Rare July Tornado Outbreak

A wall cloud over the City of Maryville in Blount Count on July 27, 2014. Photo courtesy of Jeff Weaver.
Mamattus Clouds ahead of a supercell thunderstorm highlight the sky in Chestnut Hill, TN. Photo courtesy of Jared Etherton.
A potent upper-level storm system impacted the region on Sunday, July 27th, bringing several rounds of severe weather along with locally heavy rainfall. Three tornadoes were observed across east Tennessee, one classified as a strong, EF-3 tornado with winds of 140 mph. Additionally, there were several reports of straight-line wind damage and large hail up 2 to 3 inches in diameter! A storm system of this magnitude is highly unusual for this time of year across the Southern Appalachians as strength of the front and the colder air behind it rarely make it this far south during the middle of summer. A bulk of our severe weather season occurs during the spring months of March, April and May, when tornadoes and the larger 2-3 inch diameter hail are far more common.
140727_rpts Filtered Reports Graphic
Map of all the storm reports from July 27, 2014 across the nation. Note the high density of hail, wind damage and tornado reports across our area!
Supercell thunderstorms began to develop along a prefrontal trough by the 2 PM EDT across central and southern Kentucky and advanced southeastward into the northeast Tennessee, southwest Virginia and the northern Cumberland Plateau of Tennessee between 3 to 5 PM EDT. This activity produced the three tornadoes that occurred during this event. Two of the EF-1 tornadoes occurred across northeast Tennessee in Washington and Sullivan Counties from the same supercell thunderstorm. The first of these tornadoes occurred briefly around 5:45 PM EDT in Rock Spring community with winds estimated at 110 mph. After lifting, another EF-1 tornado with winds of 100 mph tracked for nearly 11 miles across Sullivan county from 5:47 to 6:02 PM EDT, impacting the Gray community. Another intense supercell produced a stronger EF-3 tornado across Campbell and Claiborne Counties around 6 PM EDT. This tornado tracked around 5 miles and produced some significant damage along its track. Numerous trees were uprooted and snapped along the entire path. Structural damage varied from minor to extensive, with the most significant damage occuring in the Speedwell area (shown below) where the tornado was at its maximum strength. 
Radar images of the supercell thunderstorm that produced the strong EF-3 Tornado in Campbell and Claiborne Counties taken at 5:54 PM EDT. The image on the far left is of radar reflectivity and shows an intense supercell thunderstorm with a distinctive "hook" echo over the Speedwell community where the tornado was occurring. The image on the right is a new Dual-Polarization radar product called "Correlation Coefficient". The blue area circled in black is actually debris from the tornado that has been picked up by the radar. This is called a Tornado Debris Signature or TDS. Debris from this tornado was lofted as high as 10,000 feet according to our radar in Morristown.
 EF-3 Tornado damage from Speedwell, TN in Claibourne County. Photos are courtesy of WBIR-TV and WATE-TV
Another notable aspect of these severe thunderstorms was their capability to produce widespread, damaging large hail. This is something not typically observed in our area during the middle of summer due to the freezing level being so high. This results in hail melting to a much smaller size (or all together) before it reaches the ground. However, due to the presence of some cooler air aloft, a lower melting level, and very intense thunderstorm updrafts, widespread quarter to baseball sized hail was observed across the Tennessee Valley and Cumberland Mountains and Plateau. The pictures below show just how massive some of the hail was!