The Historic Tornado Outbreak of April 27, 2011 across East Tennessee and Southwest Virginia
In the predawn hours of April 27th 2011 a strong cold front that would later produce numerous tornadic supercells across the region was tracking into the lower Mississippi Valley from East Texas. Warm, moist, unstable air was in place due to strong southerly flow ahead of this front over Mississippi, Alabama, and Tennessee. This can be seen on the surface map below, with 60 to 65 degree dewpoint temperatures over these areas. An upper-level trough ahead of the main cold front pushed east into Mississippi overnight, triggering a line of thunderstorms. As this feature tracked into Alabama during the predawn hours of April 27th, it quickly intensified into a very intense line of severe thunderstorms as it encountered this very unstable air mass and an area of strong upper disturbance. Meanwhile, surface winds backed to the south-southeast as the disturbance moved into the area, while winds at the 850 mb level (around 5,000 feet) increased to between 50 and 55 knots and became more southerly. The combination of high low-level moisture and increasing shear (winds changing direction and speed with height) provided the setup for damaging winds, large hail and brief, but intense tornadoes as it tracked into the southern Cumberland Plateau around 8:00 AM EDT, and eventually the southern and central Valleys by mid-morning. As the line moved into northeast Tennessee and a slightly more stable environment, it weakened, but still produced damaging straight-line winds and some hail. By midday, the squall line had lifted northeast into Virginia and North Carolina, but a remnant outflow boundary remained in place along the I-75 corridor, setting the stage another more significant tornado event that afternoon, evening and night. Over the course of the afternoon and evening, several rounds of supercell thunderstorms marched northeastward from northern Alabama and Georgia into East Tennessee and Southwest Virginia. It was these storms that produced the 13 strong to violent tornadoes that produced significant to catastrophic damage across southeast Tennessee, the Cumberland Plateau, and areas along the foothills of the Mountains of East Tennessee and Southwest Virginia. The boundaries left behind by the morning convection help to serve as a focus for these tornadic storms, and assisting in the tornadogenesis process. The peak of this portion of the event lasted several hours from the late afternoon on April 27th into the late evening and overnight hours. Finally, a cold front sweeped through the region during the predawn hours on April 28th, bringing an end to the severe weather and tornado threat.
This outbreak began with the first wave of severe thunderstorms that moved across middle Tennessee and northern Alabama in the early morning hours of April 27, 2011. These storms continued into east Tennessee before weakening in central east Tennessee.
Radar loop from the Hytop radar (in northeast Alabama) from 6 AM EDT (5 AM CDT) until 11 AM EDT (10 AM CDT) of the initial severe thunderstorm line on the morning of April 27, 2011.
Radar loop from the Morristown radar (in northeast Tennessee) from 7 AM EDT (6 AM CDT) until 11 AM EDT (10 AM CDT) of the initial severe thunderstorm line across east Tennessee on the morning of April 27, 2011.
After this initial line of severe thunderstorms weakened, additional supercells (i.e. rotating thunderstorms) redeveloped over middle Tennessee and Alabama in the afternoon.
Radar loop from the Hytop radar (in northeast Alabama) from 1 PM EDT (12 PM CDT) until 6 PM EDT (5 PM CDT) of the supercells (i.e. rotating thunderstorms) across middle Tennessee and northern Alabama on the afternoon of April 27, 2011.
Radar loop from the Morristown radar (in northeast Tennessee) from 2 PM EDT until 8 PM EDT of the supercells (i.e. rotating thunderstorms) in east Tennessee on the afternoon of April 27, 2011.
The incredibly strong wind field kept additional supercells (i.e. rotating thunderstorms) redeveloping over east Tennessee and southwest Virginia late into the evening.
Radar loop from the Morristown radar (in northeast Tennessee) from 8 PM EDT until 2 AM EDT of the supercells (i.e. rotating thunderstorms) in east Tennessee and southwest Virginia on the evening of April 27, 2011.