1148 VOLUME13WEATHER AND FORECASTINGq 1998 American Meteorological SocietyA Baseline Climatology of Sounding-Derived Supercell andTornado Forecast ParametersERIKN. RASMUSSEN ANDDAVIDO. BLANCHARDCooperative Institute for Mesoscale Meteorological Studies, National Severe Storms Laboratory andUniversity of Oklahoma, Norman, Oklahoma(Manuscript received 21 November 1997, in final form 30 July 1998)ABSTRACTAll of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convectiveavailable potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercellthunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classi-fication is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with eachoccurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severeweather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow)and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminateweakly between the environments of the three classified types of storms. Combined parameters (energy–helicityindex and vorticity generation parameter) discriminate strongly between the environments. The height of thelifting condensation level also appears to be generally lower for supercells with significant tornadoes than thosewithout. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even withthe best discriminators, are discussed.1. IntroductionThis paper establishes a baseline climatology of pa-rameters commonly used in supercell thunderstorm fore-casting and research. The climatology is derived fromover 6000 soundings from 0000 UTC during 1992, allof which had nonzero convective available potential en-ergy (CAPE) (Moncrieff and Miller 1976).It is believed that a baseline climatology is neededto support certain aspects of operational thunderstormforecasting. For example, values of CAPE are often cit-ed in forecasts as being ‘‘marginal,’’ ‘‘large,’’ ‘‘ex-treme,’’ etc. However, no known baseline climatologyexists that is adequate to support these quantificationsfor most of the commonly used parameters [except theclimatology of Doswell and Rasmussen (1994) forCAPE]; rather, they generally are based on the subjec-tive experience and ‘‘mental calibration’’ of the fore-casters. Similar problems exist with the operational useof storm-relative helicity (SRH; Davies-Jones et al.1990): what are climatologically large or extreme valuesof SRH? At what values should forecasters become con-cerned about mesocyclone potential?There are a number of motivations for this study inthe area of convection research. Because this is a 1-yrCorresponding author address: Dr. Erik N. Rasmussen, NSSL,3450 Mitchell Lane, Building 3, Room 2034, Boulder, CO 80301.E-mail: [email protected], it contains no information on the interan-nual variability of convection-related sounding-derivedparameters. Thus, this study is suitable as a baseline forefforts to assess the interannual variability.Another motivation is similar to the forecasting con-cerns mentioned above. Certain parameters have beenestablished through theoretical or modeling work as be-ing important in supercell structure, organization, etc.[e.g., the bulk Richardson number (Weisman and Klemp1982), CAPE, SRH]. These parameters are then used incase studies and forecasting without thorough clima-tological verification. It is desirable to begin to assessthe climatological occurrence of physically importantparameters before they are proposed for use in opera-tional meteorology. Conversely, it would seem to bedesirable for those performing numerical modeling andtheoretical studies to have data that indicate whether ornot they are exploring physically relevant parts of agiven parameter space.It appears that there are no other sounding climatol-ogies of this magnitude related to the environments ofconvective storms. Other investigations have focusedmore narrowly on various types of convection. For ex-ample, Maddox (1976) analyzed 159 proximity sound-ings to assess the effects of environmental winds ontornado production. In a similar study, Darkow andFowler (1971) compared 53 tornado proximity sound-ings with ‘‘check’’ soundings farther away in the en-vironment and found that winds were most noticeablyDECEMBER1998 1149RASMUSSEN AND BLANCHARDdifferent in the 3–10-km layer. Much more exhaustiveanalyses of both wind and thermodynamic conditionsnear tornadic storms, and 6–12 h prior to their occur-rence, can be found in Taylor and Darkow (1982) andKerr and Darkow (1996).Based on some of the foregoing studies, more recentwork has tended to focus on SRH and other measuresof lower-tropospheric shear, in combination with mea-sures of potential buoyancy. In a limited sample, Ras-mussen and Wilhelmson (1983) examined the combi-nation of mean shear (related to hodograph length) andCAPE in the environments of tornadic, nontornadic se-vere, and nonsevere storms. As SRH increased in pop-ularity as a forecast tool, climatological studies ofsounding-derived parameters began to focus on com-binations of CAPE and SRH (e.g., Davies 1993). Ex-cellent summaries of the most recent climatologicalanalyses of buoyancy and shear in the environments oftornadic storms can be found in Johns et al. (1993) andDavies and Johns (1993). An examination of helicity asa forecast tool is given by Davies-Jones et al. (1990),and Davies-Jones (1993) analyzed the mesoscale vari-ation of helicity during tornado outbreaks using sound-ings from special sounding networks.Sounding climatologies have also been used to assessthe environments related to particular types of thun-derstorms. Bluestein and Parks (1983) utilized sound-ings to compare the environments of low-precipitationstorms and classic supercells, and Rasmussen and Straka(1998) investigated these and high-precipitation super-cells using a sounding climatology. Bluestein and Parker(1993) have used soundings to investigate the modes ofearly storm organization near the dryline. A climato-logical sounding analysis of the environments associ-ated with severe Oklahoma squall lines is reported inBluestein and Jain (1985) and nonsevere squall lines inBluestein et al. (1987).In section 2, the