UDC bbl.616.43:bbl.M)9.326(794)”3!22” Summer Thunderstorms Over Southern California ANTHONY M. TUBBS-Scripps Institution of Oceanography, La Jolla, University of California, San Diego, Calif. ABSTRACT-Ten-day fire weather records from forestry coast of Mexico. Tropical storm tracks, local National lookouts in the southern California mountains are used to Weather Service reports, and fire weather records are used describe summer thunderstorm activity. The distri- to show which occurrences of thunderstorms can be bution of thunderstorms during a typical season closely credited to this tropical cyclone activity and the attendant resembles rainy-day frequencies for summer precipitation influx of upper level moisture. in Arizona, thereby supporting the concept that most When the data from the summers of 1947-68 are divided southern California summer thunderstorms are caused by into two 11-yr periods using 1957/58 as the break point, a westward extension of the same circulation that brings a significant increase in thunderstorms and tropical Arizona its “monsoon” rains. cyclones affecting southern California is noted during the Thunderstorms are also induced by dissipating eastern latter period. North Pacific tropical cyclones that move up the west 1. INTRODUCTION California, that contains the bulk of its population. Northeast of the Transverse Range, and within its rain Each summer, occasional pulsations of warm, humid air shadow, is a broad, high, desert country that merges invade southern California from the south. These con- gradually with the low desert of the Colorado River to the ditionally unstable air masses usually manifest their east. The Peninsular Range is devoid of a broad, high presence by the appearance of altocumulus cloud streets desert on its eastern flank; instead it plunges steeply to advancing from the southeast, followed by cumulo- the low desert floor. nimbus buildups over the coastal mountain ranges. On Although the mountains in southern California are rare occasions, the tropical air arrives coincident with a sparsely populated, summer thunderstorms occasionally “chubasco”-a tropical cyclone off the west coast of cause loss of life and serious damage to property. Because Mexico.’ this is the driest season, lightning-set fires pose the great- Early investigators (Campbell 1906, Carpenter 1913, est threat; but in recent years, there also have been Blake 1933) described the associated thunderstorms and incidences of human injury due to close lightning strikes, their effect on the local climate. Blake designated them washed-out roads and localized flooding caused by down- as “Mexican storms” and noted that two synoptic pours, as well as plane crashes and camper trailer accidents patterns are associated with them. One pattern consists caused by extreme turbulence. of air approaching from the south and east and is tradi- Court (1960) shows an average of 60 lightning-est tionally called the Sonora storm because it was believed range and forest fires occurring annually for the 11-yr to originate in that Mexican state. The other pattern period 1946-56 in southern California. It must be assumed brings in tropical air from the south and west and is due that these are from summer thunderstorms, as lightning to a dissipating chubasco that has penetrated far enough associated with winter frontal systems is not common northward to affect southern California. and the forests are usually moistened by the winter rains. Regardless of the weather pattern, the resulting In the higher mountains] direct lightning damage to large thunderstorms occur primarily over the mountains in trees is fairly common, and cattle are occasionally killed southern California (fig. 1). These mountains form a while seeking shelter beneath these trees. Human fatal- nearly continuous chain that may be divided into two ities from lightening strikes are practically nonexistent; general ranges: the Transverse Range, which runs to the the only one the writer could find occurred in July of 1961. southeast from Point Conception, and the Peninsular Although thunderstorms form over and just to the east Range beginning at the southern end of the Transverse of the mountain crests, their movements occasionally Range, running parallel to the coast, and continuing bring them over the populous coastal regions. When this down into Mexico. This range forms the rugged backbone happens, newspapers carry the accounts of spectacular of Baja California. lightning displays and attendant severe weather. The most Seaward of these ranges is a lowland area consisting of coastal plains and intermediate valleys. It is this lowland damage they usually do, besides putting a few power trans- area, comprising less than one-half of the area of southern formers out of service, is to wet the streets and contribute to traftic accidents. As is characteristic of convectively induced precipi- 1 This definition of chubasco has been used recently by authors describing the climate of southern California. tation, the distribution of summer rainfall in southern November1972 1 799 Unauthenticated | Downloaded 10/08/21 07:11 PM UTC FIGURE1.-Southern California land form. Contours are drawn for the 2,000- and 4,000-ft levels. Names corresponding to station numbers appear in table 1. TABLEl.-station descriptions scattered throughout the mountains of southern Cali- fornia: Thunderstorms observed within a 30-mi radius are Station Precipitation Years no. Station name' Agencyt Elevation avg. with recorded along with the time the storm began (lightning (July-Sept.) data first seen or thunder first heard) and the time it ended. (ft) (in) Comments concerning direction and distance of the thun- 1 Tecate Peak LO CDF 3,887 0.4 1947-68 derstorm from the station, direction of storm movement, 2 Descanso RS USFS 3,550 0.8 1947-68 3 Laguna Mnt. GS USFS 6, 2.4 1957-68 number of cloud-to-ground lightning strikes, and resulting 4 Julian FS CDF 4,220 1.7 1947-68 fires (if any) are usually included in the reports. Other 5 Black Mnt. LO USFS 4,055 0.6 1947-68 6 Boucher Hill LO CDF 5,446 0.8 1947-68 basic observations include amount and kind of precipi- 7 Oak Grove RS USFS 2,751 1.0 1947-68 tation, temperature (dry bulb and wet bulb) , relative 8 Red Mnt. LO USFS 4,600 1.1 1947-68 9 Keenwild CIS USFS 4,800 2.8 1965-68 humidity, cloud cover, wind speed and direction, as well 10 Tahquitz Pk. LO USFS 8,828 1.6 1947-68 as indices related to the condition of vegetation when 11 Mill Creek RS USFS 2.950 0.7 1947-68 12 Butler Peak LO USFS 8,502 2.0 1958-68 viewed as a fuel source. 13 Strawberry Pk. LO USFS 6,150 1.0 1947-68 Fire lookout stations are occupied at the start of the 14 So. Hawkins LO USFS 7,782 0.5 1947-68 1 15 Valyermo RS USFS 3,700 9.7 1947-68 fire season-usually by June when the forests begin dry- 16 Vetter LO USFS 5,903 0.8 1965-68 ing after the last of the rainy season storms. The stations 17 Mendenhall LO USFS 4,650 0.3 1964-68 18 Sierra Pelons LO USFS 4, a57 0.4 196548 remain manned until rainy weather again in the fall di- 19 Chuchupate RS USFS 5,250 0.5 1947-68 minishes the fire hazard. 20 Nordhoff LO USFS 4,477 0.4 196W8 22 1947 21 La Cumbre LO USFS 3,985 0.2 1957-68 Data from stations covering the period from 22 McPherson Pk. LO USFS 5,747 0.9 19.5243 through 1968 were tabulated and encoded on punched cards. The tabulated facts concerned the occurrence of *LO-Lookout RS-Ranger Station GS-Guard Station FS-Fire Station t CDF-Califoha Division of Foredry, USFS-U.S. ForLst Service thunderstorms and/or precipitation (a trace or more) , precipitation amount, and the date. The resulting time series of 5-mo seasons (June-October) varied in length California is quite variable in space. It is not uncommon because some stations were not manned each year; 13 for a reporting station to fail to record even a trace of stations had the full 22 yr of data, five stations had 10 yr, summer precipitation when deluging rains have occurred and four had 5 yr. only a few miles distant. One of the record rainfall inten- The 22 stations mere picked from a total of approxi- sities observed in the United States took place at Campo, mately 60 occupied each year by U.S. Forest Service and Calif. (less than 5 mi from station no. 1 in fig. l), on California Division of Forestry personnel in southern Aug. 12, 1891, when 11.50 in. of rain fell in 80 min (U.S. California. Selection was based on location and length of Department of Commerce 1960). record, with those stations having unrestricted visibility and occupying the higher peaks given preference. An 2. SOURCES OF DATA attempt was made to pick stations such that the resulting network of observations covered the length of the moun- Ten-day fire weather records (WB Form 612-17) are tain ranges with not more than 30 mi separating one filed by forestry personnel manning fire lookout stations station from the next (fig. 1 and table 1). 800 / Vol. 100, No. 11 / Monthly Weather Review Unauthenticated | Downloaded 10/08/21 07:11 PM UTC 10 8 86 L s : 0 NLY RUEUST OCTOBER 10 0 8 ?S 6c !5 6 L L 8’ ‘E 5* 2f 0 0 JUNE JULY RUGUST SEPTEHeER OCTOBER FIGURE2.-Total number of thunderstorm days (.-.-.), rainy days (&-), and days having both thunderstorms and rain (- - -) occurring on each given summer day during the 22 yr from 1947 to 1968.