on the 25th , the front had surged to just north of a Midland-Abilene line, as both stations were reporting local convective development (Fig. 3). By 0600 UTC on the 25th, the front had stalled out along the higher terrain of southwest Texas, with cooler temperatures and a relatively high residual moisture content evident over West Texas and Southeast New Mexico.

The 500 mb pattern valid 0000 UTC on the 25th was characterized by a high-amplitude ridge positioned over the northern Rockies with a migratory positive-tilt shortwave trough that extended from central Minnesota southwestward into north-central Colorado (Fig. 4). Uniformly zonal winds of 15-20 ms-1 (30-40 kt) were present over West Texas and New Mexico at this time with a meridional temperature gradient and no indications of any embedded thermal troughs. At 850 mb, a well-developed baroclinic zone extended from between Amarillo and Midland eastward to near the Dallas-Fort Worth area with dewpoints of 10 C in the vicinity of this frontal boundary (Fig. 5). It will be shown that this intense baroclinic zone was ultimately responsible for the initiation of the convective complex, as well as enhancing the low-level inflow into the southernmost cell, allowing it to evolve into a supercell.

The 0000 UTC Midland sounding indicated that the atmosphere was slightly unstable with an 850 mb-based CAPE and lifted index of 500 j kg-1 and -2 C, respectively. The 2-6 km shear profile exhibited good clockwise turning with a respectable speed shear of 10 ms-1. By utilizing the WSR-88D VAD Wind Profile (VWP) and base-velocity data on the WSR-88D Algorithm Testing and Display System (WATADS), a proximity hodograph was constructed (Fig. 6). Storm-relative helicity values approaching 850 m2 s-2 were calculated for the 3-km layer immediately above the frontal zone. With an Energy-Helicity Index (EHI) (Hart and Korotky 1991) value of 2.7 (Davies 1993) and relatively weak 4-6 km storm-relative winds (Brooks et. al. 1994), one could expect that supercell storms would be possible, but the possibility of any tornadic activity would be quite low. Although law enforcement officials reported "mud falling from the sky" in central Andrews County, no confirmed tornado reports were received.

3. Model Output and Radar Observations

The 0-hr forecast of selected thermodynamic and kinematic diagnostic fields from the September 25 0000 UTC Eta run were examined using the PCGRIDDS program. Model output of the 850 mb quasigeostrophic deformation field in the vicinity of the intense baroclinic zone located over West Texas and eastern New Mexico indicated that the deformation axis of the wind field was oriented within 45 deg of the isotherms, which is suggestive of frontogenetical forcing (Petterssen 1956). Hoskins and Bretherton (1972) studied frontogenesis and have described the physical process of how quasigeostrophic deformation of the wind field acts to increase the horizontal temperature gradient, effectively forcing the atmosphere away from thermal wind balance. The atmosphere reacts to this frontogentical forcing by inducing a frontolytical response. It does this by accelerating an ageostrophic component of the wind upward in the warm air and downward in the cold air. This direct thermal circulation (hereafter DTC) helps to return the atmosphere to near thermal wind balance. It is possible that this upward branch of the DTC helped initiate the deep convection over eastern New Mexico, where moderately steep 700-500 mb lapse rates where present.

A cross-section taken over extreme eastern New Mexico indicates that a DTC was indeed present at this time (Fig. 7). The low-level forcing associated with this DTC can be evaluated by the use of Qn-vectors. The Qn-vector is that component of the Q-vector that lies across the isotherms (thickness lines). Its divergence and convergence show the atmosphere's response to the tendency of the quasigeostrophic deformation to pack or unpack the isotherms (thickness lines), i.e., frontogenesis and frontolysis (Thaler personal communication). Figure 8 is an illustration of the Qn-vector fields valid 0000 UTC on the 25th. Note the convergent Qn-vectors over extreme eastern New Mexico and western Texas. This would be the quasigeostrophic representation of the rising branch of the DTC.

Various base reflectivity, as well as base and storm-relative velocity loops of this convective event, were studied using the WATADS program. Interesting mesoscale features were observed including several mesocyclone life cycles and a detailed sampling of an inflow jet. Of particular interest was the inflow jet as seen on the base reflectivity loop (Fig. 9). As the supercell located at the southern end of the complex moved southeastward along the 850 mb thermal boundary into West Texas, this outbound low-level speed maximum could be seen curling into the storm. Velocities on the order of 20 ms-1 (40 kt) were observed within the core of this mesoscale feature. When taking into account the storm motion, storm inflow speeds of 30-40 ms-1 (60-80 kt) were calculated. This extremely strong inflow, when combined with a veering wind profile and modest instability, allowed this supercell to attain a steady-state structure as it moved across a large portion of the Midland County Warning Area.

4. Conclusion

An elevated convective complex containing at least one long-lived supercell developed in an area of 850 mb frontogenesis, possibly in the rising branch of the DTC, over southeast New Mexico on the evening of September 24, 1995. By propagating along the 850 mb thermal boundary, the southernmost cell was able to top large values of storm-relative helicity, allowing it to become well-organized as it moved into West Texas. The majority of large hail and damaging wind reports were coincident with the track of this southern cell, which exhibited several mesocyclone evolutions as it moved southeast across the western and central Permian Basin of Texas.

Developments were not entirely anticipated. This case study is a good example of how mesoscale signals can dominate in a marginally unstable synoptic scale environment. Although incipient upper-level features appeared to be relatively quiescent, those forecasters familiar with frontogenetical theory and certain lower tropospheric conceptual models would be able to more accurately anticipate similar events. Current PCGRIDDS macros and the future AWIPS technology will continue to allow for advanced quasigeostrophic diagnoses, as well as some mesoscale prognostication with some of the finer gridded numerical model outputs. Tools such as these, along with an ever-increasing knowledge of meteorological processes, will allow NWS forecasters to better anticipate convective weather events that might have been much more difficult to forecast in the past.

5. Acknowledgments

The authors wish to thank Brian Francis (SOO, NWSO Midland) and Loren Phillips (SOO, NWSFO Lubbock) for their reviews and helpful suggestions.

6. References

Brooks, H.E., C.A. Doswell, and R.B. Wilhelmson, 1994: The role of midtropospheric winds in the evolution and maintenance of low-level mesocyclones, Mon. Wea. Rev., 122, 126-136.

Davies, J.M., 1993: Hourly helicity, instability, and EHI in forecasting supercell tornadoes. Preprints, 17th Conf. Severe Local Storms (St. Louis, MO), Amer. Meteor. Soc., 107-111.

Hart, J.A., and W.D. Korotky, 1991: The SHARP workstation - v1.50. A skew-T/hodograph analysis and research program for the IBM and compatible PC. User's manual. NOAA/NWS Forecast Office, Charleston, WV., 62 pp.

Hoskins, B.J., and F.P. Bretherton, 1972: Atmospheric frontogenesis models: mathematical formulation and solution. J. Atmos. Sci., 29, 11-37.

Petterssen, S., 1956: Weather Analysis and Forecasting, Volume I: Motion and Motion Systems. McGraw-Hill, New York. 428 pp.

Fig. 1. Severe weather reports associated with the elevated supercell as it moved from Southeast Mexico into West Texas.

A = Large hail

W = Wind damage

Fig. 2. Surface analysis at 1800 UTC September 24, 1995.

Fig. 3. Surface analysis at 0000 UTC on September 25, 1995.

Fig. 4. 500 mb forecast valid at 0000 UTC on September 25, 1995. Solid lines are geopotential height contours (dm), and the dashed lines are relative vorticity (s-1 x 10-6) isopleths greater than zero.

Fig. 5. 850 mb analysis valid at 0000 UTC on September 25, 1995. Dashed lines are isotherms (C), and the solid line is the 10 C dewpoint contour. Note the isothermal packing between Amarillo and Midland.

Fig. 6. Proximity hodograph of the ambient wind profile immediately above the frontal inversion. This hodograph was constructed from the WSR-88D VAD Wind Profile (VWP) and base velocity data. By incorporating the observed storm motion, storm-relative helicity values approached 850 m2s-2 for the 0 - 3 km layer above the frontal zone.

Fig. 7. Cross section along 102 longitude, from 40 deg north to 28 deg north latitude. Solid lines are potential temperature surfaces (k), dashed lines are upward vertical velocity isopleths in microbars per second, and arrows represent the ageostrophic wind component (ms-1). The direct thermal circulation (DTC) is evident along this cross section with the rising branch of the circulation on the warm side of the frontal zone in an area of lower static stability.

Fig. 8. 850-700 mb Qn-vector forecast valid at 0000 UTC on September 25, 1995. Dashed lines are areas of Qn-vector convergence (10-19 m Pa-1 m-2s-1). The upward branch of the DTC is represented in a planar sense by the convergent area of Qn-vectors located over West Texas and extreme eastern New Mexico.

Fig. 9. 0.5 deg base velocity at 0309 UTC on September 25, 1995. The elevated supercell is located in extreme northwestern Andrews County (see Fig. 1 for county names), while the inflow jet can be seen rotating into the storm from northern Winkler and southern Andrews Counties. Radar is located at "X" (Midland).