DECEMBER 1996 NOTES AND CORRESPONDENCE 599 NOTES AND CORRESPONDENCE Lightning during Two Central U.S. Winter Precipitation Events RONALD L. HOLLE AND ANDREW I. WATSON National Severe Storms Laboratory/NOAA, Norman, Oklahoma 17 October 1994 and 15 June 1996 ABSTRACT Network-detected cloud-to-ground lightning coincident with mainly frozen precipitation (freezing rain, sleet, snow) was studied over the central United States during two outbreaks of arctic air in January 1994. During the ®rst event, the ratio of positive to total ¯ashes was 59%, ¯ashes were few and disorganized in area, and no surface observer reported thunder. For the other event the ratio was 52% during the ®rst few hours in subfreezing surface air, then decreased when ¯ashes formed in the nearby region above freezing. Also, ¯ashes in this case were linearly aligned and coincided with conditional symmetric instability; thunder was heard infrequently by surface observers. On radar, re¯ectivity cores grew from weak to moderate intensity within a few hours of the lightning during both cases. Echo area increased greatly before ¯ashes in one case, while the area increase coincided with ¯ashes in the other. Some base-scan re¯ectivities were strong in both thunderstorm regions due to the radar beam intersecting the melting level. Regions with lightning often could be identi®ed better by high echo tops than re¯ectivity. Analyses on the scale of one or two states diagnosed the strength of low-level warming that contributed to formation of thunderstorms and signi®cant frozen precipitation. Quasigeostrophic analyses showed that 850-mb temperature advection and 850±500-mb differential vorticity advection were similar in magnitude in the lightning area during both events. Once convection formed, lightning and echo-top information identi®ed downstream regions with a potential for subsequent frozen precipitation. 1. Introduction the isolated event coincided with, or were within 150 km of subfreezing air and frozen precipitation at the During January 1994, a series of arctic fronts swept ground. About 20% of the ¯ashes in the more organized from northwest to southeast across the eastern two-thirds event were in such regions. of the United States. Low-temperature records for the day, Studies of simultaneous frozen precipitation at the the month, and for all time were set across the Midwest surface and CG lightning detected by networks were and Great Lakes states, especially on 4±6 and 16±19 reviewed by Holle and LoÂpez (1993). Frequent ¯ashes January, according to the National Oceanic and Atmo- were associated with the large and intense March 1993 spheric Administration (NOAA) publication Storm Data. snowstorm on the U.S. east coast (Orville 1993). Other Storm Data is a monthly publication prepared by the Na- studies usually have been of the ¯ow of cold air over tional Climatic Data Center (NCDC) in Asheville, North large unfrozen water bodies. A few ¯ashes were found Carolina. Ahead of each arctic outbreak developed south- downwind of the Great Lakes in convective snowbands erly component winds above the surface, frozen precipi- by Moore and Orville (1990). Large numbers of tation at the ground (freezing rain or drizzle, sleet, or ¯ashes occurred as cold air ¯owed over the Gulf Stream snow), and thunderstorms. (Biswas and Hobbs 1990; Orville 1990, 1993) and the Two lightning events during January 1994 over Mis- Sea of Japan (Goto et al. 1992). souri and Arkansas coincided with subfreezing tem- Winter thunderstorms have also been examined peratures and frozen precipitation at the ground. One without lightning network data. Curran and Pearson isolated event had 27 cloud-to-ground (CG) ¯ashes (1971) calculated a mean sounding near 76 thun- over Missouri, and the other more organized event had dersnow reports and found a subfreezing layer near 2417 ¯ashes from Oklahoma into Arkansas. Flashes in the ground, topped by an inversion whose upper-half was warmer than freezing, then a deep layer with high relative humidities. Beckman (1987, 1989) and Corresponding author address: Ronald L. Holle, National Severe Elkins (1989) suggested more study of thundersnow Storms Laboratory, NOAA, 1313 Halley Circle, Norman, OK 73069. in the context of using satellite imagery for heavy E-mail: [email protected] snow forecasting. Colman (1990a,b) examined the /3q06 0226 Mp 599 Monday Dec 02 02:41 PM AMS: Forecasting (December 96) 0226 Unauthenticated | Downloaded 10/02/21 11:39 PM UTC 600 WEATHER AND FORECASTING VOLUME 11 2. Data The CG lightning data were collected by the Na- tional Lightning Detection Network (NLDN) de- scribed by Cummins et al. (1995) and Orville (1994). The NLDN during January 1994 was composed of di- rection ®nder antennas (Krider et al. 1976, 1980; Holle and LoÂpez 1993). Location accuracy for ¯ashes within the NLDN at this time was estimated as 5±10 km. Sys- FIG. 1. Missouri counties and 230-km (124 n mi) ranges from Kansas City (EAX, shaded) and St. Louis (LSX) radars. Negative cloud-to-ground lightning ¯ashes from 1003 to 1340 UTC 10 January 1994 are shown by a square; positive ¯ashes by a /. UMN is Monett, Missouri upper-air station. Position of 07C surface isotherm shown by dashed line for center of lightning period. dynamic and thermodynamic environments of ele- vated thunderstorms in the eastern United States on the cold side of fronts. Stewart and King (1990) con- sidered the region separating frozen from liquid pre- cipitation in southern Ontario; thunder was heard by observers in one of two cases. Galway and Pearson (1981) focused on winter tornadoes in the central United States that usually had widespread blizzards, heavy snow, and/or extensive glazing on the cold side of the weather systems. Grant (1995) studied severe spring thunderstorms in the cold sector north of fronts without coincident frozen precipitation at the ground. Marwitz and Toth (1993) determined the synoptic-scale kinematic ®elds and radar structure of a warm-frontal snowband over Oklahoma ahead of a surface cold front; thunder was observed but was not the focus of the study. Beckman (1989) and Colman (1990a) recommended the use of lightning data to study winter thunder observations. The National Weather Service (NWS) Storm Pre- diction Center (SPC) is being established in Norman, Oklahoma (McPherson 1994), to provide guidance, coordination, and short-term forecasting for winter weather across the United States. The extent to which lightning data at SPC and other NWS of®ces can iden- FIG. 2. Surface maps for middle Mississippi Valley at 1300 (upper tify locations and times of signi®cant frozen precipi- panel) and 1500 UTC (lower panel) 10 January 1994. Station models tation is a motivation for the present study. This study and all symbols are conventional; temperature and dewpoints in 7F; full wind barb Å 5ms01 (10 kt); half barb Å 2.5 m s 01 (5 kt). will compare lightning with radar and surface data, and Pressure contours (solid) are in mb. Shading and ZR indicate regions synoptic- and subsynoptic-scale conditions at times and with mainly freezing rain; S- for mainly snow. Warm front in south- places of ¯ashes. west part of map. /3q06 0226 Mp 600 Monday Dec 02 02:41 PM AMS: Forecasting (December 96) 0226 Unauthenticated | Downloaded 10/02/21 11:39 PM UTC DECEMBER 1996 NOTES AND CORRESPONDENCE 601 tem detection ef®ciency, the ratio of ¯ashes detected WSR-88D radar re¯ectivity and echo-top heights at by the network to the number of CG ¯ashes that actu- several sites were obtained in near-real time as products ally occurred, was estimated as 65%±80% for the from WSI Corporation (a private vendor). NWS fore- NLDN. Accuracy and detection ef®ciency are based on casts and discussions were obtained from the Experi- widely available sources such as Orville (1993, 1994), mental Forecast Facility collocated at the Norman since no independent ground truth was collected at the NWS Forecast Of®ce (NWSFO) described by Janish times and locations of the storms. et al. (1995). FIG. 3. Radar re¯ectivity from WSR-88D radar at Kansas City (EAX) at 0900 (top) and 1200 UTC (bottom) 10 January. Circle is 230-km (124 n mi) range from radar. /3q06 0226 Mp 601 Monday Dec 02 02:41 PM AMS: Forecasting (December 96) 0226 Unauthenticated | Downloaded 10/02/21 11:39 PM UTC 602 WEATHER AND FORECASTING VOLUME 11 FIG. 4. Radar re¯ectivity from WSR-88D radars at Kansas City (EAX, top) and St. Louis (LSX, bottom) at 1500 UTC 10 January. Circle is 230-km (124 n mi) range from radar. 3. 10 January 1994 case ¯ashes lowering positive charge to ground and 11 negative ¯ashes from 1003 to 1340 UTC where sur- a. Lightning face temperatures are slightly above freezing. There The NLDN detected 27 CG ¯ashes in western is no recognizable con®guration to the ¯ashes, Missouri on 10 January (Fig. 1). There are 16 which differs from thunderstorms in Colman /3q06 0226 Mp 602 Monday Dec 02 02:41 PM AMS: Forecasting (December 96) 0226 Unauthenticated | Downloaded 10/02/21 11:39 PM UTC DECEMBER 1996 NOTES AND CORRESPONDENCE 603 (1990a,b) that originated in warm air south of warm was reported within both regions at 1500 UTC. Several fronts and moved north over subfreezing surface stations in the lightning region reported sleet (ice pel- temperatures. lets) at and just above the freezing point between 1000 The ratio of 59% positive ¯ashes is much higher and 1200 UTC, and freezing rain was observed in Mis- than the 4% detected over the entire United States souri starting at 1300. Surface temperatures were above during 3 years (Orville 1994); the ratio is usually freezing where lightning occurred, but freezing tem- lowest in summer.