SR/SSD 97-17 4-15-97

Technical Attachment


William Scott Sharp
NWSO Nashville, Tennessee

1. Introduction

The severe weather event that occurred on May 18, 1995, across middle Tennessee has been considered one of the most widespread outbreaks to occur in this area since the April 3, 1974, Super Outbreak. By sunset, tornadoes had killed three Tennesseans and injured over 60. Two of the tornadoes were rated as F3, and several weaker tornadoes occurred. Due to the outstanding job of the staff on duty at NWSO Nashville, the loss of life was minimal. The WSR-88D performed well, allowing the meteorologists to issue timely warnings.

This paper briefly discusses the use of the PC-GRIDDS program (Petersen 1993) and several macros in heightening the awareness of just how serious this severe weather event was going to be.

2. Discussion

Leading up to the severe weather event, the NCEP computer models had predicted an airmass conducive to the development of severe thunderstorms as much as 24 hours in advance. The advent of PC-GRIDDS has allowed forecasters to view a variety of atmospheric parameters not available through the traditional method of using AFOS graphics. By using macros developed at NWSFO Jackson, meteorologists at NWSO Nashville were aware of the impending severe weather well in advance. Several gridded products that were used highlighted the severe weather threat.

The PCGRIDDS macro JETS. displays both the 300 mb and the 850 mb isotachs. This macro showed that the Eta run at 1200 UTC May 17 predicted a low-level jet of at least 45 kt over middle Tennessee in 30 hours (1800 UTC on the 18th), with an upper-level speed max of 100 kt crossing western Kentucky Fig. 1. This particular pattern has been recognized as enhancing supercell thunderstorm development due to the exceptional upward vertical motion.

The macro SRHE. computes the storm-relative helicity using model data for a specific forecast hour. Again using the Eta run at 1200 UTC May 17, the 24-hour forecast predicted a 0-3 km S-R helicity maximum axis stretching from the Lower Mississippi Valley through the Tennessee Valley into the eastern Ohio Valley, with a maximum of 320 m2s-2 over northeast Mississippi Fig. 2. This axis was forecast to move little during the next 12 hours, with a maximum of at least 360 m2s-2 over middle Tennessee by 0000 UTC May 19th Figs. 3 and 4. Davies and Johns (1993) reviewed 242 cases of tornadoes, classified as either strong or violent, to develop storm relative helicity threshold values. Their study indicated an assumed 0-3 km S-R helicity of 339 m2s-2 for strong tornadoes and 452 m2s-2 for violent tornadoes. These findings fit well with the severe weather outcome over middle Tennessee on May 18. The 0000 UTC May 19 Nashville hodograph, modified to depict the observed storm motion (not shown), illustrated strong veering of the winds with height in the lower layers of the atmosphere, with a computed storm relative helicity (0-2 km) of 413 m2s-2 and an unbelievable S-R helicity of 1485 m2s-2 between 0-3 km.

The macro KAPE. calculates the convective available potential energy (CAPE) values for a specified time period. Typical values of CAPE associated with moderate to strong convection range between 1000 to 3000 Jkg-1. The important feature of Fig. 5 is the axis of high CAPE values increasing over middle Tennessee between 1800 UTC on the 18th through 0000 UTC on the 19th (Figs. 6a and 6b), even though the predicted values were nowhere close to the CAPE observed from the 1200 UTC May 18 Nashville sounding (2643 Jkg1). This suggested the airmass over the region would become increasingly buoyant during the afternoon hours.

3. Summary and Conclusion

This paper discussed the use of the PC-GRIDDS program and the 1200 UTC Eta model run on May 17, 1995, in determining the severe weather threat across middle Tennessee on May 18. Using the displayed products by themselves will not provide the answer to defining whether a widespread severe weather outbreak is imminent; however, when used with other parameters (cold air advection aloft, low-level moisture convergence, etc.), confidence can be increased, allowing the forecaster to heighten public awareness to the severe weather potential.


The macros mentioned were developed by Ed Agre (lead forecaster, NWSFO Jackson, Mississippi), and Wayne Albright (currently a forecaster at NWSO Wakefield, Virginia). Thanks to Henry Steigerwaldt (SOO, NWSO Nashville) for his critical review and input.


Petersen, R. A., 1993: A PC based system for display of digital gridded WAFS data, NWS Meteorological Center, Washington D.C.

Davies, J. M., and R. H. Johns, 1993: Some wind and instability parameters associated with strong and violent tornadoes, 1, Wind shear and helicity. The Tornado: Its Structure, Dynamics, Prediction, and Hazards, Geophys. Mono. 79, edited by C. Church, D. Burgess, C. Doswell and R. Davies-Jones, American Geophysical Union, Washington, D.C., pp. 573-582.