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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Bi-monthly Tornado Counts and ENSO Phase Strength

The charts below illustrate that occasional weak cool-season links may occur between the phase strength of the EL Niño Southern Oscillation (ENSO phase strength) and EF1 to EF5 tornado reports for bi-monthly periods from 1950 through 2010 (1951 to 2011 for the last pair, Dec-Jan period). The charts display a simple running sum of EF1 to EF5 tornado events over the same bi-monthly periods that are used for the Multi-variate ENSO Index, or MEI, (Wolter, K., and M. S. Timlin, 1998).

The bi-monthly charts on the left show tornado events summed for each period with the years increasing along the positive y-axis. The charts on the right use the same bi-monthly tornado sums for each period but years are re-ordered and ranked based on MEI. The strongest bi-monthly El Niño events are at the top of the y-axis while the strongest La Niña events are at the bottom of the y-axis. Years/months between the two extremes are ENSO neutral, or transition periods between the two phases.

Cook and Schaefer, 2008, found an increase in cool-season tornado events during ENSO cool-phase (La Niña) and this finding is borne out to some extent by the charts for the two bi-monthly periods of Dec-Jan and Jan-Feb. These two overlapping cool-season periods exhibit some correlation between summed tornadoes and MEI with no more than 12 percent of the variance (R2) in the summed tornadoes being accounted for by ENSO phase strength (stronger La Niña ~= more U.S. tornado activity).

The weak connection between ENSO cool-phase strength and more tornadoes is maintained, but to a lesser extent, for the Feb-Mar and Mar-Apr bi-monthly tornado events with R2 values of 6 percent and 10 percent, respectively. All other bi-monthly periods throughout the year show very little or no correlation with ENSO phase strength indicating that there are clearly many other positive and negative influences that modulate tornado activity in the U.S. through most of the year. (The R2, or proportion of variance, value is indicated in the upper left of each chart for each bi-monthly period where the years are ordered by ranked MEI value and is also indicated in the caption for each of these charts.)

It is interesting to note that in a couple of the strongest cool-season El Niño cases (1983 and 1998) tornado events appear to be above average when compared to other cool-season El Niño cases. This is especially true of the strongest El Niño case (based on MEI) of Dec 1982 through Jan 1983 (Dec-Jan right-hand chart below). Thus, a very strong El Niño (ENSO warm phase based on MEI) during the cool season does not necessarily result in weakly suppressed downstream tornado activity in the U.S. but may actually act to support more vigorous larger scale storm systems that, in turn, result in more tornadoes.

For additional information please contact Greg Carbin at SPC.

References:

Wolter, K., and M. S. Timlin, 1998: Measuring the strength of ENSO events – how does 1997/98 rank? Weather, 53, 315-324.

Cook, A.R., and J.T. Schaefer, 2008: The Relation of El Niño – Southern Oscillation (ENSO) to Winter Tornado Outbreaks. Mon. Wea. Rev., 136, 3121-3137.

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Jan-Feb (Years in order)
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Jan-Feb (Ranked by MEI), R2=12%
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Feb-Mar (Years in order)

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Feb-Mar (Ranked by MEI), R2=6%
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Mar-Apr (Years in order)
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Mar-Apr (Ranked by MEI), R2=10%

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Apr-May (Years in order)
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Apr-May (Ranked by MEI), R2<1%
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May-Jun (Years in order)
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May-Jun (Ranked by MEI), R2=2%
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Jun-Jul (Years in order)
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Jun-Jul (Ranked by MEI), R2<1%
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Jul-Aug (Years in order)
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Jul-Aug (Ranked by MEI), R2<1%

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Aug-Sep (Years in order)
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Aug-Sep (Ranked by MEI), R2<1%
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Sep-Oct (Years in order)
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Sep-Oct (Ranked by MEI), R2=1%
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Oct-Nov (Years in order)
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Oct-Nov (Ranked by MEI), R2<1%
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Nov-Dec (Years in order)
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Nov-Dec (Ranked by MEI), R2<1%

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Dec-Jan (Years in order)
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Dec-Jan (Ranked by MEI), R2=12%
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April 4-5, 2011 Severe Weather

20110404-05-radsm

The widespread severe weather episode at the beginning of the week of April 3, 2011, resulted in over 1000 individual reports of thunderstorm wind gusts that either produced damage, or were of such magnitude that the observed wind speed was capable of producing damage. One definition the National Weather Service (NWS) uses to categorize a severe thunderstorm is: any thunderstorm producing a wind gust equal to or greater than 50 knots (58 mph), or observed wind damage resulting from estimated wind speeds equal to or greater than 50 knots (58 mph).

A truly remarkable number of wind events did occur in this recent event. In fact, while the number of wind reports across a 24 hour period remain preliminary and will need to be further reviewed by NWS meteorologists in the affected areas, it is likely that in the final analysis, the wind reports numbers alone will far exceed any other severe weather outbreak in the official records of the NWS. However, using wind reports alone can be misleading when attempting to put an event such as this into some meteorological and historical perspective.

The image above is a composite of 24 hours of base reflectivity data, taken approximately every three hours, to show the evolution of the squall line across the southern United States. The line traveled more than 800 miles in about 24 hours with an average speed of something between 30-40 mph. Clearly, surface wind speeds within the intense segments of the line must have been blowing harder to produce such a huge number of reports. However, there have been other recent examples of intense linear convection (squall lines), some moving across this same region of the country, with faster overall system motions.

4cases

At left are composites of four damaging wind events from 2009 that had overall system speeds greater than this most recent event. One big difference in these events, when compared with April 4-5, 2011, is that the linear extent of the squall line was more compact. The May 3, 2009 event covered 570 miles in eight hours at an average speed of more than 70 mph and only produced about 165 wind reports. Arguably, this was a more intense and faster-moving squall line than what occurred on April 4-5, 2011, but it was concentrated over a much smaller area.

Probably the one event of the four depicted from 2009 that comes closest in terms of areal extent to the recent outbreak is the February 11, 2009 squall line (upper left). This event produced more than 400 reports and traveled at an estimated forward speed approaching 50 mph. Even with a greater forward speed across some of the same general area of the country as the event on April 4-5, 2011, the total number of wind reports was less than half of those recently reported. This event, coming during the winter months, was in an environment characterized by generally weak instability but incredibly potent dynamics.

These examples serve to illustrate the point that severe weather episodes come in all shapes and sizes and, combined with secular trends in severe weather reporting, increasing population density, and information technologies allowing the rapid transmission of observations, make characterizing these events and placing them in historical perspective very difficult. April 4-5, 2011 was a very “big day” in terms of reports and squall line extent. But, how big a day was it when compared to other “big days”?

The greatest single-day annual wind event totals, for each year since 2005, are summarized in the table below.

Date => 6-Jun-05 2-Apr-06 19-Jun-07 11-Jun-08 11-Feb-09 5-Aug-10 4-Apr-11
Wind Reports/Sig 350/13 455/38 358/15 432/10 375/16 447/11 1160/6
Total Watches 13 19 11 6 6 5 16
Fatalities* 0 28 1 6 1 1 ~8
Injuries* 1 357 3 53 1 15 ~2
Warnings 1014** 1412** 711** 659 311 463 568
*includes tornado
**county-based

The greatest values for any of the event characteristics are highlighted in red font. Please note the numbers for this most recent event remain preliminary, as already mentioned above. While the April 2-3, 2006 event had fewer than 50% of the wind reports of the 2011 event, it ended up with considerably more “significant” wind observations. The Storm Prediction Center (SPC) considers “significant wind” as any gust of 65 knots (75 mph) or greater. The 2006 event, coming at nearly the same time of year, had 38 thunderstorm wind gusts in excess of hurricane force. And, despite encompassing a more concentrated area of the Midwest, this outbreak required the issuance of three more watches from the SPC, and had a lot more warnings associated with it. Most of the fatalities that occurred in April 2006 were associated with tornadoes, not straight-line winds. The injuries from this event far exceed those reported so far for the most recent event.

There are other methods one could use for some of these recent events, including looking at the relatively dense automated observational network where thunderstorms and wind gusts are observed. Given the 1000+ wind reports from April 4-5, 2011 over such an expansive area, one would think several airport observations would record wind gusts in excess of 50 knots, along with thunder. A preliminary investigation into this question shows a total of less than 10 observations had thunder and wind gusts observed at or above 50 knots. There are only three observations on any hour in the 24 hour period that meet severe thunderstorm criteria (thunder with wind gust equal to or greater than 50 knots). When intra-hour special observations are included in this analysis, there are only three additional thunderstorm “PEAK WIND” gusts of 50 knots or greater that can be added to the total. The ratio of thunderstorm observations, to thunderstorm observations with gusts meeting severe criteria (>= 50 knots), and to overall wind damage reports, is another aspect of “big wind” outbreaks that can be used to measure their characteristics and place them into better meteorological perspective. This particular  investigation is very preliminary and any additional information on this approach may be released here on the U.S. Severe Weather Blog.

Ultimately, the most important aspect of these events from an NWS and SPC perspective is that we get the word out to the public that trouble is on the way. The map below shows the extensive nature of the NWS watches and warnings issued on April 4-5, 2001.

20110403-0406

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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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Can tornado activity through April portend May?

Short answer: apparently not! Longer answer follows.

Few tornadoes have occurred to date in 2010, especially when compared to very active recent years such as 2008. The adjusted annual tornado trend shown below indicates that 2010, with 79 tornadoes (adjusted) through April 15, is rapidly approaching the minimum number of tornadoes expected through this time of year (about 75). While 2010 has not quite reached that adjusted minimum threshold yet, it looks like it may be only a matter of another day or two with few or no tornadoes.

tornado chart This graph shows how the number of tornadoes so far in 2010 in the United States is very near the adjusted minimum value expected. Additional information about the chart and the method used to adjust for tornado report inflation can be found here: http://www.spc.noaa.gov/wcm/adj.html%5B/caption%5D

Does the scarcity of tornadoes so far in 2010 say anything about what we can expect for tornado activity in May? Let’s review the past 30 years of tornado counts and see. Below is a chart depicting the departure from the decade’s average number of tornadoes for two periods in each year (1980-2009). The first period is the departure from the decade’s average number of tornadoes, January through April (gold). The second period is the departure from the decade’s average number of May tornadoes (light blue). In the early years of this chart (1980 through 1983), this chart would suggest that, yes, we can say that an above (below) average start to the year through April is indicative of a  above (below) normal May. However, that forecast methodology falls apart in 1984, 1985, and 1986 when, in each of those years, an above normal January-April period was followed by a below normal May. In 1987 and 1988, there is a strong correspondence between a low number of tornadoes through April, and a slow (below normal) May. Then in 1989, an inverse relationship occurs when a below normal start to the year is followed by an above normal May.

tor charg

 

With the exception 1995, there is a remarkable 13 year stretch from 1990 to 2002 when the start of the year could possibly be used as an indicator of the May to come. During this period there were nine years when normal to below normal tornado numbers in the January through April period were followed by normal to below normal tornado activity in May. There were only three years during this period (1991, 1998, and 1999) when an above normal start to the year continued into May. If you were looking at only those years you might conclude that in 2010 we will see below normal tornado activity into May.

Trouble starts again in 2003 and 2004 when low tornado counts through the early months of the year are followed by remarkably active Mays. In fact, May 2003 and May 2004 are the most anomalously active Mays of the 30-year period and both were preceded by some of the most anomalously inactive January through April periods! Some semblance of correspondence returns in 2005 (both periods below normal) before a reversal shows up in 2006 and 2007 with above normal January through April periods being followed by below normal Mays. One is entitled to be quite flummoxed by this point in the analysis but the conclusion should be that, at least in the most recent years, making a prediction about tornado activity in May, based on the character of the season to date through April, would be a tricky bet.

Last year saw an active start through April only to be followed by a somewhat quiescent May. That situation was addressed in this blog post.

Despite the fact that the correlation coefficient (r) for this entire time series is an abysmal 0.15, there is still hope (albeit diminishing) that we can say something about the predictability of the future based on what has happened in the past. So, one last chart for all of those folks hoping to capture a tornado during the upcoming VORTEX2 field program…

tor chart 2

 

The correlation coefficient on this time series for tornadoes only occurring in Texas, Oklahoma, Kansas and Nebraska is barely improved over the previous comparison for the entire United States. It’s quite possible that charts used to evaluate the stock market would exhibit a similar lack of correlation. And, perhaps it’s fitting to think of this little experiment in the same light by using an oft-quoted line from a typical mutual fund prospectus: “Past performance is no guarantee of future results.”

Stay tuned. We’ll know the answer soon enough.

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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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U.S. tornado droughts since 1990

Preliminary reports indicate that 23 days passed between November 7th and 29th, 2009 without a single tornado report in the continental United States. That period turns out to be about the average duration of an annual  tornado drought based on recent records.

The U.S. should experience tornado droughts spanning a little over 20 days about once per year. The longest tornado-free span in the last two decades occurred over a 45 day period from December 31, 2002 through mid-February, 2003.

Other tornado droughts in the longer-term record include the following:

57 days, ending on Nov. 14, 1952.
54 days, ending on Jan. 28, 1956.
52 days, ending on Feb. 2, 1986.
45 days, ending on Jan. 19, 1961.
45 days, ending on Jan. 22, 1981.

In an attempt to better visualize tornado “drought” durations, and where those episodes have occurred on the calendar, I put together a chart depicting the longest annual tornado droughts since 1990.

U.S. Tornado Droughts (1990-2009)

This chart seems to indicate that most November tornado droughts are relatively short in duration. There have been six November droughts in the last 20 years. The November 2002 drought was the longest, at 30 days, and continued into December before ending. Over half (11) of the tornado droughts began or ended in December but it is not uncommon for tornado droughts to span January/February too.

There are other times in the year when there may not be a tornado for a few days, or up to a week. Those spans are not shown here. Analysis of the records suggests that our most recent tornado-free November ended just about on schedule after 23 days.

Incidentally, the Storm Prediction Center (SPC) issued no Tornado or Severe Thunderstorm Watches during November 2009. This is the only November in the watch database (since 1970) when no watches have been issued during the month. But, that’s a subject for another post, perhaps!

Harold Brooks has been conducting a return period analysis on the tornado record for different tornado (E)F scales. He is likely to provide additional insight in an upcoming blog post on this subject so stay tuned!

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