Tornado Climatology

TORNADO CLIMATOLOGY


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The severe weather and tornado climate across our coverage region can be horrific and violent, but at other times, quiet in nature. This is largely due to the underlying fact that our coverage region lies on the northern most periphery of what is affectionately referred to as "Tornado Alley". Tornado alley is that part of the country which extends from the southern plains northward to Kansas and Nebraska, then bends east across the Mississippi river valley area. This is where tornadoes are most frequent. While every state in our coverage region obviously has reported tornadoes since accurate records began in 1950, the overall trend in tornado events seems to be slowly declining. The primary reasoning for this is due to "El-Nino" and "La-Nina" events over the equartoral Pacific ocean. This events have high impacts on the main jet stream winds and their placement over the earth's atmosphere. Recent studies have shown a decrease in the number of severe weather episodes across our coverage region in "La-Nina" events. That's because the jet stream is positioned further south and west of the coverage region, so the warm and moist air can't move far enough north to meet up with a cold air mass from the north. Yet despite this, battles still happen and severe does develop across the area. On this page, we'll discuss more about those severe weather and tornado trends across our coverage region, and present some interesting statistical facts.

If you wish to view statistical data for the country, please use this link.

2010-2015 SEVERE REPORTS
Click on tabs to reveal other data.

    COMPOSITE         |          TORNADOES         |          LARGE HAIL         |          DAMAGING WINDS

HIGHLIGHTS...Coverage Region

Typically, the season of peak tornado activity is from April to September, but tornadoes have been reported outside of the primary season too. The months of greatest tornado activity runs from March to July, but again, peak activity has also occurred in other months as well. On average, our coverage region has about 972 documented tornadoes based on a 30 year study from 1980 to 2010. The states with the greatest total tornadoes reported are Iowa and Illinois. That's because those states are within the northern periphery of "tornado alley". About 85 percent of the tornadoes which are reported in our region are of the "Weak" category, F-0 to F-1. About 12 percent are of "Strong" category, F-2 to F-3. Finally, about 3 percent are of the "Violent" category, F-4 to F-5. In addition to tornadoes, large hail and damaging winds associated with severe thunderstorms are much more common. Studies done in early 2001 by our office have shown a marked increase in the number of reported large hail and wind events since that year. However, this study did not include any potential duplicate reports. Lightning is also a major concern during periods of thunderstorm activity, and kills an average of 49 people across the country. Flash floods rank 4th and are another concern when heavy rainfall occurs. It is very noteworthy that the overall death rate from severe weather events across our region remains very low despite the number of storm events reported, which too, were lower then previous years (prior to 1980). Just from tornadoes alone, the overall total number of related deaths across our coverage region was 206 during the period from 1980 to 2010. This equates to a yearly average of 34 deaths. This is down markedly from the previous total number of deaths. For all forms of severe weather, it appears that large hail reports is trending higher, while severe wind reports is trending slowly downward for the coverage region.

Figure 1 illustrates the cumulative tornado count and related deaths from across our coverage region from 1980 to 2010.


Fig.1


SYNOPTIC ASSESMENT...

As mentioned above, while not totally related to existing weather patterns acros the lower 48 states, Pacific ocean surface temperatures have been an influence on the jet stream winds fashioned across the country. Studies from 2008 to 2013 have concluded that El-Nino/La-Nina patterns over the Pacific have had tendencies to position the subtropical jet stream across the southwest states, then turning more northeast across the Ozarks and Ohio valley. With this general position, less surges of deep rich moisture near the surface off the Gulf of Mexico can make it this far north. Also, with the polar jet being further south (running parallel with the International border with Canada), the region is more exceptionally cool and drier, although severe storms do happen from time to time as better moisture influx is realized. When compared to back in the 1960's, 1970's, and 1980's, the overall trend with strong to violent tornado outbreaks have been on a slow decline, though their have been exceptions. One such noteable exception was the "Super Outbreak" of tornadoes on April 3-4, 1974. In that two day barrage of twisters, a total of 148 tornadoes were documented. In our coverage region this outbreak affected the states of Illinois, Indiana, Ohio, and Michigan. The storms killed 315 people total, and injured 5,484 more. The total number of those killed in our coverage region was 89. This event stands as the single worst tornado outbreak on record! Still another notable event was the July 3-4, 1977 Derecho which struck Minnesota, Wisconsin, and Michigan. This Derecho event packed winds to over 115 mph in swaths producing incredable devastation. Their were other notable events which occurred across our coverage region.

So even though with the influences of El-Nino and La-Nina events, and how they affect the jet stream winds and associated weather patterns across the globe, our region still can get hammered by another barrage of tornadoes, large hail, and damaging straight line wind events. Studies done recently have shown a drop in the overall number of reported tornadoes in our coverage region, while large hail and damaging wind reports have risen sharply since 1998. Mathematically, the chances of another major outbreak of tornadoes anywhere within our coverage region is "0.013589", or about once in 500 years. That simply means that another large and violent tornado outbreak is highly unlikely within the coverage area. But don't be fooled here! This does not say that another bout of severe weather won't ever happen either! The odds don't support it, but another episode can still happen. In figure 2, the total number of strong F-3 F-4 to violent F-5 tornadoes is illustrated. Also shown is the total number of fatalities and related injuries from 1950 to 2010 in our coverage region.


Fig.2

TORNADO PEAKS...Months of peak activity and off peak months

Since reliable records were begun by the U.S. Weather Bureau back in 1950, to document tornadoes and other severe weather, studies were also done as to when tornadoes typically happened (specific months), and even the clock hours in which they developed. These studies ultimately were revised by the National Weather Service in the middle 1990's, with all new information. Based on these studies, it seems that the most likely tornado peak runs from April to July. July of course, is the month of highest tornado events. But, oddly enough, their can be offsets to this too.
In figures 3-6 below, not only is the months of peak tornado events illustrated, but also the number of Derecho events, large hail events, and damaging wind events. The totals are the cumulative number of events documented.

Tornadoes...By the month and yes, clock hour!

As most people should already know by now, that tornadoes can happen at anytime or anywhere around the world. No country is immune from them. But by far, tornadoes are most common here in the lower 48 state region. In the coverage region, tornadoes have occurred in all months except December, January, February, and March. Tornadoes can and have happened at all hours of the day or night too. Their is no favored hour of occurrance. In table "A" below, figures 7-12 respectively, we illustrate tornadoes by the month for each state.

TABLE A

In table "B" below, figures 13-17 respectively, we illustrate tornadoes by the clock hour for each state in our coverage region. (Note: for reasons which are unknown, this data is missing for Minnesota).
TABLE B

On this page, we have discussed in detail, many of the latest climatology facts about tornadoes which have occurred in recent years and decades gone by. We have also covered many of the reasons which supports these climatology trends, and thinking. But, like many other things in life, history and climatology will continue to evolve in years ahead, and then perhaps the climatology picture may change too. Stay tuned!!

For more information about tornadoes, severe thunderstorms, and other related statistics, check out these pages.
  1. Learn about tornadoes: Everything about tornadoes, what they are, look like, how they form, etc here.
  2. Wisconsin Tornado Data: The latest available tornado data for the state and local area is here.
  3. Additional severe weather statistics: Find out additional tornado and severe storm information here.
  4. Learn about tornado history: Find out some memorable storms to hit our coverage region here.


Note: Much of the data included on this and corresponding pages was obtained from official data from the NCDC, or "National Climatic Data Center", and "National Weather Service" datasets. Monthly tornado totals and deaths are "preliminary" and considered unofficial. For official information, consult the publication entitled "Storm Data", which is issued by the NCDC.

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