A preliminary climatology of tornado events with closed cold core 500 mb lows in the central and eastern United States

Tornado events that are associated directly with closed 500 mb lows having a core of cold temperatures aloft are not uncommon. Miller (1972) identified this synoptic pattern as a specific severe weather type, and similar settings associated with severe thunderstorms and sometimes tornadoes east of the Rocky Mountains have been discussed by Goestch (1988), Davies (1993), and McDonald (2000). Patterns associated with tornadic thunderstorms in California (e.g., Monteverdi et al. 2000) also at times involve closed 500 mb lows. Goetsch (1988) noted that some tornadoes associated with “cold core” upper lows produce significant damage, and observed that potential for convection and severe weather with such patterns was frequently underestimated by forecasters. In particular, operational experience during recent years has shown that potential for tornadoes with 500 mb closed low systems can be overlooked, especially when surface dew points appear marginal compared to more “typical” scenarios associated with tornadoes.

To date, little work has been done regarding climatology and frequency of tornado-producing events associated with 500 mb closed lows. This study is intended as a preliminary step in that direction, utilizing cases in the central and eastern United States because of similarity in moisture source (usually the Gulf of Mexico) and lack of mountainous topography. Convective events associated with closed cold core upper lows east of the Rocky Mountains during the period 1996-2003 were located using 500 mb charts and simple surface dew point criteria in conjunction with severe weather reports in Storm Data. Features and characteristics of these events, such as surface dew points near the 500 mb low center, 500 mb temperature, and size and movement of the 500 mb low, were cataloged. An examination of these characteristics is in progress to determine general differences between tornadic and nontornadic systems. Seasonal frequency and geographical location issues are also being examined.

Early examination results suggest that tornadoes near cold core upper lows tend to occur with 500 mb closed systems that are more compact in size and notably progressive in movement toward the east or northeast instead of southeast or stationary. Daytime heating also appears important, along with a certain amount of minimal moisture as indicated by surface dew point and time of year. This paper should provide useful information for forecasters regarding recognition of closed 500 mb systems that may have increased potential for tornadoes near the cold core upper low where surface dew points can appear marginal compared to many tornado-producing synoptic patterns.

References

Davies, J. M., 1993: Small tornadic supercells in the central plains. Preprints, 17th Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 305-309.

Goetsch, E. H., 1988: Forecasting cold core severe weather outbreaks. Preprints, 15th Conf. on Severe Local Storms, Baltimore, MD, Amer. Meteor. Soc., 468-471.

McDonald, M., 2000: Cold core tornadoes: A forecasting technique. Internal report, Prairie Storm Prediction Centre, Environment Canada, 7 pp.

Miller, R. C., 1972: Notes on analysis and severe-storm forecasting procedures of the Air Force Global Weather Central, AWS Tech. Rep. 200 (rev.), Air Weather Service, Scott AFB, IL, 190 pp.

Monteverdi, J. P., C. A. Doswell III, and G. S. Lipari, 2000: Shear parameter thresholds for forecasting California tornadic thunderstorms. Preprints, 20th Conf. on Severe Local Storms, Orlando, FL, Amer. Meteor. Soc., 522-525.