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Home , Anticyclonic storm, Outflow boundary, Supercell, Tornado, Tornadogenesis

NOVEMBER 2001 PICTURE OF THE MONTH 2805

PICTURE OF THE MONTH

First WSR-88D Documentation of an Anticyclonic with Anticyclonic Tornadoes: The Sunnyvale±Los Altos, California, Tornadoes of 4 May 1998

JOHN P. M ONTEVERDI San Francisco State University, San Francisco, California

WARREN BLIER National Service Forecast Of®ce, Monterey, California

GREG STUMPF National Severe Laboratory, and Cooperative Institute for Mesoscale Studies, University of , Norman, Oklahoma

WILFRED PI Forecast Of®ce, Monterey, California

KARL ANDERSON Second , Sunnyvale, California

16 August 2000 and 12 June 2001

ABSTRACT On 4 May 1998, a pair of tornadoes occurred in the San Francisco Bay Area in the cities of Sunnyvale (F2 on the Fujita scale) and Los Altos (F1). The parent was anticyclonically rotating and produced tornadoes that were documented photographically to be anticyclonic as well, making for an extremely rare event. The tornadic thunderstorm was one of several ``pulse type'' that developed on out¯ow boundaries on the left ¯ank of an earlier-occurring thunderstorm east of San Jose. Satellite imagery showed that the tomadic moved northwestward along a sea-breeze boundary and ahead of the out¯ow boundary associated with the prior thun- derstorms. The shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature, and by shear and buoyancy pro®les that were favorable for anticyclonically rotating updrafts. Mesoanticyclones were detected in the Monterey (KMUX) data in association with each by the National Severe Storm Laboratory's (NSSL) new Detection Algorithm (MDA) making this the only documented case of a tornadic mesoanticyclone in the that has been captured with WSR- 88D level-II data. Analysis of the radar data indicates that the initial (Sunnyvale) tornado was not associated with a mesoanticyclone. The satellite evidence suggests that this tornado may have occurred as the storm ingested, tilted, and stretched solenoidally induced associated with a sea-breeze boundary, giving the initial tornado nonsupercellular characteristics, even though the parent thunderstorm itself was an anticyclonic supercell. The radar- depicted evolution of the second (Los Altos) tornado suggests that it was associated with a mesoanticyclone, although the role of the sea-breeze boundary in the cannot be discounted.

1. Introduction port of a third funnel or possible tornado near the city of East Palo Alto. This represents the ®rst instance of On 4 May 1998, a pair of tornadoes occurred in the a tornadic thunderstorm in the San Francisco Bay region San Francisco Bay Area in the cities of Sunnyvale (F2) (2331 UTC) and Los Altos (F1) (2355 UTC) (see Fig. to be within close range (ϳ30 km) of the Monterey 1 for locations). There was an additional unveri®ed re- (KMUX) Weather Surveillance Radar-1988 Doppler (WSR-88D) site in the Santa Cruz Mountains. The Sunnyvale tornado was well documented with Corresponding author address: Dr. John P. Monteverdi, Depart- both video and still images. An analysis of videotape ment of Geosciences, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132. imagery (Fig. 2) shows that this tornado was anticy- E-mail: [email protected] clonic. Remarkable still images (e.g., Fig. 3) also reveal

᭧ 2001 American Meteorological Society 2806 MONTHLY WEATHER REVIEW VOLUME 129

FIG. 2. Sunnyvale tornado. Arrows along anticyclonic striations.

son 1993). We speculate that the ®rst tornado was non- supercellular and related to the interaction of the thun- derstorm with a surface boundary [see Markowski et al. (1998) for a discussion of tornadogenesis in intercepting surface boundaries during the Veri®cation

FIG. 1. Map showing locations of the Sunnyvale and Los Altos of in Tornadoes Experiment]. However, the tornadoes in relation to KNUQ (Moffett ) and KSJC (San Jose) evolution of the radar signatures associated with the Los METAR observation sites, and the KOAK radiosonde and KMUX Altos tornado suggests that it may have been supercel- WSR-88D locations. lular. In previous studies, California supercell tornadoes that it both was anticyclonic and had entrained consid- have been documented with the more typical right-mov- erable during its 12-min cycle. Witness re- ing cyclonically rotating storms (e.g., Monteverdi and ports suggest that the tornado was multiple and Quadros 1994; Braun and Monteverdi 1991) although damage surveys determined that its pathlength was nonsupercell tornadoes are probably the most common about 2 km. Video and radar evidence of the Los Altos in the state (Blier and Batten 1994). Although radar tornado show that this was the second anticyclonic tor- signatures have been documented for California storms nado produced by the same parent thunderstorm. This (see, e.g., Carbone 1983; Monteverdi and Johnson tornado had a pathlength of around 1 km and produced 1996), the hooks seen thus far were cyclonic. an injury as it moved through the campus of Los Altos There are published cases of both anticyclonic tor- High School. nadoes and anticyclonic supercells in the refereed lit- The National Severe Storms Laboratory's (NSSL) erature. Fujita (1977) provided the ®rst documentation Mesocyclone Detection Algorithm (MDA; Stumpf et al. 1998) detected mesoanticyclones several during the life cycle of the parent storm (hereafter referred to as the Sunnyvale storm) that had formed on the left ¯ank of an earlier thunderstorm. The motion of the Sun- nyvale storm to the left of the hodograph brought it into a shear environment favorable for the development of an anticyclonically rotating updraft. The radar evidence suggests that the Sunnyvale F2 tornado did not originate from the traditional ``supercell cascade''1 process (as outlined in Wicker and Wilhelm-

1 The ``supercell cascade'' leading to tornadogenesis is a conceptual model that starts with the development of the midlevel mesocyclone, advection of by the mesocyclone around the updraft area, development of a rear-¯ank downdraft (RFD) simultaneous with the development of the mesocyclone at lower levels, and interaction FIG. 3. Unusual photograph shot from directly underneath the Sun- of the RFD with the low-level shear to produce the low-level tornado nyvale tornado. Note debris in upper left and anticyclonic features and, eventually, the tornado. in the funnel structure. (Photo by D. Misaki.) NOVEMBER 2001 PICTURE OF THE MONTH 2807

minor enhancement of the cyclonic ¯ow over California having occurred. Although the very weak synoptic-scale ¯ow throughout the would suggest minimal, if any, contributory quasigeostrophic (QG) forcing, comprehensive Eta and Aviation (AVN) Model based QG analyses were constructed and examined (not shown). In all cases, and at both the model initialization of 1200 UTC 4 May and the approximate time of the tornadic event (0000 UTC 5 May), this forcing was found to be quite weak throughout the region of interest.

b. Buoyancy and shear A rare thermodynamic environment occurred in the FIG. 4. AVN Model initialized 500-hPa geopotential height (m) San Francisco Bay Area on 4 May 1998. The Oakland and (ЊC) for 1200 UTC 4 May 1998. Selected radiosonde (KOAK) 0000 UTC 5 May 1998 sounding (not shown) observations are also shown. showed a great departure from the typical warm thermal and moisture structure in the San Francisco Bay region. The usually seen marine inversion was absent, of anticyclonic tornadoes but did not postulate a relation due to the in¯uence of the offshore cold upper-level of the tornadoes to the storm-scale rotation. Fujita and low. The relatively moist nature of the surface air was Grandoso (1968) ®rst hypothesized that leftward-prop- evidenced by atypically high surface dewpoints of near- agating, anticyclonic thunderstorms occur as part of ly 16ЊC. The superposition of relatively cold middle- storm splitting. This was veri®ed in modeling studies and upper-tropospheric over an unchar- by Wilhelmson and Klemp (1978) and Weisman and acteristically warm and moist marine intrusion created Klemp (1982), who showed that storms propagating to strati®cation, which, for California, was highly unstable. the left of the hodograph rotate anticyclonically. How- Surface-based convective available potential ever, no case study of an anticyclonic tornado in as- (CAPE) for the observed KOAK sounding of 2137 sociation with an anticyclonic supercell has yet appeared JkgϪ1 underscored the potential for strong-to-severe in the reviewed literature. convection on this day. The purpose of the present manuscript is to document The shear environment on the afternoon of 4 May what was a remarkable event. This is the ®rst study that 1998 was not remarkable. The KOAK 0000 UTC 5 May provides documentation of the rare com- 1998 hodograph (not shown) was straight and of rela- bination of an anticyclonic tornado associated with an tively short length. Both deep-layer shear and that in anticyclonic supercell. The fact that this unusual tor- the lowest layers were weak and unremarkable. The nadic storm was photographed and also occurred in a buoyancy and shear combination at KOAK, as estimated region in which tornadoes and supercells are themselves by the (BRN) of 173, was fa- relatively infrequent further establishes the uniqueness vorable for multicellular or pulse-type convection. of this case. The evolution of the afternoon subsynoptic environ- ment in the southern portion of the San Francisco Bay 2. Storm environment area was dominated by a surge of marine air into the region from the northwest, due to the typical thermally a. Synoptic overview induced onshore pressure gradient in the boundary layer The synoptic-scale environment associated with this (with lowest surface pressures in the Central Valley in tornadic event was characterized by weak cyclonic ¯ow the afternoon; not shown). This was evidenced by a through the depth of the troposphere, signi®cantly cool- shift and a decrease in temperature (Fig. 5) at er than normal temperatures aloft, and relatively abun- Moffett Field (KNUQ; see Fig. 1). Virtual temperatures dant moisture. At 1200 UTC 4 May 1998, a 500-hPa were 2.2 K colder north of this sea-breeze boundary. low was centered about 1100 km west-southwest of San The marine surge advected air with atypically high Francisco, with a central height of approximately 549 dewpoints (undoubtedly related to unusual warm sea dm; a very weak upper trough extended from the low surface temperatures offshore) southward into the north- center through north-central California (Fig. 4). The sys- ern Santa Clara valley. Thus, this channeled ¯ow was tem had an equivalent barotropic structure, with anal- associated with an unstable strati®cation since it brought yses of sea level pressure and 850-hPa height (not air with relatively high equivalent potential tempera- shown) appearing qualitatively similar to the 500-hPa- tures underneath lower midtropospheric ¯ow character- height analysis. Over the ensuing 12 h, the system ized by cold advection. moved eastward but, at 0000 UTC 5 May (not shown), The Sounding Hodograph Analysis Research Pro- still lay well to the west of the coast, with only very gram (Hart and Korotky 1991) was used to construct a 2808 MONTHLY WEATHER REVIEW VOLUME 129

FIG. 5. KNUQ meteogram from 2050 to 0050 UTC showing wind shift, temperature, and dewpoint increase at about 2100 UTC asso- ciated with southward-moving sea-breeze front. Arrival of the thun- derstorm out¯ow at about 0000 UTC marked by another wind shift, and temperature and dewpoint decrease. FIG. 6. Proximity sounding (modi®ed from KOAK radiosonde at 0000 UTC 5 May 1998) for KNUQ at 2300 UTC 4 May 1998. Sound- ing obtained by modifying KOAK temperature, dewpoint, and wind proximity sounding and hodograph by insertion of the pro®le with surface temperature, dewpoint, and wind observation for 2300 UTC observation from KNUQ into the 0000 UTC KNUQ. Dashed line is ascent curve for a lifted surface parcel. Wind 5 May 1998 KOAK radiosonde observation. CAPE was speed in kt. calculated on the basis of a lifted surface parcel. In this case, insertion of the surface temperature and dewpoint continued deviate motion of the storm to the left of the temperatures at KNUQ into the KOAK sounding re- hodograph. The deviate motion then would contribute sulted in a lowering of the CAPE from its initial 2137 to negative storm relative helicity in the 0±3-km layer JkgϪ1 to 1730 J kgϪ1 (Fig. 6). The shear environment into which the Sunnyvale indicating the potential for at least the development of storm moved was much different than that which was a weak mesoanticyclone. observed at KOAK (as described above). The strong The relatively minimal cyclonic curvature evident in northwest wind associated with the created the KNUQ hodograph was consistent with the marginal a basically unidirectional shear pro®le for KNUQ (Fig. values of low level negative shear seen in Table 1. This 7) but with some cyclonic curvature. However, the sur- also suggests that the supercell would not be tornadic face wind at KNUQ contributed to large negative shear2 in the 0±0.5-and 0±1-km layers and a deep-layer shear magnitude marginally favorable for supercells as indi- cated by the BRN of 50 (contrasted to that for KOAK of 173). The shear evident in the KNUQ hodograph (Table 1) for the 0±0.5- and 0±1-km layers was only marginally consistent with the values associated with tornadic storms in California. For example, Monteverdi et al. (2000) have shown that tornadic events in California typically are associated with very strong shear (Ͼ15 ϫ 10Ϫ3 sϪ1) in the 0±1-km layer. However, the 0±6-km negative shear of ϳ3.0 ϫ 10Ϫ3 sϪ1, although weak to moderate, was suf®cient for supercells (Weisman and Klemp 1982). The parameters calculated on the basis of the modi®ed KOAK hodograph indicated that the environment in the northern Santa Clara valley into which the Sunnyvale storm moved was characterized by shear that would favor both in the updraft and the FIG. 7. Proximity hodograph (modi®ed from KOAK radiosonde at 0000 UTC 5 May 1998) for KNUQ at 2300 UTC 4 May 1998. Ho- dograph obtained by modifying KOAK wind pro®le with surface wind 2 The term negative shear refers to shear that creates streamwise for KNUQ. Storm motion is for Sunnyvale storm. Hodograph points negative relative vorticity that can be converted into negative relative plotted at 500-m intervals from surface to 7 km. Hodograph points vertical vorticity by the updraft. at surface, 1, 2, 3, 4, 5, 6, and 7 km indicated. in kt. NOVEMBER 2001 PICTURE OF THE MONTH 2809

TABLE 1. Shear and storm-relative environmental helicity (SREH) This Sunnyvale storm also propagated along the sea- values extracted from KOAK hodograph modi®ed for surface con- breeze front boundary (Figs. 8e and 8f). It is interesting ditions at Moffett Field (KNUQ) at 2300 UTC 4 May 1998. SREH calculated on the basis of the Sunnyvale storm motion as shown in to note that at the time of the Sunnyvale tornado an Fig. 7. intersection of the out¯ow boundary and sea-breeze front was apparently collocated with the updraft area of Layer Negative shear Storm relative helicity (km) (10Ϫ3 sϪ1) (m2 sϪ2) the storm. Similar intersection of boundaries near the updraft areas of strong supercells were observed by 0±1 14.7 Ϫ70 Markowski et al. (1998) for the tornadic storms in the 0±2 6.9 Ϫ90 0±3 5.1 Ϫ111 Panhandle on 2 June 1995. They hypothesized 0±4 3.5 Ϫ111 that the updraft of these storms tilted and stretched the 0±5 3.6 Ϫ126 horizontal vorticity associated with the boundaries up- 0±6 3.1 Ϫ128 ward shortly before tornadoes occurred. In the case of the Sunnyvale storm, the evidence suggests a similar set of circumstances, since the motion of the storm or, at least, that the traditional ``supercell cascade'' to would have had the effect of tilting horizontal anticy- tornadogenesis would not be expected to occur for this clonic vorticity solenoidally generated on the sea-breeze case, since the strongest and mesoanti- front into the updraft in much the same manner. Thus, tend to occur with curved hodographs (Blue- this essentially nonsupercellular process could have had stein 1993, p. 475). This is veri®ed by the relatively an important role in the formation of the ®rst tornado small storm-relative environmental helicity (SREH) val- early in the life cycle of the Sunnyvale tornado. ues (e.g., Ϫ111 m2 sϪ2 in the 0±3-km ``in¯ow'' layer). 4. WSR-88D radar imagery 3. Satellite imagery To the authors' knowledge, the WSR-88D images An examination of satellite imagery and surface ob- from KMUX presented in this section are the ®rst im- servations shows that the Sunnyvale storm interacted ages of an anticyclonic tornadic supercell ever recorded with a sea-breeze boundary that had passed through and analyzed. Analysis of WSR-88D radial velocity data KNUQ earlier in the afternoon. The motion of the sea- for the storm showed -scale vortices breeze front past KNUQ could be noted in the hourly with vertical and time continuity. WSR-88D re¯ectivity observations (Fig. 5), and was marked by a wind shift data revealed the storm's supercell structure, but as a from southerly to northerly, a decrease in temperature, mirror image to that of the typical and an increase in dewpoint between 2000 and 2100 cyclonic supercell. The storm motion was to the left of UTC [1300±1400 Paci®c daylight time (PDT)]. the mean ¯ow, opposite to that of a classic right moving The sea-breeze front was marked by a discontinuous cyclonic supercell. The supercell, and both tornadoes arc of cumulus that extended from west to east across associated with it, moved from southeast to northwest. the northern Santa Clara valley by 2200 UTC (Fig. 8a), These observations are consistent with the nature of the where it intersected a developing thunderstorm east of storm as expected from the discussions of the shear San Jose (Figs. 8a±c). This thunderstorm (labeled 1 in environment in section 4b above. Fig. 8) was the ®rst to form in the afternoon hours. An experimental version of the NSSL MDA,3 tuned Another thunderstorm (labeled 2 in Fig. 8) developed to detect clockwise-rotating storm-scale vortices using on the out¯ow boundary extending out from the left WSR-88D radial velocity data, was run on the KMUX ¯ank of the initial storm (Fig. 8c) and then propagated level-II data. The level-II archive, which is the highest- west-northwestward. The Sunnyvale storm (labeled 3 in resolution data archived from the WSR-88D, began with Fig. 8) can be seen both on radar (not shown) and on a volume scan collected at 2331 UTC 4 May 1998, the the satellite imagery (Fig. 8d) to be an apparently in- time of the F2 Sunnyvale tornado. Therefore, using the dependent development on the sea-breeze boundary. level-II data alone, it was dif®cult to ascertain the or- The passage of the out¯ow boundary associated with igins of the vortex that produced this initial tornado. storm 2 through the San Jose area could also be noted However, the radar velocity characteristics of this ®rst in the hourly observations at San Jose International Air- tornado at 2331 UTC were similar to other nonsupercell port (KSJC) (Fig. 9). This was marked at around 2200 tornadoes, in that only a very small gate-to-gate shear UTC (1400 PDT) by a wind shift from northerly to couplet (albeit anticyclonic) was observed, with no ac- easterly, temperature and dewpoint temperature falls, companying or surrounding larger mesoanticyclonic and observations of , , and passing vortex (Fig. 10a). In fact, the vortex was too small to from southeast to northwest. The progression of the out- be accurately detected by the MDA at this time and, at ¯ow boundary was clearly noted in the satellite imagery (Figs. 8b±f) and the composite radar imagery (not 3 At the time of this publication, the NSSL MDA has been rec- shown) and intersected the Sunnyvale storm at about ommended for inclusion as an of®cial operational algorithm for the the time of the ®rst tornado. WSR-88D system. 2810 MONTHLY WEATHER REVIEW VOLUME 129

FIG.8.GOES-9 visible images from 2200 to 2353 UTC showing location of initial thunderstorm development in relation to sea-breeze boundary; development of new storms on out¯ow boundary; motion of Sunnyvale storm along the sea-breeze boundary; sense of solenoidal circulation along boundary, as explained in text; and locations of Sunnyvale tornado at 2331 UTC (T in 2341 UTC image) and Los Altos tornado (T in 2353 UTC image) in relation to storm. Light contours represent 200-m elevation. Imagery courtesy of U.S. Navy. this range (31 km), was more typical of a weak tornadic volume scan at the lowest elevation scan (0.5Њ), a pen- vortex signature (TVS). The subsequent volume scan, dant echo, most likely a degraded view of a , at 2337 UTC (not shown), continued to show the an- was apparent on the left ¯ank of the anticyclonic su- ticyclonic Sunnyvale vortex; this was near the time of percell, as opposed to the typical right ¯ank of cyclonic the tornado's demise based on visual observations. storms. An in¯ow notch was also apparent on the left Radar re¯ectivity characteristics at 2331 UTC de- side of the re¯ectivity echo. picted this ``mirror image'' supercell (Fig. 10b). For this The KMUX WSR-88D was sited at an elevation of NOVEMBER 2001 PICTURE OF THE MONTH 2811

ticyclone,'' a vortex detection that is de®ned as occu- pying at least 25% of the storm depth, but is less than 3 km tall. The storm depth was about 9 km, which was in the general range of low-topped storms. Also at 2355 UTC (Fig. 11), the MDA detected two- dimensional anticyclonic vortex features with vertical continuity. The radar data also suggest that this second tornado was characterized by a gate-to-gate shear cou- plet at the lowest elevation, encompassed by a larger 3±5-km mesoanticyclonic vortex at the higher elevation angles. The time evolution and vertical structure of the radar vortex suggest that the second tornado in Los Altos formed in the traditional supercell cascade process. Since the storm was still interacting with the sea-breeze FIG. 9. KSJC meteogram from 2050 to 0050 UTC showing wind boundary at that time, the tornado's formation may have shift, temperature, and dewpoint temperature decrease at about 2200 been a hybrid of supercellular and nonsupercellular pro- UTC associated with westward-moving out¯ow boundary. Arrival of cesses. thunderstorm noted in portions of METARs included for 2245±2347 UTC by -to-ground lightning initially east of and, ®nally, north- west of KSJC, rain and hail (). 5. Conclusions Temperature and wind patterns in the San Francisco 1077 m above mean sea level (MSL) to minimize terrain Bay Area on the afternoon of 4 May 1998 contributed blockage, yet with a trade-off that the radar horizon is to conditions favorable for strong-to-severe pulse-type much higher than that for ¯atland . At the 30-km thunderstorms throughout the area. An unusual wind range to the tornadic storm, the KMUX radar beam at pro®le developed in the southern portion of the Bay its lowest elevation angle of 0.5Њ was intersecting the Area in the midafternoon when a strong sea breeze storm at about 1350 m above ground level (AGL). This moved southward into the Santa Clara valley producing height is typically too high to observe any re¯ectivity a vertical wind pro®le characterized by backing wind ®ne lines associated with thunderstorm out¯ow bound- with height and negative deep layer and boundary layer aries and sea-breeze boundaries so it was not possible shear. to verify on the radar imagery the location and evolution Several thunderstorms developed on an out¯ow of the boundaries discussed in section 4.4 boundary associated with a thunderstorm near San Jose. Nonetheless, the WSR-88D data suggest that this ®rst This out¯ow boundary and one of the thunderstorms tornado may have formed from the nonsupercell tor- developing along it moved to the left of the hodograph nadogenesis mechanisms discussed in previous sections and intersected the sea-breeze boundary in the northern of this paper, even though the overall storm re¯ectivity Santa Clara valley. structure was beginning to assume supercell character- The Sunnyvale storm formed on the left ¯ank of this istics (nonsupercell tornadoes are occasionally observed thunderstorm on the intersection of the out¯ow bound- in conjunction with ¯anking lines of developing super- ary and the sea-breeze boundary. Analyses of the sat- cells). ellite and radar evidence suggest the storm both moved WSR-88D radial velocity data show the development along the sea-breeze boundary (strongly to the left of of a second separate vortex, northwest of the Sunnyvale the hodograph) and, possibly, tilted the solenoidal vor- vortex. The second vortex was associated with the sec- ticity associated with the boundary into the updraft. It ond (Los Altos, F1) tornado. The ®rst indications of the is interesting to note that both the deep-layer shear and second vortex were observed as early as 2337 UTC (not the solenoidal vorticity associated with the sea-breeze shown) to the northwest of the dissipating Sunnyvale boundary would have produced midlevel and low-level gate-to-gate shear vortex. This second vortex was com- anticyclonic rotation, respectively, in this storm. posed of a larger region of anticyclonic shear with a The radar evidence corroborates the fact that when diameter of 3±5 km. The second anticyclonic tornado the ®rst tornado occurred, its signature resembled a was born from this larger vortex at 2349 UTC. At 2355 TVS, with an unrelated nascent larger-scale rotation in- UTC (Fig. 10c), the tornado was associated with a more- dicative of a mesoanticyclone at midlevels of the storm. pronounced vortex signature in the radial velocity ®eld. Thus, the Sunnyvale tornado was a nonsupercell anti- This radar vortex was detected by the MDA (overlaid cyclonic vortex that did not form in the classical su- in Fig. 10d) and classi®ed as a ``low-topped mesoan- percell cascade even though the parent storm was an anticyclonic supercell. 4 Since KMUX WSR-88D level-II data were not archived before The radar evidence suggests that the second clockwise 2331 UTC, it is possible that these boundaries may have exceeded vortex associated with the Los Altos tornado experi- this depth prior to the ®rst volume scan available for analysis. enced the more conventional progression expected, in 2812 MONTHLY WEATHER REVIEW VOLUME 129

FIG. 10. Radar imagery from KMUX WSR-88D: (a) 2331 UTC 4 May 1998 re¯ectivity at 0.5Њ elevation angle showing cyclonically curved pendant, as well as an in¯ow notch on the west side of the storm, near Mountain View; (b) 2331 UTC 4 May 1998 radial storm relative velocity showing 2.4Њ elevation angle location of anticyclonic vortex associated with the F2 Sunnyvale tornado (small yellow dot), along with the direction of motion (arrow); (c) 2355 UTC 4 May 1998 re¯ectivity at 0.5Њ elevation angle at time of F1 Los Altos tornado with red-in-yellow circle indicating MDA output of a ``low-topped mesoanticyclone''; and (d) 2355 UTC 4 May 1998 radial 0.5Њ elevation angle storm relative velocity with locations of anticyclonic vortex associated with the F2 Sunnyvale and F1 Los Altos tornadoes (small yellow dots), along with the directions of motion (arrow). this case, with an anticyclonic supercell. The larger- from the traditional supercell cascade, the in¯uence of scale anticyclonic vortex appeared ®rst at midlevels and the tilt of the solenoidal-generated vorticity associated was followed by the development of a smaller-scale with the sea-breeze boundary cannot be discounted as vortex visible at the lower elevation scans. Although an important in¯uence as well. the second vortex, therefore, apparently was formed The mesoanticyclones identi®ed by the MDA on the NOVEMBER 2001 PICTURE OF THE MONTH 2813

FIG. 11. The 2355 UTC 4 May 1998 multipanel radial storm relative velocity image, at 1.1, 3.0, 4.9, 6.6, 8.5, 11.8, 19.2, and 28.0 ϫ 103 ft AGL, from the KMUX (WSR-88D). Blue circles indicate the 2D anticyclonic feature output from the MDA (as detected for elevation scan) associated with the F1 Los Altos tornado. The algorithm-estimated diameter of the circles corresponds to the diameter of the vortices in the radar velocity data. Note the small-diameter TVS signature at the lowest elevation scan, surmounted by the larger-radius mesoanticyclonic vortices at other elevations, suggestive of tornadogenesis supercell cascade for the Los Altos tornado.

KMUX data for this case represent the ®rst WSR-88D Braun, S. A., and J. P. Monteverdi, 1991: An analysis of an meso- documentation of anticyclonic mesocirculations in com- cyclone-induced tornado occurrence in northern California. Wea. Forecasting, 6, 13±31. bination with anticyclonic tornadoes. Although this case Carbone, E., 1983: A severe frontal . Part II: Tornado parent was very unusual, it also underscores the utility of the vortex circulation. J. Atmos. Sci., 40, 2639±2654. latest experimental suite of WSR-88D algorithms (e.g., Fujita, T., 1977: Anticyclonic tornadoes. Weatherwise, 30, 51±64. the NSSL MDA) in detecting both cyclonic and anti- ÐÐ, and H. Grandoso, 1968: Split of a thunderstorm into anticy- clonic and cyclonic storms and their motion as determined from cyclonic rotation in thunderstorms, even in the more numerical model experiments. J. Atmos. Sci., 25, 416±439. typical low-topped supercells common in the Paci®c Hart, J. A., and J. Korotky, 1991: The SHARP WorkstationÐA skew- coast states. T/hodograph analysis and research program for the IBM and compatable PC. Users' guide. NOAA/NWS NWSFO, Charles- town, WV, 62 pp. Acknowledgments. This research was partially funded Markowski, P. M., E. N. Rasmussen, and J. M. Straka, 1998: The by the Department of Geosciences, San Francisco State occurrence of tornadoes in supercells interacting with boundaries University, the National Severe Storms Laboratory, and during VORTEX-95. Wea. Forecasting, 13, 852±859. the National Weather Service. The authors gratefully Monteverdi, J. P., and J. Quadros, 1994: Convective and rotational acknowledge David Misaki, Jan Null, and Kathy Pagan parameters associated with three tornado episodes in northern and central California. Wea. Forecasting, 9, 285±300. for their contributions to this work. ÐÐ, and S. Johnson, 1996: A supercell with hook echo in the San Joaquin Valley of California. Wea. Forecasting, 11, 246±261. ÐÐ, C. Doswell III, and G. S. Lipari, 2000: Shear parameter thresh- REFERENCES olds for forecasting California tornadic thunderstorms. Preprints, 20th Severe Local Storms Conf., Orlando, FL, Amer. Meteor. Blier, W., and K. A. Batten, 1994: On the incidence of tornadoes in Soc., 522±525. California. Wea. Forecasting, 9, 301±315. Stumpf, G. J., A. Witt, E. D. Mitchell, P. L. Spencer, J. T. Johnson, Bluestein, H. B., 1993: Observations and Theory of Weather Systems. M. D. Eilts, K. W. Thomas, and D. W. Burgess, 1998: The Na- Vol. II, Synoptic±Dynamic Meteorology in the Midlatitudes, Uni- tional Severe Storms Laboratory mesocyclone detection algo- versity Press, 594 pp. rithm for the WSR-88D. Wea. Forecasting, 13, 304±326. 2814 MONTHLY WEATHER REVIEW VOLUME 129

Weisman, M. L., and J. B. Klemp, 1982: The dependence of nu- Its Structure, Dynamics, Prediction, and , No. 79, Geo- merically simulated convective storms on vertical phys. Monogr., Amer. Geophys. Union, 75±88. and buoyancy. Mon. Wea. Rev., 110, 504±520. Wilhelmson, R. B., and J. B. Klemp, 1978: A three-dimensional nu- Wicker, L. J., and R. B. Wilhelmson, 1993: Numerical simulation of merical simulation of splitting that leads to long-lived storms. tornadogenesis within a supercell thunderstorm. The Tornado: J. Atmos. Sci., 35, 1037±1063.