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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 hodograph 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).

᭧ 1996 American Meteorological Society

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The fact that mesocyclones can be associated with thunderstorms in California's Central Valley was documented 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 outlined in previous paragraphs are responsible for the fact that no additional direct radar corroboration of a mesocyclone-induced tornado has appeared in the refereed literature until the present time. However, indirect evidence 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 severe 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 mesocyclone 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 supercells. 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 1/2Њ 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 recent 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.

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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-

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FIG.3.(Continued) (c) PC-Grids analysis of RAFS 700-mb heights (dam, thick solid), temperatures (ЊC, 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 . . .'' with regard to the development of such storms strong midtropospheric cold advection. More detailed in their region. In particular, only forecasters familiar description of the synoptic aspects of this weather type, with the regional qualitative implications of the syn- including a discussion of the nature of the quasigeo- optic pattern associated with severe thunderstorms will strophic forcing, may be found in Monteverdi and Qua- be able to anticipate the initial threat to their area of dros (1994). responsibility. The risk for supercellular development Terrain-induced quasi-stationary mesoscale troughs then can be localized only by those who are familiar or low pressure areas often develop in California's with the techniques of surface subsynoptic analysis and Central Valley subsequent to the passage of surface who are able to assess the combined roles of buoyancy cold fronts. Such troughs form under the area of syn- and shear in in¯uencing thunderstorm type.

2. Dynamic and thermodynamic controls on the Central Valley severe storms 2 Note that in estimating the ®rst forcing function for the quasigeostrophic v in the lower midtroposphere that the assumption may a. Overview of synoptic and mesoscale controls be made that surface vorticity advection is minimal. Hence, the differential vorticity advection forcing term for the surface to 500-mb layer may be approximated by the 500-mb CVA pattern. Thus, qua- ``Cold sector'' convection in California occurs most sigeostrophic v is proportional to 500-mb CVA but only if the forc- often north or northwest of surface cold fronts advanc- ing function related to the shape of the temperature advection ®eld ing from the Paci®c. In the typical sequence associated is negligible.

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FIG. 4. As in Fig. 3 except for 0000 UTC 6 March 1994. optically forced upward motion associated with the Valley, served as foci for the formation of both the main middle- and upper-tropospheric trough when 1986 Vina and the 1992 Oroville±Marysville tornadic there is signi®cant cross-mountain ¯ow in the lower supercellular thunderstorms (Braun and Monteverdi third of the troposphere (Weaver 1962; Braun and 1991; Monteverdi and Quadros 1994). Monteverdi 1991). The eastern portions of such sur- Other important and common features associated face features are associated with southerly or south- with the pattern described above (see Fig. 2) include easterly ¯ow, while the western portions often are char- strong (ú15ms01) low-level southeasterly jets in the acterized by dry, subsident ¯ow from the Coast Range. 600±1500-m layer AGL in the eastern portion of the Such ``leeside troughs,'' centered in the Sacramento Central Valley (adjacent to the foothills of the Sierra

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FIG.4.(Continued)

Nevada) (Parish 1982; Staudenmier 1994) and the ver- updraft in strong-shear environments. For situations tical motion ®eld associated with mobile surface sub- of moderate and strong unidirectional shear (char- synoptic troughs (usually attributable to the upper-tro- acterized by a straight hodograph) the effect is sym- pospheric divergence in the left-front quadrant of ad- metric about the shear vector, leading to the devel- vancing jet streaks). Destabilizing in¯uences include opment of mirror-image supercell storms that move moderate-to-strong midtropospheric cold advection ``off the hodograph.'' 3 Such storm splitting takes (Monteverdi and Quadros 1994; Braun and Monteverdi place when the midlevel rotational couplet on the 1991) and surface sensible heating (Reed and Blier ¯anks of the original cell produces dynamic vertical 1986; Monteverdi et al. 1988). All of these phenomena pressure gradients that generate new updrafts be- act together to produce low static stabilities (which in neath both the cyclonic and anticyclonic members of turn increase the ef®ciency of the quasigeostrophic the couplet. The new updrafts strengthen the vorticity forcing for vertical motion) and/or shear characteris- further through stretching, which in turn enhances tics favorable for severe storm genesis. the dynamic vertical pressure gradients on the new cell ¯anks. Thus, the dynamically induced vertical b. Overview of buoyancy and shear pressure gradient forces are responsible for the observed deviate motion of supercells. Cold sector thunderstorms are often associated with Johns and Doswell (1992) (and many others) point very low tropopause heights and equilibrium levels and out that certain values of the environmental shear and rather small values of buoyancy. For example, the larg- storm motion are associated with an updraft that tilts est value of convective available potential energy existing horizontal vorticity into the vertical producing (CAPE) or positive buoyancy (B/) for the three win- a ``helical'' updraft (i.e., the mesocyclone). One useful ter tornadic storms documented in Monteverdi and 01 measure of such mesocyclone potential is the storm- Quadros (1994) was estimated to be 552 J kg . Even relative environmental helicity (SREH) of the 0±2- or the warm season Vina event (Braun and Monteverdi 0±3-km layer, obtained easily from analyses of the ho- 1991) was associated with only moderate buoyancy 01 dograph if the storm motion is known or can be esti- (1804 J kg ). Most of the positive area on such mated. soundings is contained beneath the 500-mb level, with the 700-mb lifted index (LI) a better ``indicator'' of buoyancy than is the traditional 500-mb LI. As shown in modeling studies completed by Weis- 3 Off the hodograph means not parallel to the mean shear vector. man and Klemp (1982), Rotunno and Klemp Storm motion vectors that lie to the right or the left of the mean shear (1982), Weisman and Klemp (1986), and others, vector when plotted on hodograph paper are off the hodograph. Such shear-induced pressure forces augment the buoyant storm motion is also termed ``deviate.''

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FIG. 5. (a) Simulated FAT sounding, 2000 UTC 5 March 1994, as produced on SHARP. Dashed line shows surface lifted parcel. Numbers next to plotted winds are the actual wind speed observations (kt). Information on left margin relates to various parameters calculated by SHARP. (b) Simulated FAT hodograph (winds at 250-m intervals), 2000 UTC 5 March 1994, as produced on SHARP. Straight lines show SREH (m2 s02 ).

Johns et al. (1993) and Johns and Doswell (1992) 3. Dynamic and thermodynamic controls on the show that strong and violent supercellular tornadoes Fresno storm occurred in low-buoyancy environments with high values of SREH. Similarly, McCaul's (1991, 1990) results a. Synoptic controls prove that rotating convective columns can be sus- tained in landfalling hurricane environments with The authors adopted the approach of Doswell (1987) buoyancy values as small as those observed for cold in discussing this case study. The threat for severe thun- sector thunderstorm events in California when shear- derstorm development can be related to a cooperative induced vertical pressure gradient forces are great. interrelationship between ``dynamics'' and ``thermo-

FIG. 6. (a) NLC sounding, 2000 UTC 5 March 1994, as produced on SHARP. Dashed line shows surface lifted parcel. Numbers next to plotted winds are the actual wind speed observations (kt). Information on left margin relates to various parameters calculated by SHARP. (b) NLC hodograph (winds at 500-m intervals), 2000 UTC 5 March 1994, as produced on SHARP. Information on left margin relates to various parameters calculated by SHARP for the right mover. Motion of left mover also shown (discussed in section 4b).

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FIG. 7. Reproduction of PPI scope at 2225±2237 UTC (1425±1437 PST) 5 March 1994 radar traces at 21/2Њ tilt. Range rings indicated at 32-km (20 mi) intervals. (Courtesy T. Henderson, Atmospherics Incorporated, Fresno, CA.) dynamics.'' Here, the term dynamics encompasses all and/or creating a focus in a small-scale area where the synoptic-scale (quasigeostrophic, i.e., related to mobile wind shear pro®le and/or buoyancy is liable to become troughs and ridges, etc.) and some subsynoptic-scale most favorable for convective development (Doswell (i.e., jet streaks) vertical motion ®elds that contribute 1985). to layer lifting, lowering of the level of free convection, On 5 March 1994, a surface cold front passed and thus, increases in buoyancy. Thermodynamics re- through the northern two-thirds of California (not fers to the buoyancy available regionally, as quanti®ed shown). The front was associated with a middle- and by CAPE. upper-tropospheric short-wave trough that had moved The dynamics may also contribute to the wind-shear southeastward from the Gulf of Alaska around a long- pro®les that together with the buoyancy determine the wave ridge axis situated at approximately 135ЊW. The general type of convection (i.e., single cell, multicell, middle- and upper-tropospheric pattern differed in or supercell), as described above. Subsynoptic analy- some respects from that associated with typical Central ses of hourly surface data may be used to isolate mech- Valley funnel cloud and tornado events. Although pro- anisms (i.e., fronts, trough lines, out¯ow boundaries) gressive, strongly dif¯uent, negatively tilted troughs that can localize the convective threat by providing lift were important synoptic features in the events docu-

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in a region of 700-mb cold temperature advection. Both the upward motion ®eld and the destabilization associated with the midtropospheric cold temperature advection could be expected to pass over the San Joaquin Valley during the time of peak diurnal afternoon heating. During the 12-h period ending 0000 UTC 6 March 1994, the 500- and 300-mb trough deepened considerably (Figs. 4a and 4b). Strong 500-mb CVA and CIVA (not shown) occurred over the central portions of the state. The 700-mb upward motion ®eld (Fig. 4c) was centered over the central portions of California including the San Joaquin Valley. Note that 700-mb cold advection continued over this area. Thus, destabilization both due to the dynamically induced layer lifting and differential temperature advection was strong over the Central Valley during the time of peak diurnal heating. An additional point should be made about the upper- tropospheric wind patterns for this case (Figs. 3b and 4b). While southwesterly ¯ow was evident over the entire Central Valley at both times, wind speeds slack- ened considerably over the Sacramento Valley by 0000 UTC. Thus, the shear environment over the Sacramento Valley became less conducive for supercell development during the day, while that in the San Joaquin Valley remained favorable.

b. Buoyancy and wind shear parameters

FIG. 8. Radar re¯ectivity summary for northern and central Forecasters familiar with the potential for severe California 1935±2235 UTC 5 March 1994. weather in California in the synoptic pattern described above must estimate conditions in the Central Valley on the basis of modi®cations of the Oakland (OAK) mented in Monteverdi et al. (1988) and Braun and and/or Vandenburg Air Force Base (VBG) radiosonde Monteverdi (1991), the trough in this case was not data. As pointed out in Braun and Monteverdi (1991) negatively tilted. In addition, although the trough for and Monteverdi and Quadros (1994), the initial OAK this case was progressive, it intensi®ed and gained am- and VBG soundings almost always underestimate the plitude as it moved toward California. In fact, by 1200 buoyancy in the Central Valley, and the hodographs UTC 6 March (not shown), a closed circulation at all similarly underestimate the low-level wind shear in the levels above 850 mb had developed off the coast of portions of the Central Valley east of the quasi-station- south-central California. ary leeside trough that develops in this synoptic pattern. At 12 UTC 5 March, the axis of the main middle- Forecasters utilizing desktop computer programs and upper-tropospheric trough (Figs. 3a and 3b) was such as the Skew T/Hodograph Analysis and Research located nearly 800 km (500 mi) west of San Francisco. Programs (SHARP) (Hart and Korotky 1991) are able Two separate areas of 500-mb CVA and, in this case, to estimate afternoon conditions on the basis of CIVA (not shown) were associated with the distur- thoughtful alteration of the 1200 UTC information. A bance at the latitude of central California. The area of common valuable operational technique is to modify weak CVA over south-central California generally cor- the original data by inserting hourly surface tempera- responded to the position of the surface front. The sec- ture (Hales and Doswell 1982) and wind data for the ond stronger area of CVA was associated with a 36 region of interest and by modifying the wind infor- ms01 (70 kt) 300-mb wind maximum and probably mation of the lowest portion of the hodograph [de- accounted for the surface subsynoptic trough that ad- pending upon the height of the Coast Range at the lat- vanced through central California (discussed below). itude of interest; see Monteverdi and Quadros (1994) The 1200 UTC 5 March 700-mb vertical velocity for details]. ®eld (Fig. 3c) indicated that moderate upward motion Since time constraints are signi®cant in an opera- was taking place just off the central coastal sections of tional environment, no attempt to modify the sound- the state. Also, this vertical velocity ®eld was located ing±hodograph information beyond the changes out-

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FIG. 9. Reproduction of 2235 UTC composite radar re¯ectivity summary for a central California as received by San Francisco State University's Weather Graphics and Simulation Laboratory. Three-digit numbers (e.g., 380) are heights of the radar echo tops (100s of ft). Arrows show cell motion vectors. Note that while total re¯ectivity pattern (shading), northern storm cell motion, and northern storm re¯ectivity top were from SAC WSR-57, cell motion and re¯ectivity top for the southern storm were from Atmospherics Inc. 2-cm radar in Fresno. lined above were performed for this study. In addition, 2 km, it is very nearly straight. As such, mirror image soundings and hodographs as they appear in the storm splitting, as outlined in Weisman and Klemp SHARP output are used in this study to illustrate the (1986), could be expected. The shear through the 0± buoyancy and shear discussions below, since these are 6-km layer was 2.3 1 10 03 s 01 and that in the 0±3-km the graphical aids used commonly by forecasters. layer was 2.2 1 10 03 s 01 . Although shear values õ3.0 As an example, the 1200 UTC OAK sounding mod- 1 10 03 s 01 are considered ``weak'' (Weisman and i®ed in the manner described above for the surface con- Klemp 1986), the bulk Richardson number (BRN) of ditions at FAT for 2000 UTC (the time of genesis of 40 was within the range (5±40) for observed super- the Fresno storm) is given as Fig. 5a. The dashed line cells. shows a surface lifted parcel, and the environmental Storm motion for the right mover was estimated by lapse rate and dewpoint lapse rate are shown as solid the SHARP as a function of the winds (plotted as dots lines. The CAPE calculated for this sounding is 1961 at 250-m intervals). The 0±3-km layer SREH of 143 Jkg01, a value that is probably near the upper bound m2 s 2 suggests that the updraft rotation in the right- of buoyancy for California storms. This is re¯ected in moving supercell would achieve values observed for a very high equilibrium level of 10 000 m (30 700 ft) weak mesocyclones (Davies-Jones et al. 1990; Johns for a winter thunderstorm in California. and Doswell 1992; Moller et al. 1994). The OAK mandatory and signi®cant level wind data Forecasters examining this information might at least modi®ed for conditions at FAT for 2000 UTC is given expect some of the convective cells in the San Joaquin as Fig. 5b. As pointed out in Monteverdi and Quadros Valley to have the steadiness and propagation charac- (1994), the morning OAK hodograph can be used as teristics of supercell storms. Since middle- and upper- an estimate of the afternoon wind pro®le for the Central tropospheric wind speeds and vertical wind shear Valley because the synoptic features described in sec- would be expected to increase slightly over the Central tion 2b are progressive; over short time periods, the Valley as the disturbance west of central California ap- wind environment characteristic of the OAK morning proached (see section 2b), forecasters should expect RAOB can be assumed to translate eastward to the val- the possibility of afternoon supercell development ley area by early afternoon. (This methodology should somewhere in the valley to increase. be used with caution for fast-moving weather systems.) The hodograph shown in Fig. 5b illustrates the dan- Although this estimated hodograph shows some ten- ger of prejudging the shear environment and the threat dency to be curved in a clockwise sense in the lowest for supercellular development by cursory glances at

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was obtained at NLC (see Fig. 1) at 2000 UTC. The plot of this data (Fig. 6a) can be considered a ``proximity sounding,'' since the RAOB sampled the environment in the Fresno area while the storm was forming (discussed below). Figures 5a and 6a are similar, a fact that tends to justify the use of the modi®cation proce- dure discussed above. The CAPE and lifted indexes calculated for the NLC sounding of 1743 J kg 01 and 06ЊC are only slightly smaller than those calculated for the estimated FAT sounding discussed above (1961 Jkg01 and 08ЊC). The 2000 UTC NLC hodograph is given as Fig. 6b. Storm motions for the right- and left-moving storms (discussed in section 4b) are true motions as obtained from half-hourly radar screen plots from both the 2-cm radar operated by Atmospherics Incorporated in Fresno and from the SAC WSR-57. As is the case for Fig. 4b, the hodograph shows the shear through the 0±7-km layer. In this case, winds are shown at 500-m intervals. Considerably greater shear is evident in the NLC hodograph than in the estimated FAT hodograph, with a 0±3-km shear of 4.6 1 10 03 s 01 , a 0±6-km shear of 3.9 1 10 03 s 01 , and a SREH in the 0±3-km layer of 2 2 FIG. 10. AVHRR visible satellite image for 1607 UTC 5 March 201 m s . Both values of shear are considered mod- 1994. Positions of leeside trough (dash±dotted line) and subsynoptic erate to strong (Weisman and Klemp 1986), and the trough (dashed) are indicated. SREH value is indicative of mesocyclone development with weak tornadoes (Davies-Jones et al. 1990). The hodograph still presents a rather straight aspect, wind information. In the Central Valley tornado and with some clockwise turning of the wind evident to 3 funnel cloud cases documented in the literature cited km suggesting some slight favoring of the cyclonic in this study, surface southeasterly ¯ow dominated the member of any splitting storm pair. Buoyancy and boundary layer. This is in agreement with the ``con- shear together contribute to a BRN of 21, well within ventional wisdom'' that wind shear favorable for se- the range documented for supercells (Weisman and vere thunderstorm development (and, in some cases, Klemp 1986). Forecasters examining this information supercell development) often is associated with south- should expect splitting supercellular storms, with pref- easterly or southerly winds at the surface, veering to erential development of the southern member. southwesterly, westerly, or northwesterly in the midtroposphere. In fact, the only necessary condition for 4. Radar and subsynoptic analyses supercell development is that there is suf®cient envi- a. Discussion of the radar evidence ronmental shear present in combination with some min- Even when California thunderstorms are near radar imum value of buoyancy (Weisman and Klemp 1986). sites, the rugged and elevated topography often limits Actual wind directions are signi®cant only in that they the usefulness of radar observations in all but a few in¯uence the shape of the hodograph, which in turn locations. This, together with the factors discussed in plays a role in whether cyclonic or anticyclonic super- section 1, accounts for the fact that there has been little cells are favored. direct documentation in the refereed literature of the In the case of the wind pro®les discussed for the existence of mesocyclones in California thunderstorms. Fresno storm, the hodograph characteristics were such The siting of WSR-88D Doppler radars in several lo- that the observed surface northerly winds were neces- cations of the state, particularly in the Fresno area sary to support a wind shear environment favorable for (Hanford) and in Davis, should begin to provide direct the development of mirror-image storms. In fact, a sur- observation of severe storm features in the Central face southeasterly wind inserted into the hodograph Valley. shown in Fig. 5b would have created a backing of the While the Fresno storm was near the ``horizon'' for wind shear vector with height in the lower levels with the Sacramento (SAC) WSR-57 (since decommis- attendant suppression of the cyclonic right-moving up- sioned), 4 it passed nearly directly over the 2-cm draft and enhancement of the left-moving updraft of the split pair. Forecasters were aided in their diagnosis of the prestorm environment for this case when a special RAOB 4 The Davis WSR-88D was operating in test mode only, and the

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oes or by apparent split of the initial echo. According to Weisman and Klemp (1986), the process of storm splitting involves the production of new updraft cells (and radar echoes) on the ¯anks of the initial cell. One way in which such a split may be evidenced is that radar may show formation of distinct new echoes with subsequent weakening or dissipation of the ®rst. While both of the new storms appeared on radar traces for the SAC WSR-57, by the time the southern storm was affecting the Fresno area it was over 200 km (120 mi) away from the SAC WSR-57 radar. Thus, re¯ectivity intensity for the southern storm was considerably underestimated, and intensity estimates from the Atmospherics Incorporated radar were substituted. The discontinuous motion of the northern storm evident in Fig. 8 probably was related both to the decreasing wind speeds and shear in the northern half of California (discussed above) and to the complex interactions between out¯ow boundaries and the rugged topography of the Sierra Nevada. Figure 9 is the actual composite re¯ectivity, radar tops (ft 110 3 ), and cell motion vectors (in knots) from the SAC WSR-57 at 2235 UTC as received by San Francisco State University's Weather Graphics and Simulation Laboratory. The motion vector of the southern storm as obtained from the Atmospherics Incor- porated radar traces is plotted to illustrate the divergent motion of the two main cells. It should also be pointed out that the echo top for the southern storm was estimated as 11.6 km (38 000 ft) by the Atmospherics In- FIG. 11. Subsynoptic analysis of altimeter settings (0.02-in. inter- corporated radar. vals) for 19 UTC 5 March 1994. Dashed lines show postfrontal The radar evidence summarized schematically in trough entering coastline and leeside trough in Central Valley; dash± dotted line indicates out¯ow boundary. Figs. 8 and 9 suggests that the two cells initially were mirror images that had split from the ®rst storm. It is clear that operational meteorologists in California should become familiar with all facets of severe storm weather radar operated by Atmospherics Incorporated behavior (e.g., splitting, training, bowing, etc.). in Fresno (see Fig. 1). The SAC WSR-88D indicated a re¯ectivity greater than 55 dBZ at 2232 UTC while the storm was passing through the Fresno area but pro- b. Subsynoptic analyses vided no corroboration of rotational features. However, The surface subsynoptic features for the prestorm the Atmospherics Incorporated radar showed that the environment included surface lows at the northern and storm had a re¯ectivity core of greater than 60 dBZ for southern ends of the Central Valley, a leeside trough more than 35 min. More importantly, a well-de®ned that extended between these two lows and a post-fron- hook echo was observed roughly simultaneously with tal trough that was related to the southern vorticity public reports of funnel clouds and possible tornado maximum discussed in section 2b. A cloud band that touchdown (Fig. 7). Considering the radar site's dis- marked the position of the subsynoptic trough could be tance of 16 km (10 mi) from the hook and the beam followed on satellite imagery as it moved southeast- tilt of 21/2Њ, the hook was occurring at an elevation of ward across the state [see Fig. 10, the visible Advanced approximately 760 m (2500 ft). Very High Resolution Radiometer (AVHRR) satellite The Fresno storm was the south member of a pair of image for 1607 UTC]. storms forming on the ¯anks of an initial cell that had The subsynoptic cloudband was associated with a developed southeast of the San Francisco Bay region weak surface low that moved across the southern San (Fig. 8). The radar data available does not clearly show Francisco Bay region at about 1900 UTC (Fig. 11). whether the cells formed discontinuously as new ech- The initial echo formed near the intersection point between the postfrontal trough and the leeside trough in Hanford and San Francisco Bay region WSR-88Ds were not yet op- the valley (indicated by I in Fig. 11). Such intersec- erational. tions of subsynoptic and leeside troughs also were

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FIG. 12. (a) Analysis of CAPE (J kg01) at 2100 UTC and (b) of BRN at 2100 UTC.

shown to be important foci for the Central Valley tor- ternoon hours indicated that relatively clear skies were nadic storms discussed in Braun and Monteverdi observed in the south-central San Joaquin Valley, al- (1991) and Monteverdi and Quadros (1994) (see lowing boundary layer temperatures there to increase Fig. 2). more during the afternoon hours than in the regions The environment in the storm genesis area was char- south of the nearly stationary cold front.5 Thus, a more acterized by temperatures in the middle- and upper- buoyant air mass (associated with higher values of 50ЊsF and dewpoints in the lower to middle 50ЊsF. Note CAPE than those calculated for the genesis area of the that southerly winds were occurring east of the leeside storm) was pooled in the south-central portions of the trough in the Sacramento Valley but that northerly San Joaquin Valley (Fig. 12a). winds were observed for most of the San Joaquin As the southern storm moved through the valley at Valley. around 2100 UTC, it ingested this relatively warm The analyses for 2000 and 2100 UTC (not given) moist air; this undoubtedly contributed to the severity show that the low south of FAT had intensi®ed as the of the cell in the Fresno area. Also, the combination of main area of CVA and CIVA associated with the mid- CAPE values and shear in the San Joaquin Valley gave dle- and upper-tropospheric trough approached the cen- BRN values in the range documented for supercells tral coast, as discussed in section 2b. In addition, the (Fig. 12b). It should be pointed out that plots shown leeside effects (discussed in section 2a) produce in Figs. 12a and 12b were obtained by substitution of troughing in the Central Valley in proportion to the hourly surface observations into the NLC sounding/ height of the Coast Range to the west and the strength hodograph information. Similar plots can be generated of the cross-mountain ¯ow (Weaver 1962). Since el- quickly by desktop computer-based software in the op- evations are highest west of the north end of the Sac- erational setting. However, only forecasters familiar ramento Valley and west of the south end of the San with the signi®cance of the values of CAPE and BRN Joaquin Valley, lowest surface pressures are often ob- to severe thunderstorm development would be able to served near Redding in the north and Bakers®eld in the use such products thoughtfully. south when cross-mountain wind¯ow is uniform from north to south. Relatively high dewpoints (mid-50ЊsF to low 60ЊsF) and warm temperatures (high 60Њs to low 70Њs) char- 5 The 1000±500-mb thickness gradients (not shown) still sup- acterized the south-central portion of the San Joaquin ported the position of the cold front over Southern California, how- Valley by 2100 UTC. The subsynoptic data for the af- ever.

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The northern storm had moved into the data-void area of the Sierra Nevada. No inferences can be made about the characteristics of the air mass that this storm was ingesting late in its life cycle. However, as discussed in section 3a, the shear environment over northern California had weakened dramatically since the early morning hours. It could be anticipated that the BRN in northern California also would have passed out of the range favorable for supercellular convection de- spite increases in buoyancy due to diurnal heating effects. In addition, the motion vector of the storm plotted on the NLC hodograph (Fig. 6b) indicates that the left mover was propagating on the hodograph, close to the mean wind. This would also suggest that the left mover was not supercellular for this case and that it more nearly presented the left-¯ank multicellular growth described for many cases in Weisman and Klemp (1986). The AVHRR 2335 UTC infrared satellite image (Fig. 15) shows the positions of the left- and right- moving storms. Overlaid on the image are schematic thunderstorm tracks and the positions of new cells that had developed between 2200 and 2300 UTC. Other cloudiness and/or cells can be seen in the area west of the Nevada border and somewhat east of the leeside trough line. This cloudiness forms on the windward

FIG. 13. As in Fig. 11 except 2200 UTC 5 March 1994. Shaded areas show radar echoes from Atmospherics Inc. 2-cm radar in Fresno.

Figure 13 shows the surface pattern at about the time of greatest damage in the Fresno area. Between 2200 and 2300 UTC (Figs. 13 and 14), the mesocyclone passed just north of FAT, as did the hail swath and the damaging winds. The two public reports of a tornado were made at about the time of Fig. 13. Between 2200 and 2300 UTC the gust front±out¯ow boundary from the storm passed FAT and NLC. This was marked by dramatic drops in temperature of 13Њ and 8ЊF, respec- tively, at the two stations. The radar echo for the southern storm as seen by the Atmospherics Incorporated radar is indicated by gray shading in Fig. 13. In addition, a new storm that had formed on the south ¯ank is also shown. Over the next several hours, several new cells seemed to develop on the out¯ow boundary of Fresno storm as it evolved into a multicellular convective system. Since succeeding subsynoptic analyses indicated that temperatures had continued to increase in the extreme southern San Joa- quin Valley, it is likely that buoyancy increased to such a degree in that area that the ratio of buoyancy to shear as measured by the BRN would have passed into the range for multicellular convection. FIG. 14. As in Fig. 11 except 2300 UTC 5 March 1994.

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sonal communication; E. J. Null, WFO San Francisco± Monterey, 1994, personal communication]. Buoyancy and shear parameters for this case were comparable to those documented in the literature for splitting supercellular storms. The dominance of the southern storm was partly due to the ingestion of air with higher CAPE in the southern San Joaquin Valley. In addition, wind shear pro®les remained consistent with possible preferential development of the southern member but, as the day progressed, became progres- sively more unfavorable for development of the northern member. Slight clockwise curvature shear in the lowest layers may also have favored the southern storm. This study also shows that useful estimations of afternoon thermodynamic and wind-shear environments in California's Central Valley can be made on the basis of alterations of the 1200 UTC OAK radiosonde data. Buoyancy and hodograph aspect based upon a ``proximity'' sounding taken at Lemoore Naval Air Station were similar to those inferred for Fresno by substitution of surface hourly weather information into the morning OAK sounding±hodograph. Although the magnitude of the shear was underestimated, forecasters examining the inferred sounding at least would have recognized the potential for splitting storms in the Central Valley FIG. 15. AVHRR infrared satellite image for 2325 UTC 5 March and would have been concerned about possible super- 1995. Positions of leeside trough (dash±dotted line), inferred out¯ow cellular development. boundaries (dash±dot±dot), and subsynoptic trough (dashed) are indicated. This study documents yet another California supercell event associated with signi®cant damage. Blier and Batten (1994) summarized many other damaging Cal- ifornia severe thunderstorm events associated with tor- side of the Sierra Nevada and often complicates storm nadoes, some mesocyclone induced. They pointed out tracking from the Central Valley eastward. The evolv- that forecasters and members of the public alike need ing mesoscale convective system can be seen on the to maintain an awareness that such storms are features extreme southern end of the image. of the climatology of the state. The present study underscores the fact that it is only a matter of time before 5. Discussion and conclusions loss of life occurs in California from such events. It is clear that forecasters, in order to fully serve the public, This study documents a supercellular storm in Cal- must be able to diagnose the patterns that give rise to ifornia's San Joaquin Valley. The mesocyclone asso- such storms and must be able to apply current knowl- ciated with this storm was observed on radar as a hook edge on evolution of storm structure. echo. This represents only the second radar-corrobo- rated mesocyclone in California to be reported in the Acknowledgments. The authors thank the reviewers refereed literature. Previous studies (Monteverdi and for their helpful suggestions. We particularly acknowl- Quadros 1994; Braun and Monteverdi 1991) pointed edge referees Warren Blier and Morris Weisman; this out that shear pro®les favorable for supercellular de- manuscript is much improved because of their pains- velopment were observed with damaging thunder- taking reviews. Radar data from Atmospherics Incor- storms in California. However, no direct radar corrob- porated was courtesy of Mr. Tom Henderson. Other oration of mesocyclone development or occurrence radar information, sounding data, PC-Grids data, and was available for these cases. storm information was provided to the authors by Mr. Since the 5 March 1994 Fresno storm, the Davis, David Reynolds, science operations of®cer; Mr. E. Jan Hanford, and San Francisco Bay region WSR-88D ra- Null, lead forecaster; and Mr. John Quadros, warning dars have been commissioned. Both of the valley radars preparedness meteorologist, of the WFO in San Fran- have since reported long-lived rotational couplets and cisco±Monterey. Thanks are given to Dr. David Demp- mesocyclones with damaging thunderstorms in the sey, site manager, Weather Graphics and Simulation Central Valley [M. Staudenmaier, National Weather Laboratory, San Francisco State University, for design- Service Forecast Of®ce (WFO) Sacramento, 1994, per- ing scripts to produce the subsynoptic plots.

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