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Severe Storms; Fire Danger; and Strong Pacific Storms

More strong storms on tap for the Pacific Northwest this weekend. Heavy rains could produce flooding and mudslides. Saturday, a cold front moving into the central U.S. will bring potential for severe thunderstorms containing damaging winds, large hail, and isolated tornadoes; and localized flooding potential. Dry winds in southwestern California will keep fire weather threats high. Read More >


Q: Was the Bridge Creek/Moore/OKC area tornado on May 3, 1999, rated F6?

A: No. The tornado has been rated F5 (minimal F5, in fact), and will not be "upgraded" to F6.

There was some speculation in the media of an F6 rating after "Doppler on Wheels" (DOW) researchers announced that their radar measured 318-mph winds in the tornado while it was near Bridge Creek. However, the jump in reasoning to rating this tornado F6 can not be made, for many reasons:

  • F-scale ratings are assigned based on the severity of the damage caused, *not* on wind speed. Although some of the damage was incredible (as it is with any F5 tornado), the most severe damage from the May 3 tornado was comparable to, but not worse than, other documented F5 tornadoes.

  • Wind speeds used in the F scale have not been scientifically calibrated to the severity of damage that defines each F scale level. They are, essentially, only estimates.

  • Even if the F-scale wind speed ranges were reliable estimates, the DOW measurement of 318 mph is still in the F5 range (261-318 mph) as defined by Dr. Fujita.

  • The data obtained by the DOW team are still in the process of being reviewed scientifically, and so the validity of the 318-mph wind measurement is still open to some question at this point. Early results of this review process suggest that the maximum speed actually may be less than 318 mph (although it likely will remain above 300 mph).

  • In reality, there is no such thing as an F6 tornado. When Dr. Fujita developed the F scale, he created a scale that ranges from F0 to F12, with estimated F12 winds up to mach 1 (the speed of sound). But he added that "tornadoes are not expected to reach F6 wind speeds." This leaves only the F0 to F5 range as the actual tornado F scale. For a tornado to be given an unprecedented F6 rating, it would have to produce damage more severe than has ever been observed. As stated above, there was nothing unusual or unprecedented in the damage from the May 3 tornado as compared with other F5 tornadoes in the past.

Q: Was the May 3 tornado the strongest, most violent tornado ever?:

A: Probably not, but we really have no way of knowing. It is impossible to make direct, objective comparisons between the May 3 tornado and most other violent tornadoes of the past. Factors such as inflation, varying population and property density over time and space, and the lack of direct wind speed measurements from all but a very few other tornadoes, prevent such comparisons.

We do know that other tornadoes have been wider, many have been more deadly, some have been longer lived, and many have produced longer damage tracks. So the May 3 tornado was *not* the "biggest" tornado. Nor was it the widest, the deadliest, the longest lived, or the longest track tornado on record.

The May 3 tornado currently is the most expensive tornado ever recorded. The damage estimate (roughly $1 billion) exceeds that of all other past tornadoes, even when inflation is taken into account. If we adjust our damage figures from other violent tornadoes in the past to account for inflation, we find that several (Lubbock,TX - May 11, 1970; Omaha, NE - May 6, 1975; Wichita Falls, TX - April 10, 1979, to name a few) were very close to the May 3 tornado in terms of dollar amount of damage. But to make such comparisons truly representative, we must also account for population growth, and the fact that people generally own more today than they used to. (So not only is property worth more today, but there also is more of it.) It's impossible to adjust for these factors objectively, and hard to even estimate their impact. But efforts to account for these factors suggest that the May 3 tornado probably was *not* the most damaging, at least in terms of material losses.

The May 3 tornado likely does have the highest recorded tornado wind speeds. But since typical wind measuring equipment does not survive a strong tornado, there are about three dozen tornadoes on record from which wind speeds have been obtained. Those measurements usually have been obtained from special research projects, such as the Doppler on Wheels (DOW) project, and have been obtained from only a dozen or so tornadoes (less than 0.1 percent of all tornadoes). So while the DOW data indicate the highest *recorded* tornado wind speed, there have been tens of thousands of tornadoes throughout history for which no wind speeds were ever obtained. Some of them easily could have had stronger winds than the May 3 tornado.

Without wind measurements, tornado wind speeds can only be estimated by examining the damage that was done. If we compare damage caused by the May 3 tornado with that of other tornadoes in the past, we find that several other F5 tornadoes have produced damage comparable to, or even more severe than, the May 3 tornado. Some famous examples of other F5 tornadoes that produced comparable or worse damage - and therefore may have been stronger than the May 3 tornado - include the "Tri-state" Tornado of March 18, 1925, the Woodward, OK tornado of April 9, 1947, the Xenia, OH tornado of April 3, 1974, and the Andover, KS tornado of April 26, 1991.

It's also important to know that although the tornado was rated F5, very few areas affected by this tornado actually experienced F5 damage. There were only a few small, narrow areas near the center of the damage path where intermittent F5 damage was found: in the Bridge Creek area (Grady County), and in a few parts of south Oklahoma City and Moore (Cleveland County). This is less than 1 percent of the approximately 15 square miles of damage that this tornado produced. The maximum damage in Oklahoma and McClain Counties was rated F4 (which, of course, is still very devastating damage).