|Intro||Table||Figure 1||Figure 2||Figure 3||Figure 4||Figure 5||Appendix||References|
Note: This paper was originally written in 1994 as NOAA Technical Memorandum NWS SR-160. This version was last updated in March 2015 to include 1 tornado which touched down in southwestern OKC and moved into Moore, OK on March 25, 2015. The tornado table has been updated with preliminary information. Research on this tornado in ongoing, and will be updated in the near future.
Oklahoma City (OKC), by virtue of its large areal extent and location near the heart of "tornado alley," has earned a reputation over the years as one of the more tornado-prone cities in the United States. In the 1960s and 1970s, news and magazine articles sometimes quoted the number of times OKC has been struck by tornadoes; that number typically was in the 30s or lower 40s, depending on the year of publication. The source of the number most likely was a local listing, kept on station at the National Weather Service Forecast Office (WSFO OKC). That list included 43 tornadoes as of 1978. Sometime later, record keeping ceased for some reason and the existing list was relegated to a binder that was lost among the growing masses of other records that accumulated at the office.
In 1991, a project was initiated to revise the list by adding recent tornadoes (since 1978), validating existing entries, and searching for any others that might have been overlooked. This turned out to be a formidable task, for several reasons. One was the lack of reliable sources for events that occurred before 1950. The only readily- available data source prior to 1950, other than the local records on station at WSFO OKC, was Grazulis (1990). The compilation by Grazulis includes all tornadoes of F2 intensity or greater, and all killer tornadoes, from 1880 through 1989. Thus there are no records on weak tornadoes prior to 1950 (unless they happened to make it into the WSFO listing), and so the true number of tornadoes probably is much higher than the total arrived at herein. After 1950, records were cross-checked using Storm Data and the severe weather database compiled at the Storm Prediction Center (SPC).
A second obstacle arose in determining exactly where the OKC city limits are - or were. OKC covers a large area today, but the city limits have changed over the years such that some areas that are now part of OKC were not always within the city limits. Peripheral townships have emerged as well over the years, some of which are now wholly surrounded by OKC city land. In order to establish a consistent area, it was decided to include the present OKC limits and all other surrounding cities and towns that are contained largely or wholly within those limits. The resulting area (Figure 1, hereafter called the "immediate OKC area") includes roughly 600 square miles. All recorded tornado events occurring wholly or partly within this area are included in the list. In addition to OKC, the following jurisdictions are included: Bethany, Choctaw, Crutcho, Del City, Forest Park, Jones, Midwest City, Moore, Mustang, Nichols Hills, Nicoma Park, Valley Brook, the Village, Warr Acres, Witcher, and Yukon (Figure 1). The cities of Edmond, El Reno, and Norman are not included, which means that tornadoes striking within their city limits are not listed unless they also affected the immediate OKC area.
Each of the 160 tornado listings contains the date and time of initial touchdown (note that all times are CST) or a total time range (if available), maximum tornado path width and length, maximum F Scale or EF Scale rating (see Appendix for descriptions of the F Scale and EF Scale), number of people killed and injured (from the entire event), counties included in the path, and a brief description of the path. A narrative describing what is known about the event follows the other data. A dollar estimate of damage (if available and not adjusted for inflation) and the data sources used for each tornado entry are included at the end of each narrative.
Many of the events were found in more than one data source. In general the multiple sources were in good agreement. However, there were occasional differences, especially with F-scale ratings. The extensive research conducted by Grazulis (1990) suggests his ratings to be the most reliable, and they have been used when the ratings differed among the available sources. Since Grazulis lists only events of F2 intensity or greater, any event not found in his listing has been given a rating of either F0 or F1. Events listed as F2 in the SPC database, but not appearing in Grazulis 1990, are listed here as F1. Notes are included in the narrative of an event if differences were found in the F-scale ratings among the available sources. Weak events (F0 and F1) before 1950 had no assigned F scale in any of the available sources, and thus have been assigned an estimated rating based on the available data.
May is the peak month for all tornadoes, followed closely by April and June (Figure 2). About two thirds of all tornadoes in OKC have struck during those three months. Strong and violent tornadoes tend to occur slightly earlier, with April the peak month. Note that 18 of the 27 April tornadoes were F2 or greater, and that 5 of the 12 F4/F5 tornadoes on record occurred in April. However, since 1999, 5 of the violent tornadoes have occurred during the month of May. Frequencies level off during the summer and autumn before dwindling during the winter. (December and January are the only two months in which the immediate OKC area has not been struck.)
Tornadoes striking OKC have formed most frequently between mid-afternoon and early evening (2 to 7PM CST, or 3 to 8 PM CDT; see Figure 3). The period of peak activity also appears in the distribution of strong (F2/F3) and violent (F4/F5) tornadoes, and the peak of F2 or greater tornadoes appears to occur as a broad peak centered around 4 PM CST/5 PM CDT. Other notable findings include a general lack of early morning events (only one between 5 and 7 AM CST), a prevalence of weak events during the late morning and midday hours (with one notable exception), and a rapid increase in frequency during the early afternoon (1 to 2 PM CST, or 2 to 3 PM CDT).
Note: The time distribution in Figure 3 is based on tornado start time, which may be up to an hour or so earlier than the actual "strike time" in OKC in the case of long-track, long-lived tornadoes. For example, the F5 tornado of 3 May 1999 began around 6:26 PM CDT (5:26 PM CST), but entered the OKC city limits around 7:12 PM CDT, and lifted in Midwest City around 7:48 PM CDT. Since such events constitute only a small fraction of the total, the difference is not considered to be significant.
Tornado distribution by decade (Figure 4) is largely a reflection of the data sources used. The relatively small number of tornadoes through the 1930s is dominated by strong and violent events (F2 or greater), while the increase in frequency in the 1950s, 60s, and 70s is due mainly to a larger number of weak (F0 and F1) events. The frequency of weak events almost certainly has been more constant than the data indicate, but most of the weak events before 1950 probably were not documented.
The decrease in frequency from the 1960s/70s through the 1980s/90s may be due to changes in quality control of severe weather reports. (Note: Eleven of the 15 tornadoes of the 1990s occurred on only 3 dates: 13 June 1998, 4 October 1998, and 3 May 1999.) More attention was placed on downbursts (Fujita 1985), resulting in closer inspection of wind damage and an increasing number of events being classified as straight-line (downburst) winds instead of tornadoes. If this is the case, then some of the reported tornadoes in earlier years may actually have been downbursts, and the decrease in frequency in the 1980s may actually reflect an adjustment toward more representative numbers (i.e. the totals from the 1960s and 70s may be too high).
Figure 5 shows the approximate locations and tracks of the tornadoes listed. Exact tracks are difficult to plot in many cases, especially the early ones, since the available data do not provide enough detail. (Also, many of the referenced landmarks no longer exist). Tracks of tornadoes prior to about 1970 are drawn subjectively from the information available, and may be accurate only within a few miles. This level of accuracy is still enough to reveal a slightly higher concentration of events on the south side of OKC. This trend is especially evident in F2/F3 tornadoes (shown in blue). Although several tornado "corridors" are suggested, it is likely that these distributions are no more than statistical fluctuations. Therefore, past tornado frequencies in a given location do not imply similar frequencies in the future.
Most of the tornadoes moved northeast, while a few moved east or east- southeast. Only one long-track event showed no eastward component of motion; it traveled north to north-northwest through the center of Oklahoma County and the eastern parts of OKC. Speeds, based on available reports, generally ranged from 15 to 40 mph. However, several events (especially weaker ones) displayed slow and erratic movement, and a few were nearly stationary during their short durations.
Some other interesting facts about OKC tornadoes:
Note that all but five events were between March and June, and that four were on April 30.