A Severe Weather Threats Checklist to Determine Pre-Storm Environment

1. Introduction

Determining the pre-storm environment is a critical step in assessing severe weather potential. To aid in this assessment, a severe weather threats checklist and value interpretation guide was developed, incorporating 16 of the most useful severe weather parameters for analysis. These parameters can be obtained from soundings available in AWIPS or the SHARP program (Hart and Korotky 1991) using local upper air soundings.

This paper documents two cases in which the use of this checklist either alerted or would have alerted forecasters to the short term severe weather threat. The cases documented include the derecho of 5-6 May 1999 across middle Tennessee and the tornado outbreak of 22-23 February 1998 in east-central Florida. Examples of the severe weather checklist from these events are provided.

Application of the 16 sounding and hodograph parameters provided in this checklist gives forecasters a systematic method for the identification of potential storm-producing environments. Daily use of the checklist led to increasing forecaster recognition of short term severe weather threats at NWSO's Nashville, TN and Melbourne, FL. The compilation of severe weather parameters has also helped forecasters improve communication of short term severe weather threats to the public, especially by utilizing the Hazardous Weather Outlook (Special Weather Statement) and Short Term Forecast (Nowcast).

It must be noted that this severe weather checklist has operational utility especially for high shear environments when the greatest potential for life threatening severe weather exists.

2. Middle Tennessee Derecho of 5-6 May 1999

During the evening and early morning of 5-6 May 1999, a bow echo complex formed ahead of a vigorous cold front and moved across middle Tennessee. This complex produced extensive wind and hail damage, flash flooding, and isolated tornadoes, including an F4, which killed 2 people in Linden (Perry County).

Preliminary damage assessments from the Tennessee Emergency Management Agency estimated at least $4.7 million in damage across the Nashville County Warning Area (CWA). More than $2.7 million of damage occurred in Davidson County alone. The Nashville International Airport (OHX), where several aircraft were either damaged or destroyed, was especially hard-hit. Overall, severe weather was verified in 37 of the 43 counties in the Nashville CWA.

Although synoptic and mesoscale conditions on 5 May indicated the potential for severe weather, utilization of the storm environment checklist gave forecasters a "quick look" a the particularly unstable environment in place that day. Initially, model forecasts indicated the primary threat from this storm system would be flash flooding. However, as the day progressed, the focus shifted somewhat from heavy rainfall to severe thunderstorms and tornadoes. In fact, between 1200 UTC and 0000 UTC, the precipitable water value at Nashville actually decreased slightly, from 1.29 in to 1.25, and the 0000 UTC K-Index of 27 simply did not indicate the presence of the deep moisture needed to produce widespread heavy rainfall. Although heavy rainfall did occur in many places across middle Tennessee, reports of flash flooding were isolated.

The comprehensive list of critical parameters taken from the 1200 UTC, 5 May, and 0000 UTC, 6 May OHX soundings indeed heightened forecasters' awareness of the strength of the approaching storm system. Such advanced knowledge prompted issuance of a Hazardous Weather Outlook (HWO) at 0916 UTC to increase public preparedness. This HWO included the wording "middle Tennessee residents should be on the lookout for damaging wind and large hail overnight, although flash flooding remains the primary threat."

A second HWO was issued at 2145 UTC which highlighted the increased risk of severe thunderstorms and tornadoes. Although a flash flood watch remained in effect, this HWO mentioned that "isolated tornadoes are still possible. The primary threat, however, will be large hail and damaging winds." Extra staffing was maintained at the National Weather Service Office in Nashville in preparation of this event.

3. East Central Florida Tornado Outbreak of 22-23 February 1998.

The tornado outbreak during the late night and early morning of 22-23 February 1998 resulted in 42 deaths and 260 injuries in east-central Florida (Sharp et.al. 1998). The existence of an El Nino pattern during February enhanced atmospheric instability (Hagemeyer 1997); (Hagemeyer 1998). Seven tornadoes, three of which were rated F-3 or greater (Fujita 1981), occurred between 0400 UTC and 0730 UTC on 23 February.

The synoptic situation during the evening of 22 February indicated a high probability of severe weather across east-central Florida. A strong upper level trough with a 140 kt jet streak was approaching the Florida peninsula from the Gulf of Mexico. A surface low was located over Alabama, with a trailing cold front extending across the eastern Gulf of Mexico. A warm front was located north of Florida (Sharp et. al. 1998). East-central Florida was in the favored sector of a 50 kt low-level jet.

Analysis of the parameters taken from the 0000 UTC 23 February sounding from Tampa (TBW) indicated that environmental conditions four hours prior to this event supported development of tornadoes in east-central Florida. The total totals value of 49 indicated a moderate severe weather potential as proposed by Giordano (1994) and Miller (1972). A lifted index -8 was also a good indicator of the severe weather potential, as shown by Giordano (1994) and Miller (1972). Furthermore, instability parameters which indicated a moderate to high severe weather potential included a CAPE of 2687 J kg^-1, which, as suggested by Weisman and Klemp (1986), indicates a high potential for severe weather.

The SWEAT index of 387 was near the high potential index value as proposed by Miller (1972). The theta-e index value of 18.3^oC also indicated a high potential for severe weather (Moore 1992); (Elson 1991). The Bulk-Richardson number (BRN) of 38, as per Weisman and Klemp (1986), was indicative of multicell potential. The storm relative helicity (SRH) values within the 0-2 and 0-3 km layers of 272 m²s^-2 and 282 m²s^-2 indicated the potential for at least F-2 tornado occurrence (Johns and Doswell 1992). The 0-2 km positive shear of 4.3 and energy helicity index (EHI) of 4.60 was very high, which indicated that tornadoes to F-4 intensity were possible (Davies 1989); (Davies 1993b); (Hart and Korotky 1991).

These sounding parameters, as indicated by SHARP (Hart and Korotky 1991), led forecasters at NWSO Melbourne to issue an HWO at 0340 UTC 23 February to heighten public awareness to the seriousness of the severe weather threat. Within the HWO was the statement that "another round of severe weather is expected." The statement also suggested that "emergency management officials and law enforcement agencies are urged to coordinate with the local National Weather Service. This is a dangerous situation! Spotters are encouraged to keep an eye to the sky and relay all information back to the National Weather Service. Area residents should be ready to react quickly if a tornado warning is issued for their area. Remain informed of the latest weather situation for your county by listening to NOAA weather radio or other local news media" (NOAA 1998).

4. Conclusions

These two severe weather outbreaks represent good use of the severe weather checklist in quickly and accurately assessing the short term storm environment. The usefulness of such parameters varies geographically, as severe weather thresholds vary region-by-region, season-by-season and system-by-system. The use of this checklist does not necessarily stress the use of specific thresholds, but by following atmospheric trends.

5. References

Davies, J., 1989: On the use of shear magnitudes and hodographs in tornado forecasting. Preprints, 12^th Conference on Weather Forecasting and Analysis, Amer. Meteor. Soc., Monterey, CA.

__________, 1993b: Hourly helicity, instability, and EHI in forecasting supercell tornadoes. Preprints, 17^th Conference on Severe Local Storms, Amer. Meteor. Soc., St. Louis, MO.

Elson, D., 1991: Computing the theta-e index (TEI). NOAA Eastern Region Computer Programs NWS ERCP 12MC, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 13pp.

Fujita, T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.

Giordano, L., 1994: A fingertip guide to key stability and shear index values used in evaluating severe weather and flash flood potential. ERH Technical Attachment 94-4. 1-7.

Hagemeyer, B., 1997: Peninsular Florida tornado outbreaks. Wea. Forecasting, 12, 399-427.

__________, 1998: Significant extratropical tornado occurrences in Florida during strong El Nino and strong La Nina events. Preprints, 19^th Conference on Severe Local Storms, Amer. Meteor. Soc., Minneapolis, MN.

Hart, J., and J. Korotky, 1991: The SHARP workstation v1.50, A skew-t/hodograph analysis and research program for the IBM and compatible PC, user's manual. National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 62 pp.

Johns, R., and C. Doswell III, 1992: Severe local storms forecasting. Symposium on Weather Forecasting, Amer. Meteor. Soc., Atlanta, GA.

Miller, R., 1972: Notes on analysis of severe storm forecasting procedures of the Air Force Global Weather Central. AFGWC Tech. Rep. 200 (Rev.), Air Wea. Serv., Scott AFB, IL, 181 pp.

Moore, J., 1992: Isentropic analysis and interpretation: operational applications to synoptic and mesoscale forecast problems. St. Louis University, St. Louis, MO, 88 pp.

NOAA, 1998: Service assessment of central Florida tornado outbreak February 22-23, 1998. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland. 1-23.

Sharp, D., A. Cristaldi, S. Spratt, and B. Hagemeyer, 1998: Multifaceted general overview of the east central Florida tornado outbreak of 22-23 February 1998. Preprints, 19^th Conference on Severe Local Storms, Amer. Meteor. Soc., Minneapolis, MN.

Weisman, M., and J. Klemp, 1986: Characteristics of isolated convective storms. Mesoscale Meteorology and Forecasting, P.S. Ray, Ed., Amer. Meteor. Soc., 331-357.