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May-June 2012

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Tornadoes 2011: An Unprecedented Year

Tornado statistics are like baseball statistics. There are many ways to describe a season and the performance (or lack thereof) from the players. Unlike baseball, however, tornadoes and the atmosphere play by rules that we do not completely understand. The impacts that tornadoes have on mankind are determined by demographics and underlying societal vulnerability—slower-changing factors that are also difficult to predict and track. The fact that weather is highly variable allows us to state unequivocally that the tornadoes of 2011 were unlike any tornadoes that came before, and will remain different from tornadoes in the years ahead. Nonetheless, there are historical analogs to some of the 2011 tornado events that can provide insight into how often significant tornadoes, and especially significant tornado outbreaks, may occur in the future.

For people from Oklahoma to Arkansas and Missouri, and from Mississippi to Alabama, Georgia, Tennessee, North Carolina, Virginia, and even Massachusetts, 2011 will be remembered as a year of catastrophic devastation, and unfortunate and untimely death. Even for seasoned severe-storm forecasters, the events of 2011 are sobering and difficult to place in historical context. In 2011, 550 people lost their lives to 59 killer tornadoes. This makes 2011 the fourth-deadliest year for tornadoes in recorded history. Only 1925, with over 700 deaths; 1936, with 552 fatalities; and 1917, with 551 deaths, exceed the stark death toll of 2011. Over the past 50 years, the United States has averaged 72 tornado deaths from 22 killer tornadoes per year. The 59 killer tornadoes of 2011 are surpassed only by the 73 killer tornadoes of 1974. The 2011 events that culminated in an eightfold increase over the average annual number of tornado fatalities, and the historical significance of 2011, will be studied and discussed for years.

Annual tornado fatality figures have shown a steady decrease in the last century (see Figure 1). Enhanced understanding of severe weather meteorology, and subsequent advances in forecasts and warnings, as well as significant improvements in detection and communications technologies, have all surely contributed to the downturn in tornado deaths in the United States. So why were the tornadoes of 2011 so deadly? One explanation is that, like category 5 hurricanes striking the U.S. mainland, widespread and violent tornado outbreaks are infrequent meteorological events—perhaps even more infrequent than major hurricanes striking the United States mainland. Even with impressive advances made in forecasting and warning for these events, no technology currently exists to change the formation, intensity, or track of the most locally extreme examples of severe weather on Earth—tornadoes.

There were 1,690 tornadoes reported across the United States in 2011. This annual total is exceeded only by 2004 when 1,817 tornadoes occurred. There were tornadoes in 48 states in 2011, with only the largest state (Alaska) and the smallest state (Rhode Island) recording no touchdowns. The only other year when 48 states had at least one tornado recorded was 1989. Tornadoes occurred on 179 days in 2011 and, despite being a prolific year in other ways, this daily count is very close to the 50-year average of 177 days per year with at least one tornado reported. The year with the most tornado days (211) was 2001.

Caption: Figure 1. United States annual tornado death toll per 1 million in population, 1875–2008.

Caption: Figure 1. United States annual tornado death toll per 1 million in population, 1875–2008.

January

The year began ominously, as a significant New Year's Eve tornado outbreak (see the May/June 2011 Weatherwise) continued into early New Year's Day 2011. The year's first tornado struck Attala County, Mississippi, at two minutes after midnight local time. This EF3 twister, with estimated peak winds of 145 mph, was the earliest recorded tornado of any year for at least the past 61 years. There were two injuries, as well as significant home and building damage reported, but fortunately no fatalities occurred.

There were three other days and two other states (Florida and Texas) with tornadoes reported in January. The 16 tornadoes tallied for the month compare to a 10-year January average of about 22 tornadoes on five days.

February

The month had 63 tornadoes on six days in 16 states, including a brief tornado on February 11 in Maui, Hawaii. While this tornado total is about double the 10-year average for the month (32), the number of tornadoes in February can vary considerably from year to year. For example, only one tornado was reported in the United States in February 2010, and nearly 150 occurred in February 2008. The most prolific tornado day of February 2011 came on the last day of the month. On February 28, strong wind shear developed coincident with thunderstorms ahead of a cold front spreading across the Midwest, and eventually the South. The advancing squall line produced 25 tornadoes across seven states. A 79-year-old man in Tennessee became the first tornado victim of 2011, when his mobile home was picked up and thrown 70 yards by an EF2 tornado.

March

About one-half of the days during March 2011 had at least one tornado somewhere in the country. These events were scattered across 15 states, and the 75 tornadoes tallied for the month were mostly weak—rated EF0 or EF1. While the 15 days with tornadoes was above the 10-year average of about 11 days per month, the total number of tornadoes for the month came in below the decade's average number of 83 tornadoes in a typical March.

April

April is usually when tornadoes increase in intensity and frequency, as the vestiges of the winter season (strong winds and cold air) battle it out with the rapid onset of the southern spring (warmer air and more moisture available over much of the Lower Mississippi Valley and Southeast). A typical April may experience around 150 tornadoes, with the busiest April months exceeding 200 tornadoes. However, there was nothing normal about the tornadoes in April 2011, as the month established many new and disturbing records. No other month on record has experienced as many tornadoes as the number reported in April 2011. The 758 tornadoes during the month constitute a nearly fivefold increase over what might be considered typical. (May 2004, with 542 tornadoes, now holds a distant second place for the most tornadoes in any month on record.) The 360 direct tornado fatalities in April 2011 make it the deadliest month for tornadoes in the United States in 75 years. The tragic 316 fatalities that occurred on April 27 not only set a new modern record for tornado deaths in a single day, but the 200 tornadoes on this day also set a new record for the number of tornadoes in any 24-hour period. While 2011 statistics are still under review, it is clear that the tornadoes of April, and especially those that moved across 16 states on April 27, can be considered extreme. Only the “Super Outbreak” of April 3–4, 1974, previously uncontested as the most significant tornado outbreak in the modern era, comes close to being a meteorological and historical analog to the events of April 27.

There were 21 days with tornadoes in April 2011, compared to a 10-year April average of 18 days during the month with tornadoes. Most of the tornadoes in April 2011, however, occurred during the second half of the month. There were over 200 tornadoes during the first two weeks of April 2011, with over half of those occurring during a three-day (serial) tornado outbreak that straddled the middle of the month on April 14–16. Tornadoes with a deepening low pressure system commenced in the late afternoon of April 14 over Eastern Oklahoma. By early evening, a powerful EF3 tornado, with winds of 150-165 mph, produced significant damage and killed two people in the town of Tushka.

The same weather system went on to produce widespread tornadoes across the South on April 15, when five tornadoes killed 10 people (seven in Alabama, two in Arkansas, and one in Mississippi). North Carolina bore the brunt of this severe weather episode on April 16, as the state had its worst tornado outbreak since March 1984. Before departing the East Coast, the storm system spawned over 50 tornadoes from Georgia to Maryland, with 24 fatalities occurring in North Carolina and two in Virginia (see Figure 2).

Caption: Figure 2. National Weather Service radar reflectivity indicating a mature supercell with a tornado indicated to the southwest of Raleigh, North Carolina, on April 16, 2011.

Caption: Figure 2. National Weather Service radar reflectivity indicating a mature supercell with a tornado indicated to the southwest of Raleigh, North Carolina, on April 16, 2011.

One of the more publicized tornadoes of the year occurred during the evening April 22. While not part of a larger outbreak like the other memorable events of the month, this EF4 tornado, with winds estimated around 170 mph, is notable for two reasons. First, it struck a major airport (Lambert-Saint Louis International) and then moved across portions of the Saint Louis metropolitan area. Second, and amazingly considering the densely populated areas affected, no one was killed.

It was during the second half of April 2011 when more than just previous tornado records were unfortunately demolished. By far, the most destructive tornadoes and widespread devastation took place during the period April 26–28, when a very intense and dynamic storm system moved from the Great Plains to the East Coast. Warm air and moisture were pulled into this system from the Gulf of Mexico and provided the fuel for widespread thunderstorms from Arkansas, Texas, and Louisiana on April 25–26, across the Deep South on April 27, and finally the Eastern Seaboard on April 28.

Early on April 27, intense thunderstorms produced damaging winds and tornadoes, with one death in Mississippi and two in Alabama before daybreak. Even though the tornado event was very well forecast on this day, no one anticipated that the death toll would increase by over 100 times by the end of the day. As the morning storms weakened and moved away, daytime heating across Mississippi and Alabama contributed to the development of strongly unstable conditions. With the powerful jet stream and developing surface low pressure providing lift and shear for new thunderstorm development, clusters, or families of tornado-producing supercell storms, spread east and northeast from Mississippi to Alabama, Tennessee, and Georgia during the afternoon and through the late evening. Some of the tornadoes were on the ground for well over 100 miles and produced swaths of complete devastation almost a mile wide in many places. A far-reaching event, tornadoes occurred as far north and east as upstate New York by the late night hours of April 27. By the end of this horrible day, 316 lives were lost to tornadoes across five states—the greatest one-day death toll since the Super Outbreak of April 3, 1974.

May

Coming on the heels of the historic and deadly tornado outbreak of late April 2011, the weather pattern that evolved during early May became very unfavorable for tornadoes. It was as if the tornado-producing atmosphere that dominated the second half of April had been switched off at the start of May. During the first two weeks of May 2011, there were 35 tornadoes and no fatalities. When compared to a typical May, the month experienced the fewest number of tornadoes for this period of the year in the past 25 years. The month was on course to establish a new record low in terms of tornado activity, and this immediately following a month in which a number of extreme tornado activity records had been set. Unfortunately, the quiescent conditions did not last.

Joplin, a city of about 50,000 people, sits in the southwest portion of Jasper County, in the far southwest corner of Missouri. Sunday, May 22, 2011, began like any typical late spring day across this part of the country, and severe storms were expected across a broad swath of the Great Plains from Texas to Minnesota. Unlike the April tornado outbreaks, this day was different, meteorologically speaking. While the overall pattern supported scattered to locally concentrated severe weather, the dynamics and ingredients influencing thunderstorm formation and character were more subtle than the powerful systems of April.

The first tornado of May 22, 2011, touched down hundreds of miles away from Joplin, very near the city of Minneapolis, Minnesota, around 2:30 p.m. CDT. This EF1 tornado with wind speeds around 100 mph claimed one life and injured nearly 50 people. This tornado was associated with an area of surface low pressure that was moving across the upper Midwest in the presence of strong instability. The instability, or fuel for thunderstorms, was readily available across much of the Central United States, and over the next few hours, intense thunderstorms became widespread across much of this expansive region. The thunderstorm that spawned the eventual Joplin EF5 tornado began in Southeast Kansas during the late afternoon. In addition to abundant instability, the wind shear across this part of the United States was enhanced by a mid-level wind max, or jet streak, that was moving through the base of the larger upper level trough associated with low pressure well to the north. Radar imagery depicted an unusual evolution to the Joplin tornadic supercell, as numerous storms converged on Southwest Missouri just after 5:00 p.m. CDT. After the merger of two powerful thunderstorm updrafts, the Joplin tornado intensified into a monster of unimaginable fury in a very short period of time. The tornado widened to over five football fields and produced EF5 estimated wind speeds of 220–250 mph over the densely populated area of Joplin just south of the city center (see Figure 3).

Caption: Figure 3. Post-event survey of the tornado path across Joplin, Missouri. Areas depicted in red experienced “catastrophic damage.”

Caption: Figure 3. Post-event survey of the tornado path across Joplin, Missouri. Areas depicted in red experienced “catastrophic damage.”

The Joplin tornado is now in the record books as the most destructive tornado in United States history, with estimated total losses between $2 and $3 billion. With 158 direct fatalities, a deadlier single tornado does not exist in the official NOAA records back to 1950. The most recent tornado with a comparative death toll in the historical record was the Woodward, Oklahoma, tornado of April 9, 1947, which claimed 181 lives.

Less than two days later, during the afternoon and evening hours of May 24, 2011, a classic Southern Plains tornado outbreak, similar in many ways to a year earlier (see the May/June 2011 Weatherwise), struck portions of Oklahoma, Kansas, Texas, and Arkansas. The 48 tornadoes on this day (coincidentally, the same number of tornadoes as May 22), included an EF5-rated long-track tornado in Central Oklahoma that resulted in nine fatalities. This was the first May since 1985 to experience two EF5 tornadoes during the month, and the only other May since May 1953 in which those two EF5-rated tornadoes occurred on different days during the month.

In summary, the 326 tornadoes on 24 days during May 2011 compare to the recent May average, with 298 tornadoes on 24 days.

June

The year was far from finished with the destructive surprises that tornadoes can bring, when on the first day of June 2011, a rare New England tornado swept across the Connecticut River and into Springfield, Massachusetts, at rush hour. While not immune to the danger of tornadoes, New England only rarely experiences the violence these whirlwinds can bring. Three people were killed by the EF3 Springfield tornado that went on to track for over 30 miles across south-central portions of Massachusetts. With peak wind speeds possibly exceeding 140 mph, the tornado caused considerable damage to homes, businesses, and over 10,000 acres of woodlands. This was the deadliest tornado in Massachusetts since a twister killed three in May 1998.

Only scattered and relatively weak tornado events filled out the rest of June, with 160 tornadoes on 24 days. June 2011 ended with far fewer tornadoes than the decadal average of 236 tornadoes on 27 days.

Caption: Figure 4. 2011 Preliminary tornado maps with state totals. Tracks plotted by EF-scale. State totals sum greater than United States totals due to counting state border crossing tornadoes once in each state.

Caption: Figure 4. 2011 Preliminary tornado maps with state totals. Tracks plotted by EF-scale. State totals sum greater than United States totals due to counting state border crossing tornadoes once in each state.

Table: Table 1. Annual tornado information chart.

July

As the Southern United States baked under a scorching heat wave, the pattern became favorable for a close relative to the tornado, called a derecho. These straight-line thunderstorm wind events are known to bring havoc from the Northern Plains to the Midwest, and sometimes even into the East and Southeast, primarily during the summer months. The most significant thunderstorm wind events of the month came on July 10–11, when a jet stream disturbance tracked from the Dakotas to the Ohio Valley. Peak thunderstorm winds during this event exceeded 100 mph in Iowa. During the month, there were five deaths and over $64 million in destruction produced by straight-line thunderstorm winds. Of the 101 tornadoes on 25 days, most were generally weak and caused little damage. The July 2011 tornado numbers are nearly average when compared to the 10-year figures of 112 tornadoes on 24 days.

August

While wind damage events were much more common during late summer, a total of 57 tornadoes occurred on 20 days in August 2011. This compares to a decadal average of 87 tornadoes in 21 days. A tornado fatality in Oklahoma, especially rare for the state in August, occurred with an EF2 tornado that struck near Locust Grove, in association with a line of thunderstorms, early on August 10. The highest rated tornado (EF3) of the month occurred on August 11 in Cherry County, Nebraska, as part of an early evening cluster of tornadoes in North-Central Nebraska near a warm front. The second of the two killer tornadoes in August occurred on August 19 when an EF1 tornado struck near Wausaukee in far Northeast Wisconsin.

As Hurricane Irene impacted the Eastern United States in late August, several tornadoes occurred on August 25–27 across the mid-Atlantic coast and Long Island, in association with the outer bands of the hurricane.

September

A slow-moving tropical storm named Lee moved inland across the Louisiana coast in early September. The circulation and tropical moisture associated with the remnants of Lee contributed to two dozen tornadoes (all rated EF0 or EF1) from the Gulf Coast to mid-Atlantic during the period September 3 through 7. On September 25, around a half dozen tornadoes occurred from the lower to middle Mississippi River Valley to the lower Ohio Valley in association with weak surface low pressure but a strong upper level disturbance. September 2011 saw 51 tornadoes on 16 days as compared to a 10-year average of 92 tornadoes on 16 days.

October

The first half of October had a few days of relatively isolated and mainly weak tornadoes. Early in the overnight on October 9, a rogue tornado produced EF1 damage when it touched down near Lackland Air Force Base on the west side of San Antonio, Texas. A few days later, around a half dozen relatively weak tornadoes occurred on October 13 near a warm front across East-Central Virginia. As part of a cluster of tornadoes across South Florida on October 18, the most damaging tornado of the month, an EF2 with estimated $4 million damage, occurred during the evening. A below-average month finished with 23 tornadoes in nine days, which compares to the 10-year average of 65 tornadoes on 10 October days.

November

After a quiet start to the month of November, portions of Oklahoma and Northwest Texas were impacted by severe thunderstorms on November 7. There were at least six tornadoes across Southwest Oklahoma, including an EF4 tornado near Tipton that destroyed an Oklahoma State Agronomy Station. This is the first and only EF4 or greater tornado ever reported in Oklahoma in November since official records began in 1950. An Oklahoma Mesonet site near Tipton measured an 86 mph wind gust before it was destroyed. Another tornado destroyed the Oklahoma Mesonet station at Fort Cobb after the wind instruments measured a 91 mph gust. This was the second Oklahoma Mesonet site to be hit by a tornado during 2011. (A site near El Reno was struck by a tornado during the May 24 outbreak.)

An active pattern for severe thunderstorms presented itself again on November 14, when a strong upper-level jet stream combined with adequate autumnal season moisture across the Ohio River Valley and lower Great Lakes. Two rare late-season EF2 tornadoes were reported in Western New York with this system. As an upper-level system emerged from the Southwestern United States, a widespread severe weather threat extended from the Gulf Coast to the Southeast with more than a dozen tornadoes on November 15–16. This round of severe weather brought with it the last tornado fatalities of the year, when two separate EF2 killer tornadoes moved over parts of upstate South Carolina (three deaths) and Western North Carolina (two deaths). In all, the month saw 44 tornadoes on six days, which compares to a 10-year average of 68 tornadoes on seven days.

December

The final tornadoes of the year occurred across the Central Gulf Coast and Southeast on December 22, which was the most active day of the month in an otherwise quiet end to 2011. Aside from several weak tornadoes in Southeast Louisiana and Central Alabama, Northwest Georgia was especially impacted on December 22, when EF2 and EF3 tornadoes caused seven injuries and significant damage to over a dozen homes. The month ended with 16 tornadoes on four days, as compared to the 10-year average of 36 tornadoes on seven days.

2011 Wrap-Up

The ferocity and violence of the tornadoes during 2011 make the year stand out as one of the deadliest and most destructive in U.S. history. Many people may have been surprised at such widespread tornado destruction occurring in the modern era. But history provides examples of what happens when the atmosphere mixes the necessary ingredients together for widespread and deadly tornado outbreaks. These extreme events may only occur once or twice a generation, over relatively modest-sized areas of the United States, if we are lucky. There is high confidence, however, that the more significant events can be anticipated to some degree, at least a few days in advance, despite the continuing elusive and deadly nature of individual tornadoes.

Some of the unfortunate United States tornado records in 2011 include the second greatest annual number of tornadoes (1,690), the second greatest annual number of tornado injuries (5400), and the fourth deadliest year for tornadoes (550) in U.S. history. Given these figures, it is difficult to find adjectives that adequately describe the tornadoes of 2011, and the impacts those tornadoes had on people's lives.

All tornado numbers for 2011 remain preliminary pending further review by SPC and the NOAA National Climatic Data Center. The tallies indicated here are the best estimates at the time of publication.

GREG CARBIN is the Warning Coordination Meteorologist at the NOAA/National Weather Service Storm Prediction Center in Norman, Oklahoma.

JARED GUYER is an Outlook/Mesoscale Forecaster at the NOAA/National Weather Service Storm Prediction Center in Norman, Oklahoma.

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