A large and destructive tornado outbreak struck the Midwestern and Southern United States between January 21–23, 1999. The outbreak mostly took place across the Mississippi River Valley. The outbreak as a whole was the largest tornado outbreak on record to occur during the month of January. Over the course of roughly two days, 128 tornadoes touched down across the region, resulting in widespread damage. Nine people were killed by the tornadoes. The outbreak is best known for an F3 that struck Little Rock, Arkansas, killing three people. __TOC__
Meteorological synopsis On January 21, the Storm Prediction Center (SPC) issued a High risk of severe weather across much of central and eastern Arkansas, southwestern Tennessee, northeastern Louisiana, and much of northern and western Mississippi. The organization warned of the potential for a widespread severe weather outbreak, including several long-tracked and intense tornadoes. The meteorological setup unfolded as a broad upper-level trough, with its axis across the Central United States, rapidly amplified in conjunction with a jet streak across the southern Rockies. Substantial height falls—implying cooling in the mid layers of the atmosphere—coupled with the sharpening trough allowed for strong divergence to overspread the highest risk area. At the surface, a deepening area of low pressure initially centered near the Texas–Oklahoma border early on January 21 deepened and accelerated eastward, reaching the Arkansas–Missouri border during the overnight hours. A dry line extended southward across eastern Texas, providing increased mechanisms for rising air as it combined with a cold front and pushed eastward into Arkansas.
As the upper-level trough rapidly intensified, 500mb winds increased to and 850mb winds topped across the High risk area. The combination of speed shear and adequate directional shear (noted by curved low-level hodographs) led to 0–3 km storm relative helicities of 300–500 m2/s2, favorable for rotating thunderstorms. A capping inversion, qualified by cold and dry air aloft, initially prevented the formation of thunderstorms and instead allowed substantial surface heating. Surface-based CAPE values of 1500–2000 J/kg and most unstable CAPE values of 1000–2000 J/kg became prevalent across the broad warm sector. Low-level moisture transported northward from the Gulf of Mexico resulted in dew points in the low- to mid-60s °F by the afternoon hours. With continually strengthening wind fields, the capping inversion steadily weakened, and several tornadic supercells formed across Arkansas. In addition to the tornado outbreak, steep 850–500mb lapse rates, ranging from 6.5 to 7.5 °C/km, contributed to a favorable environment for gigantic hail and widespread damaging winds. Nine reports of hail over were received by the SPC.
Confirmed tornadoes
January 21 event