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2011 Tuscaloosa–Birmingham tornado

2011 EF4 tornado in Alabama, U.S.A

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CCTV video of the Tuscaloosa EF4 tornado
CCTV · Public domain

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First touching down around 4:43 p.m. CDT in Greene County, Alabama, the tornado rapidly intensified and eventually reached a maximum path width of 1.5 miles (2.4 km) when it crossed I-65 north of Birmingham, and attained estimated wind speeds of 190 mph (310 km/h) shortly after passing through the city. It then went on to impact parts of Birmingham at high-end EF4 intensity, before rapidly weakening and eventually dissipating. In total, the tornado was on the ground for approximately an hour and a half, leaving catastrophic damage in its wake. Multiple television stations captured the tornado live during its track, and it is one of the most documented tornadoes of the entire Super Outbreak due to its proximity to the Birmingham and Tuscaloosa metropolitan areas.

In total, the tornado killed 64 people and injured well over 1,500. This was the third tornado to strike the city of Tuscaloosa in the prior decade, and the second in two weeks. The tornado is the second deadliest tornado in Alabama's history, just behind the Hackleburg–Phil Campbell tornado that occurred on the same day. The tornado caused approximately $2.4 billion (2011 USD), making it the costliest tornado in Alabama history and, at the time, the costliest tornado in the United States, before it was dethroned a month later by the Joplin EF5 tornado in Missouri. Following the storm, then-President Barack Obama toured the city of Tuscaloosa on April 29 and the wreckage.

Meteorological synopsis

Setup

The environmental conditions leading up to the 2011 Super Outbreak were among the "most conducive to violent tornadoes ever documented". On April 25, a vigorous upper-level shortwave trough moved into the Southern Plains states. Ample instability, low-level moisture, and wind shear all fueled a significant tornado outbreak from Texas to Tennessee; at least 64 tornadoes touched down on this day. An area of low pressure consolidated over Texas on April 26 and traveled east while the aforementioned shortwave trough traversed the Mississippi and Ohio River valleys. Another 50 tornadoes touched down on this day. The multi-day outbreak culminated on April 27 with the most violent day of tornadic activity since the 1974 Super Outbreak. Multiple episodes of tornadic activity ensued with two waves of mesoscale convective systems in the morning hours followed by a widespread outbreak of supercells from Mississippi to North Carolina during the afternoon into the evening.

Tornadic activity on April 27 was precipitated by a 995 mbar (hPa; 29.39 inHg) surface low situated over Kentucky and a deep, negatively tilted (aligned northwest to southeast) trough over Arkansas and Louisiana. A strong southwesterly surface jet intersected these systems at a 60° angle, an ageostrophic flow that led to storm-relative helicity values in excess of 500 m2s−2—indicative of extreme wind shear and a very high potential for rotating updrafts within supercells. Ample moisture from the Gulf of Mexico was brought north across the Deep South, leading to daytime high temperatures of 77 to 81 °F (25 to 27 °C) and dewpoints of 66 to 72 °F (19 to 22 °C). Furthermore, convective available potential energy (CAPE) values reached 2,500–3,000 J/kg−1.

Forecast

On the morning of April 27, a strong cold front with several areas of embedded low pressure extended from the Texas Hill Country northeastward towards the Arklatex and the Ozarks, and later into the lower Ohio Valley. Warm moist air was in place due to strong southerly flow ahead of the front over Mississippi, Alabama, and Tennessee. An upper level disturbance sparked a broad area of showers and thunderstorms as it moved across the frontal boundary on the previous evening. The eastern edge of the line of showers and storms continued to move eastward, in concert with the upper disturbance, reaching the northwest Alabama border around 2:00 a.m. CDT.

This produced the last and most violent round of severe weather, which began around 2:30 p.m. CDT for northern Alabama as supercells began to line up to the southwest of the area. During the early afternoon hours, the potential for destructive tornadoes was highlighted by the Storm Prediction Center's upgrade to a high risk for severe weather around 1:00 p.m. CDT. This prompted a particularly dangerous situation (PDS) tornado watch, which was issued for northern Alabama and portions of southern Tennessee at 1:45 p.m. CDT. The bulletin that accompanied the watch read:

THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF: MUCH OF ALABAMA, NORTHWEST GEORGIA, SOUTHEAST MISSISSIPPI, SOUTHERN MIDDLE TENNESSEE, EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 145 PM UNTIL 1000 PM CDT.

...THIS IS A PARTICULARLY DANGEROUS SITUATION...

DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER. THUNDERSTORM WIND GUSTS TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

The potential for tornadoes ramped up from noon through 9:00 p.m. CDT. During this period, much of Alabama experienced numerous supercell thunderstorms that produced violent tornadoes, one being the Tuscaloosa tornado. Shortly thereafter, at 3:09 p.m. CDT (20:09 UTC), the National Weather Service office in Jackson, Mississippi issued the first tornado warning on the supercell that would eventually produce the Tuscaloosa–Birmingham tornado.

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