246 WEATHER AND FORECASTING VOLUME 11 A Supercell Thunderstorm with Hook Echo in the San Joaquin Valley, California JOHN P. MONTEVERDI Department of Geosciences, San Francisco State University, San Francisco, California STEVE JOHNSON Association of Central California Weather Observers, Fresno, California (Manuscript received 30 January 1995, in ®nal form 9 February 1996) ABSTRACT This study documents a damaging supercell thunderstorm that occurred in California's San Joaquin Valley on 5 March 1994. The storm formed in a ``cold sector'' environment similar to that documented for several other recent Sacramento Valley severe thunderstorm events. Analyses of hourly subsynoptic surface and radar data suggested that two thunderstorms with divergent paths developed from an initial echo that had formed just east of the San Francisco Bay region. The southern storm became severe as it ingested warmer, moister boundary layer air in the south-central San Joaquin Valley. A well-developed hook echo with a 63-dBZ core was observed by a privately owned 5-cm radar as the storm passed through the Fresno area. Buoyancy parameters and ho- dograph characteristics were obtained both for estimated conditions for Fresno [on the basis of a modi®ed morning Oakland (OAK) sounding] and for the actual storm environment (on the basis of a radiosonde launched from Lemoore Naval Air Station at about the time of the storm's passage through the Fresno area). Both the estimated and actual hodographs essentially were straight and suggested storm splitting. Although the actual CAPE was similar to that which was estimated, the observed magnitude of the low-level shear was considerably greater than the estimate. The bulk Richardson number for the observed conditions (BRN Å 21) was well within the range observed for supercells elsewhere in the country. Both the estimated and observed 0±3-km storm- relative helicities were in the range observed for mesocyclone development. Forecasters familiar with the relation of buoyancy, shear, and hodograph characteristics to the development of severe storms could have made useful inferences about the potential for severe weather in the Central Valley on the basis of simple modi®cation of the morning OAK sounding. 1. Introduction reports of funnel clouds and straight-line winds in ex- cess of 30 m s 01 (58 knt). Damage (mostly related to Between 2200 and 2245 UTC 5 March 1994, severe the hail swath) was estimated at $12 million in the thunderstorms passed through central Fresno County in Fresno area alone (Johnson 1994). California's San Joaquin Valley. Three reports of fun- Severe thunderstorms in California remain poorly nel clouds were tallied (USDC 1994) at Kerman, west- documented features. Moller et al. (1994) point out that ern Fresno, and Lemoore (NLC), 24 km (15 mi) west, supercells often go undetected in sections of the coun- 5 km (3 mi) west, and 20 km (12 mi) southwest, re- try in which such storms are rare because of precon- spectively, of Fresno Air Terminal (FAT, see Fig. 1 for ceived notions (both on the part of forecasters and the locations). The hail swath associated with the storm public) that such storms simply do not occur there. (hereafter referred to as the Fresno storm) extended Blier and Batten (1994) and Monteverdi (1993) verify from Kerman through Fresno to Clovis and left drifts that this has been a problem in California as well.1 In over 1/3 m (1 ft) deep in places. Hailstones as large as addition, the detection of supercells tends to be com- 2.9 cm (11/8 in.) were observed in Kerman and in plicated in California by the fact that the cold season Fresno, while there were uncon®rmed reports of larger severe thunderstorms in this state occur in environ- stones in downtown Fresno and points east (Johnson ments characterized by low equilibrium levels and have 1994). There were two uncon®rmed public reports of tops generally below 10 000 m (30 000 ft). Using the a tornado striking a business in Fresno, and numerous 1 The authors hasten to point out that local forecaster awareness of Corresponding author address: Dr. John P. Monteverdi, Depart- the severe thunderstorm forecasting problem in California has in- ment of Geosciences, San Francisco State University, 1600 Holloway creased dramatically (E. J. Null, WFO, San Francisco±Monterey, Ave., San Francisco, CA 94132. 1994, personal communication; M. Staudenmaier, WFO, Sacra- E-mail: [email protected] mento, 1994, personal communication). q 1996 American Meteorological Society /ams 3q04 0212 Mp 246 Friday May 24 10:58 PM AMS: Forecasting (June 96) 0212 Unauthenticated | Downloaded 09/30/21 06:43 PM UTC JUNE 1996 MONTEVERDI AND JOHNSON 247 The fact that mesocyclones can be associated with thunderstorms in California's Central Valley was doc- umented as early as 1982. The reader is referred to Carbone (1982, 1983) for comprehensive analyses of Doppler radar±derived re¯ectivity and velocity ®elds documenting a deep mesocyclone associated with a damaging tornado near Sacramento. The factors out- lined in previous paragraphs are responsible for the fact that no additional direct radar corroboration of a me- socyclone-induced tornado has appeared in the refereed literature until the present time. However, indirect ev- idence on the basis of calculated or inferred buoyancy and shear characteristics has been reported in recent years. For example, Hales (1985), Braun and Monte- verdi (1991), and Monteverdi and Quadros (1994) show that the interaction of topographic factors and low-level ¯ow patterns in certain cold sector weather types create an environment favorable for mesocyclone development. Finally, forecasters accustomed to anticipating se- vere thunderstorms in patterns resembling the Great Plains ``warm sector'' (i.e., associated with air with FIG. 1. Location map. Shaded areas have elevations greater than 300 m (approximately 1000 ft). Radars in operation at time of Fresno storm (large solid squares) were conventional weather radars: SAC WSR-57 and a 2-cm operated by Atmospherics Inc. in Fresno. WSR- 88D radars not operating at time of Fresno storm, but subsequently commissioned, are shown as open crossed squares: Santa Cruz Mountains, Davis, and Hanford. Location of 2000 UTC proximity sounding at Lemoore Air Force Base (NLC) shown as solid circle. Locations of other severe weather reports shown as small solid squares. criterion that a thunderstorm is supercellular if a me- socyclone extends through a substantial depth of the storm (more than 1/3) (Moller et al. 1994), even low tilt scans of storms by the existing WSR-88D radar sites will ``overshoot'' rotational signatures in California su- percells. In the case of the San Francisco Bay area, WSR-88D in the Santa Cruz Mountains [base elevation of nearly 1000 m (Ç3000 ft) above ground level (AGL)], the lowest tilt angle of 1/27 means that no features beneath an elevation of 4000 m (Ç13 000 ft) will be detected for thunderstorms in the northern portion of the San Francisco Bay region and the nearby portions of the Central Valley. Both of these are regions in which re- cent mesocyclone-induced tornadoes associated with thunderstorms having relatively low tops have been ob- FIG. 2. Schematic chart showing important mesoscale features served [i.e., tops on the order of 8000 m (Ç26 000 ft); associated with severe thunderstorm development in the Central Monteverdi and Quadros (1994)]. Valley. /ams 3q04 0212 Mp 247 Friday May 24 10:58 PM AMS: Forecasting (June 96) 0212 Unauthenticated | Downloaded 09/30/21 06:43 PM UTC 248 WEATHER AND FORECASTING VOLUME 11 FIG. 3. (a) PC-Grids analysis of Regional Analysis Forecast System (RAFS) 500-mb heights (dam, solid) and absolute vorticity (1005 s01, dashed) for 1200 UTC 5 March 1994. (b) PC- Grids analysis of RAFS 300-mb heights (dam, solid) and isotachs (tens of kts) for 1200 UTC 5 March 1994. high equivalent potential temperature in the boundary without mesocyclones (e.g., Halvorson 1971; Cooley layer) may have dif®culty envisioning supercell devel- 1978). Recent studies by Hales (1985), Braun and opment in California. Most severe thunderstorms in Monteverdi (1991), and Blier and Batten (1994), northern California, for example, occur in a cold sector, however, point out that severe thunderstorms are an low-buoyancy environment that, until recently, was important feature of the climatology of certain regions thought to be characterized only by thunderstorms in California and that some of these storms undoubt- /ams 3q04 0212 Mp 248 Friday May 24 10:58 PM AMS: Forecasting (June 96) 0212 Unauthenticated | Downloaded 09/30/21 06:43 PM UTC JUNE 1996 MONTEVERDI AND JOHNSON 249 FIG.3.(Continued) (c) PC-Grids analysis of RAFS 700-mb heights (dam, thick solid), tem- peratures (7C, thin solid), and vertical velocities (mbs01, negative values indicate rising motion) for 1200 UTC 5 March 1994. edly are supercellular (Braun and Monteverdi 1991; with severe thunderstorm events in the Central Valley Monteverdi and Quadros 1994). of California (Monteverdi and Quadros 1994), a mod- The purpose of this paper is to add to the sparse erate-to-strong surface disturbance passes through literature on California supercells. The present study northern and central California. The surface distur- also presents the ®rst documentation of a hook echo for bance is typically associated with a middle- and upper- a California storm since the Carbone studies cited tropospheric short-wave trough moving southeastward above. Finally, this study underscores the observation along the upstream side of a long-wave trough. The made in Moller et al. (1994) that forecasters in portions short-wave trough may often be negatively tilted and of the country where supercells are infrequent need to is almost always associated with moderate-to-strong make special efforts to acquaint themselves with ``. midtropospheric cyclonic vorticity advection (CVA), 2 the pertinent meteorological questions to be asked cyclonic isothermal vorticity advection (CIVA), and .