改良藤田スケール
On April 23, the Storm Prediction Center (SPC) began monitoring the potential for a substantial severe weather outbreak in the extended range. As a shortwave trough tracked across portions of the Mid-South and southeastern United States, moderate instability and strong wind shear ahead of a trailing cold front was expected to promote the development of supercell thunderstorms capable of producing tornadoes, large hail, and damaging winds. Two days before the event, on April 25, the SPC issued a moderate risk of severe weather encompassing portions of central and eastern Kentucky, middle and eastern Tennessee, northeast Mississippi, central and northern Alabama, and northwest Georgia. Due to the combination of rich low-level moisture, strong shear, and focused large-scale ascent, there was relatively high confidence across the outlined area for strong tornadoes – a tornado rated EF2 or higher on the Enhanced Fujita scale – and widespread damaging winds. By the morning of April 27, the SPC upgraded to a high risk of severe weather, noting that a dangerous tornado outbreak capable of producing several violent – EF4 tornado or stronger on the Enhanced Fujita scale – and long-track tornadoes was expected.