Low Tornado Numbers and Low Tornado Deaths, May 2012-April 2013

Updated 10 May 2013 to add new information from April 2013

Updated 2 May 2013 to correct typo on date of previous low tornado count

The 12-month period from May 2012 to April 2013 was remarkable for the absence of tornado activity and tornado impacts in the United States.

We can start by looking at the number of EF1 and stronger tornadoes during that period. A final count is available through January 2013 and we have a pretty good estimate of how many occurred in February through April, although final numbers won’t be available until July. Although the 12 month total may change a little bit with the final data, it’s unlikely to change enough to affect the results here.

From May 2012-April 2013, the estimate is that there were 197 tornadoes rated EF1 or stronger. Where does that stack up historically? Well, we have pretty good data back to 1954. During that time, the previous low for (E)F1 and stronger tornadoes in a 12 consecutive calendar month period was 247, from June 1991-May 1992. The next lowest (ignoring the overlapping periods, such as April 2012-March 2013) was 270 from November 1986-October 1987. The lowest non-overlapping 12 month counts on record from 1954-present, with the starting month, are:

217 May 2012 (preliminary)
247 June 1991
270 November 1986
289 December 2001
298 June 2000


This apparent record was set less than two years after the record for most EF1+ tornadoes in a 12-month period was set, with 1050 from June 2010-May 2011. The time series showing the evolution of the number of (E)F1+ tornadoes since 1954 is below. The number of (E)F1+ tornadoes in the 12 months beginning with the time on the x-axis is plotted for every month starting in January 1954 and ending in May 2012, the most recent point.

The death toll from May 2012-April 2013 was 7. National Weather Service official statistics go back to January 1950, but we can extend that by using the work of Tom Grazulis from the Tornado Project, who has collected tornado fatality information back into the 17th century. The data are reasonably good back to 1875, but it’s still possible that there are some missed fatalities, particularly as we go back farther in time. So, where does 7 fatalities in 12 consecutive calendar months stack up? Again, here are the lowest totals, going back to 1875, for 12 consecutive months, with the starting month. (For overlapping periods, such as April 2012-March 2013 and May 2012-April 2013, only the lowest period is listed.)

5 September 1899
7 May 2012
8 August 1991
12 November 1909
12 May 1940


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The Tornado “Drought” of 2012

UPDATE (14-Aug-2012): Graph corrected to indicate 2006 as previous 15 Apr-31 Jul minimum.

The 2012 tornado season in the United States got off to a quick start with well-above average numbers in January, February, and March. Later, over 80 tornadoes occurred on 14 April. Since then, the number of tornadoes in the US has been unusually low. In order to understand how low, we need to look at the long-term history of tornado occurrence. The most reliable portion of the tornado data begins in 1954 but, even after that, we have to be careful in how we interpret it. Since the mid-1950s, the number of tornadoes reported has increased by an average of 14 per year. The increase has been almost entirely in the weakest tornadoes (F0) and is highly likely that the causes are non-meteorological. We can think of this increase in the same way we think of inflation in economics and estimate its impact by adjusting historical tornado counts to account for it. This process, and how it can be applied to part of the year, is discussed here.

That inflation-adjustment process allows us to look at historical data, but a problem still remains of how to look at recent reports. Preliminary, eyewitness reports of tornadoes are collected by local National Weather Service Forecast Offices and the offices then evaluate those reports and produce a list of “final” tornado reports. This process of evaluation takes a few months to complete, so it can be challenging to answer the question “how many tornadoes occurred” shortly after an event. Over the last several years, a simple relationship between the preliminary and final reports has been observed with the number of final reports being approximately 85% of the preliminary reports. As a result, when looking at the preliminary reports in recent months, we can a pretty good estimate of the final reports simply by multiplying the preliminary reports by 0.85.

Let’s look at how many tornadoes we would expect based on the inflation-adjusted tornado count and compare this year’s tornadoes to that long-term expectation. To emphasize the small number of tornadoes since the middle of April, we’ll start on 15 April and add up the number of tornadoes each year through the end of July. In the accompanying chart, we see the distribution of the accumulated number of inflation-adjusted tornadoes as we got from 15 April-31 July. The distribution is based on the period from 1954-2011. The maximum and minimum of any of those years are shown in blue (note that the year associated with the maximum and minimum can change from day to day along the way). The heavy black line is the median of the distribution, the gray lines are the 25th and 75th percentiles (half the years will be between them), and the dashed lines are the 10th and 90th percentiles (4 out of 5 years will be between them). For comparison, the estimated number of final tornado reports from 2012 are shown in red.

Accumulated number of tornadoes from 15 April-31 July from 1954-2011 with 2012 compared to it.
Accumulated number of tornadoes from 15 April-31 July from 1954-2011 with 2012 compared to it.

Through the end of May, the tornado count for the period from 2012 goes along at approximately the 10th percentile of the long-term distribution but, after that, falls well below the previous low. To put this into perspective, the estimated number of final reports from June for 2012 is approximately 100. The previous inflation-adjusted low for any previous June is 94 in 1988. (Remember that the blue line represents the fewest number of tornadoes from any of the 58 years from 1954-2011.) The median number of June tornadoes in 1954-2011 was approximately 270.

July was even more remarkable than June. Only 24 preliminary reports were received, leading to an expected number of final reports of a little over 20. The lowest number of inflation-adjusted tornado reports from 1954-2011 is 73 (1960). Even without inflation adjustment, the fewest number of tornadoes in any July in that time period is 42 (1960), emphasizing the extraordinary nature of this July. The median number of July tornado reports is about 150.

When we look at the whole period from 15 April-31 July, the median tornado count in the record is 850, compared to 2012, with a little under 300. The 850 represents almost 2/3 of the usual annual total of about 1300. One way of thinking about the late spring and early summer tornado season is that the atmosphere missed more than 40% of a typical year’s tornadoes in 3 1/2 months. Compared to 2003, the comparable period in 2012 had more than 900 fewer tornadoes. 2011 had the second highest number of tornadoes in this part of the year, so in the last two years, the US has experienced the extreme high end of the distribution of the number of tornadoes and the extreme low end of the distribution.

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No Tornado Deaths in May 2012

In May 2012, there were no tornado deaths in the United States. How unusual was that?

We can look at the record of tornado deaths, discussed here, dating back to 1875. The last time there were no deaths in the month of May was in 2005. Prior to that, it was 1994. Overall, there have been 15 years in the 138 years of the record (1875-2012) with no deaths in the month of May, so we’d expect that to happen about once every decade.

May 2012 stands in dramatic contrast to May 2011, when 178 people died in tornadoes, 158 of them in the Joplin, Missouri tornado of 22 May. 178 deaths is the fifth highest death toll in the period 1875-2012, and the largest since 211 people died in 1933. The deadliest May on record was 1896, when 502 people were killed, including 255 in the Saint Louis, MO-East Saint Louis, MO tornado of 27 May. Adjusted for wealth of the country, that tornado was the costliest in US history, with damage adjusted to 2011 dollars of over $6 billion.

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10 May 2010 tornado outbreak

(This will be updated as more information and time to post become available. Given the local nature of the event, information may be obtained quickly, but time may be at a premium.)

A significant outbreak of tornadoes occurred over Oklahoma and southern Kansas on 10 May 2010. Numerous damaging, long-track tornadoes have been reported from the Red River on the southern Oklahoma border up through southern Kansas. As of 6 AM CDT, 11 May, there have been 37 preliminary tornado reports. This is likely to change following damage surveys that will begin later today. A very preliminary summary of information from the National Weather Service Forecast Office in Norman covering their region is available here. The Tulsa Forecast Office also has a briefing on the eastern Oklahoma portion of the outbreak.

At this time, there are five fatalities reported in Oklahoma, 2 from the Choctaw area of Oklahoma County and 3 from Tecumseh in Pottawatomie County.

VORTEX2 collected data on the storm that produced the Norman tornado east of Norman.

There were media reports of damage at the National Weather Center. These reports are untrue, although the tornado was visible from the NWC and debris could be seen as the tornado moved east of the NWC. In addition, some staff members suffered damage at their residences.

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Some brief notes on the 24 April 2010 long-track tornado

Preliminary information on the long-track tornado that went from eastern Louisiana across much of Mississippi.

1. It had a path length of at least 97 miles. Over the last 40 years, we’ve averaged about one 100 mile path length tornado every 2 years.
Update (27 April): The path length is now given as 149 miles, the 6th longest since 1970. A crude estimate is a once per 8 year event.
2. There were 10 direct fatalities with the tornado. The last double-digit death day was 25 May 2008 (the day that Parkersburg, Iowa was hit.) The last double-digit fatality tornado was on 10 May 2008 (Picher, OK and southwestern Missouri.
3. It’s the most fatalities in a tornado in Mississippi since 21 November 1992 (Brandon-12 fatalities)
Update (27 April):
4. Deaths by circumstance: 6 mobile home, 2 outdoors, 2 vehicle.

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One tornado in February 2010 (UPDATED)

UPDATE: A late report has been received. At 445 PM on 27 February, a tornado 15 miles northeast of Taft, California occurred. It was weak, lasted approximately 3 minutes, caused no damage, and was rated EF0. So, instead of no tornadoes in the calendar month, we have had one tornado reported in the US for February 2010.


There were no tornadoes reported in the United States in February 2010.  Assuming that no late reports are received, it will be the first time in the National Weather Service’s database that starts in 1950 that there has been a February without a tornado.  If we include Tom Grazulis’s database of F2 and stronger tornadoes, the last time it’s possible there wasn’t a February tornado was 1947.  The last tornado reported in the US was on 24 January, in north-central Tennessee.  The last calendar month without a tornado was January 2003.

What does this tell us about the rest of the 2010 tornado season?  Somewhere between a little and nothing at all.  Most years that have started out with few tornadoes have ended up average or below.  However, there have been big exceptions.  Most notably, in 2003, we started out with no tornadoes in the first 45 days of the year.  Even as late as 29 April, it was the slowest start in the database (after adjusting for report inflation, as discussed here.) By the 11th of May, however, 2003 was well above normal following a remarkably active week. So, even though it’s been a slow start to the season, people still need to be aware of the threats that may happen later on.

What does it tell us about long-term trends? Again, essentially nothing. The large-scale atmospheric pattern that persisted over the US for the month of February was unfavorable for tornadoes. There’s nothing in the scientific literature that provides information on any changes to expect with tornadoes in the future, so the no-tornado February can’t be interpreted in that light.

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The “shape” of the 2009 tornado season

One of the interesting questions that comes up every year is how to compare a tornado season with previous seasons. In 2009, that question has come up particularly in light of the quiet conditions lately. One way to do this is to start by adjusting historical reports for the long-term “inflation” in reports. If we assume that the “shape” of the annual progression of tornadoes for each year is true and that only the total number would be adjusted, inflation-adjusted annual cycles can be constructed for each year.

In order to compare any particular year to the average year, we can see how far ahead or behind that year is to the average for each day of the year. In other words, if there are 500 tornadoes at the end of May, that would be about 100 behind the long-term average of about 600. If there are 700, it would be about 100 ahead of the average year. A time series of this departure through the year gives an idea of how far ahead or behind the season is throughout the year.

Tornado anomalies through the year
Tornado anomalies through the year

On this graphic, positive departures indicate above average and negative departures are below average. If the line is horizontal, that period of the year is exactly average. The years shown include the biggest years in the tornado record (2004, 2008) and the smallest years (1987, 1988), as well as 1973 and 1992, which demonstrate different ways that a big year can happen. In 1973, there aren’t any really large periods for tornadoes, but a steady increase ahead of normal throughout the year. In 1992, the early part of the year looks a lot like the really slow years of 1987 and 1988, reaching more than 300 below normal by 14 June. After that, a short period of frequent tornado occurrence got the season back to normal within a month and then ending up more than 200 ahead by the end of the year.

The black dots represent 2009 at the end of each month and mid-June. Reports after March 2009 have been adjusted as discussed here. The gray line is for 1996, the closest analogue to 2009, where the season was ahead of normal through early May, then fell almost 100 below normal by the middle of June before finishing about 50 ahead.

In reality, despite the slow period that coincided with the VORTEX2 field project when, for a 5-week period, there were over 100 tornadoes fewer than normal, the 2009 season has stayed within 100 of the normal throughout the year.  Normally, by 22 June, more than 60% of the median 1290 tornadoes per year have occurred.  We’ll have to wait to see how 2009 turns out.

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US Annual Tornado Death Tolls, 1875-present

Updated 23 May 2013 to include data through 2012

One of the reasons that we started this blog was to provide basic information on severe weather and its impacts.  Frequently, we get questions along the lines of “What’s the average X per year?” For many different “X”es, this is not as easy of a question as it might appear.  In large part, this is because many of the things we deal with have large trends in them (severe weather reports per year), so that the “average” depends on how far back you go in the record.  In order to make it easy for people who want the data to get the answer they need, we hope to put some of those numbers online here.

The first dataset is the number of deaths per year from tornadoes in the United States. The National Weather Service archive goes back to 1950. Brooks and Doswell (2002) discussed the long-term history of tornado deaths, drawing on the work of Grazulis (1993) (Grazulis, T. P., 1993: Significant Tornadoes, 1680-1991. Environmental Films, 1326 pp.). Reasonably reliable estimates of deaths per year can be made back to about 1875 by using the Grazulis data.

The Brooks and Doswell paper had a graph of the annual death toll normalized by population of the US through 2000.  Here is an updated version (through 2008) of that figure, showing that the death toll per million population appears to have leveled off in the last decade or so.

The purple points are the annual death rates, the red line is a simple smoother, the solid black line is a long-term trend in two sections (1875-1925, 1925-2000) and the cyan lines are estimates of the 10th percentile and 90th percentile from 1925-2000. Brooks and Doswell (2002) have an extensive discussion of the record and its possible implications.

US Tornado Deaths Per Million People

At the end of this post, we have a table of the annual death tolls going back to 1875 (1875-1949 from Grazulis, 1950-2008 from the National Weather Service.)  Although the data represent our best understanding at this time, it is possible that the numbers could change, if additional information was found.  Occasionally, it’s discovered that a fatality associated with a tornado was missed or double-counted. We’ll correct such entries if we find about about them, but that will likely be a rare event.  The death tolls are for direct deaths, i.e., someone killed directly by the impact of the tornado.  It does not include indirect deaths, which might not have occurred if the tornado had not happened, but the tornado was not an immediate cause.  Examples of indirect deaths that have occurred include a heart attack upon seeing damage to a neighbor’s house, falls when going to shelter, and a fire caused by a candle lit when the power went out after a tornado.

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The relationship between preliminary and final tornado reports

One of the most frequently asked questions about tornadoes during 2008 was “How many tornadoes have there been this year?”  As with many of the questions we get, it sounds like a simple question that should have a simple answer but, unfortunately, it’s not.

First, it’s important to understand how tornado information is collected. Reports come into local National Weather Service Forecast Offices from a wide variety of sources (trained spotters, emergency personnel, the media, the general public.)  Those reports get put out relatively quickly and form the basis of the preliminary, so so-called “rough”, log of tornadoes.  As time goes on, more reports may come in, increasing the count, but the local Warning Coordination Meteorologist (WCM) may also find that some of the reports are of the same tornado, lowering the count.  Within a couple of months of the end of the month, the WCM submits a list of tornadoes that the National Climatic Data Center and the National Weather Service’s Storm Prediction Center’s WCM go through and make sure that tornadoes that cross from one office’s area of responsibility into another office’s area only get counted once and do some quality control.  Eventually, this produces the final, or “smooth”, log of tornadoes. Asking the question “How many tornadoes have there been this year?” right after an event gives us the problem of only having the preliminary data avaiable.  Answering someone’s question with “Ask us again in a few months” is not usually seen as satisfactory by the questioner.

The approach we’d like to take is to look at the historical relationship between preliminary reports and final reports. In general, our naive expectations would be that there might be increases from the preliminary log to the final log on days with very few, if any preliminary reports, and there might be decreases on days with long-track tornadoes that might be reported many times. Which effect wins out, and by how much?

In the spring of 2008, we started looking at this question in detail and started by looking at monthly and annual totals.  The ratio of the final total to the preliminary total for each month from January 1998 to November 2008 with at least 30 preliminary reports is shown in red in the figure below.  A running annual total for the next twelve months is shown in black.

Monthly/12-monthly Final to Preliminary Tornado Count Ratio
Monthly/12-monthly Final to Preliminary Tornado Count Ratio

The result was surprising.  A rather sudden change took place in early 2006.  Prior to that, the final log tended to have about 10% more tornadoes than the preliminary log.  The ratio had, perhaps, trended slightly downward, but starting in March 2006, it dropped precipitously, such that for 2007 and 2008, there were 20% fewer tornadoes in the final log than in the preliminary log.  This, obviously, was a huge change and meant that our intuition of what the preliminary log meant in terms of the number of tornadoes was wrong.

We then looked at the differences on a day by day basis.  The figure below shows the change from preliminary to final tornado count as a function of the preliminary count.  As we might expect, there’s not much difference on the “small” days and some of those have more in the final than in the preliminary and some have fewer. For “big” days, the final log tends to have fewer.  We broke things down in the periods 1998-2005 and 2006-2008. Linear regression lines are shown as dashes for each of the two periods.  For the early period, for 100 preliminary reports, the final total, on average, was about 10 lower.  For the later period, it was about 30 lower.  Only once in that three-year period did a preliminary count of at least 40 end up with more in the final count.  In the earlier period, that was a common event. The effect on the 2008 totals was that, instead of adding about 200 tornadoes from the preliminary to the final totals, you had to subtract more than 400!

Changes in final report count as function of preliminary reports
Changes in final report count as function of preliminary reports

We admit we’re not entirely sure what happened in March 2006.  A number of small changes in software for collecting reports were implemented about then, but it’s not clear if that’s enough to explain what happened.  We plan to monitor the situation and use the regression from the recent data to estimate the final count, based on the preliminary count.

This is another example of how challenging it can be to deal with what seems to be a simple dataset.  Caveat lector!

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The rarity of the Oklahoma tornadoes of 10 Feb revisited

One of the questions we get asked a lot about significant tornado events boils down to “How unusual was this?”  The answer is, “That depends.”  It depends on exactly what aspect you’re asking about.  Sometimes, it’s very easy to quantify.  Other times, it’s not.  The case of the 10 February 2009 Oklahoma tornadoes is an excellent example of how difficult it can be to answer.  A lot of the discussion I’ve seen has dealt with the number of tornadoes in the state of Oklahoma during the month of February.  Although that provides a very neat and well-defined answer, it’s probably not all that relevant.  February is a month of very strong gradients in space and time in the occurrence of tornadoes, especially in the southern Plains.  Most of the tornadoes in Oklahoma during that month occur in the eastern part of the state and the late part of the month.

To illustrate, I’ve put together a series of “postage-stamp” maps showing tornado touchdown locations by week for each of the first nine weeks of the year (e.g., 1-7 January, 8-14 January, etc.) for 1950-2007, with the Oklahoma tornadoes of this week overlaid on the 6th week’s map (5-11 February).

Postage stamp maps of tornado locations
Postage stamp maps of tornado locations

In general, there’s a fairly sharp boundary on the northwest side of the locations of tornadoes. For the 6th week (middle panel on the right side), the recent tornadoes are right at the edge of that boundary. Over the next three weeks, however, tornado occurrence becomes much more frequent west of where that boundary is earlier in the year. Thus, a qualitative answer to the question of rarity would be that the tornadoes would have been much more likely if they had been either a couple of weeks later or a couple of hundred miles southeast. Quantitative answers are much harder to develop.  I’ve developed techniques in the past to look at that question, but for events like this that are right at the edges of what we’ve observed in the past, the answer you get is very sensitive to the assumptions you make.

Another point that can be seen is that the 7 January 1992 tornadoes in central Nebraska (see the upper left panel) are a very long distance in space and time from anything else. They are likely to be among the rarest of tornadoes.

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