"April 25-27, 2011 Tornado Outbreak"
Melissa Watson , Undergraduate Geographer/EIUWC Weather Analyst
February 21, 2012
Introduction
The tornado outbreak of April 25-27, 2011 is considered to be a historic outbreak of severe weather and tornadoes. This tornado outbreak is considered to be the second deadliest tornado outbreaks in US history, with 342 people perishing. The number one deadliest tornado outbreak occurred on March 18, 1925, also known as the “Tri-State tornado outbreak”, with 747 deaths. Even though this event took place for 3 days, the most active day was on the 27th with 122 confirmed tornadoes occurring on that day.
This report will discuss the significant factors that caused the tornado outbreak of April 27, 2011. Along with that, this report will also discuss the three days that this event took place, and the statistics of this event. Surface maps and pictures will be used in this synoptic analysis.
April 25, 2011
The first day of this tornado outbreak was on Monday April 25, 2011, where the outbreak started across northeast Texas and spread across the mid-south and into portions of Louisiana that day. During this day there were numerous reports of large hail, destructive winds and tornadoes. Across Arkansas, Texas and Oklahoma there were 7 tornadoes on this day. All of them were given an Enhanced Fujita rating of EF0 or EF1. The Enhanced Fujita scale ranges from EF0 to EF5, where EF0 is the lowest/weakest tornado and EF5 is the strongest tornado. Figure 1 shows all the tornadoes and severe weather events that were reported for April 25, 2011.
April 26, 2011
As the second day of this tornado outbreak progressed, there wasn’t too much severe weather that occurred, that is, until during the afternoon and continued through till early morning of the 27th. This tornado event started out as supercells that produced large hail and tornadoes, expanding across northeast Texas and portions of Arkansas before developing into a squall line. Figure 2 is a surface map early in the morning (7 AM) on Wednesday April 27, 2011 that shows the squall line that formed from the supercells. This line of severe storms moved across multiple states before dissipating and was extremely efficient at producing wind damage as the storms tracked east. Unfortunately, this severe storm line produced 21 tornadoes of the total number of tornadoes for this event. From those 21 tornadoes, 11 were rated as strong tornadoes and had fairly long path lengths. A strong tornado would be classified as EF3 to EF4 on the Enhanced Fujita scale. Figure 3 shows the number of tornadoes and severe weather events that happened on April 26, 2011.
April 27, 2011
The third, and most active day of this tornado outbreak, started with the atmosphere becoming increasingly favorable for more severe storms the morning of Wednesday April 27, 2011. The key component to the third day’s events was a strong shortwave and a deep surface low that intensified as the day progressed on Wednesday. As shown in figure 4 the winds in the mid level atmosphere increased, approximately to between 80 and 100 miles per hour. This helped to further deepen the surface low which also caused the low level winds to become stronger. As the winds in the upper atmosphere increased, it caused a wind shear and a shift in the winds from a southerly to westerly direction. Furthermore, high low level moisture returned to the area in the wake of the morning. This, along with morning sunshine, caused a very unstable airmass. The instability in the atmosphere, the wind shear, and the lift from the strong upper disturbance, eventually caused the historic tornado outbreak of April 27, 2011. Futhermore, by early afternoon several supercell thunderstorms were beginning to develop across the Jackson, Mississippi forecast area. These supercells did not take long after development to start producing tornadoes.
The first tornado for Jackson, Mississippi on April 27th started in Neshoba County and was rated as an EF5 tornado with a 29 mile long track. This set the tone for the rest of the day, because there was a plethora of tornadoes that developed from several supercell storms, and nearly all of the tornadoes had long tracks. One such tornado started in the Jackson, MS area and proceeded into Alabama and had a path length of 92.3 miles. In this event there were 15 violent tornadoes, meaning the tornadoes received a ranking of EF4- EF5. From these 15 tornadoes, 8 of them had a path length that exceeded 50 miles. This tornado outbreak devastated the southeastern portion of the United States. The death tolls that day were very high at 316. Mississippi had 31 deaths, 234 in Alabama, Tennessee had 32, Georgia had 15 and Virginia had 4 deaths. Not only did the fatalities run high, but there were also 2,400 injuries and over $4.2 billion in damages.
In addition, all the tornado deaths occurred within the boundaries of issued tornado watches that were then followed up by tornado warnings. The time from the first three watch issuances to the first tornadoes in those watches, is what makes up the lead time. The lead time for this event averaged approximately 2.4 hours. For tornado warnings, the mean lead time for this event was 22.1 minutes with the probability of detection at 89% and a false ratio at 49%. Figure 5 shows the number of tornadoes and severe weather events on April 27, 2011.
Conclusion
The tornado outbreak of April 25-27, 2011 was an historic event. It is considered to be the second deadliest tornado outbreak in U.S. history, only coming behind the March 18, 1925 tornado outbreak. The April 2011 outbreak killed 342 people and had thousands of people injured. Along with that, this event had over 200 tornadoes in 5 southeastern states. But the deadliest day of this event occurred on April 27th where there were 122 tornadoes, 316 deaths and over 2400 injuries.
On the day of the 27th the atmosphere’s instability, wind shear and the lift from the strong upper disturbance, eventually caused the outbreak. Then by mid-day, supercells were beginning to form, and shortly after, started producing tornadoes. This would set the trend for the rest of the day.
