The 17 January 2012 Tornado Outbreak Across the Ohio Valley
Linda Gilbert, John Denman, and Michael Paddock
National Weather Service Louisville, Kentucky
There are many things that stand out in local severe weather history which make the January 17, 2012 tornado outbreak noteworthy. Across central Kentucky and south-central Indiana, the record for the most number of tornadoes confirmed in the month of January was broken with nine. The previous record was six tornadoes in January 2006. While this event did not surpass the February 5-6, 2008 outbreak of eighteen tornadoes, it does place second for the area's winter outbreaks. Most of the tornadoes were rated as EF-0 or EF-1 with only one EF-2 tornado found near the Kentucky-Tennessee border. Thankfully, there were no reported fatalities and only one injury. All of the tornadoes were a result of a quasi-linear convective system (QLCS) that had formed in Missouri and Illinois, strengthening as it traversed up the Ohio River Valley.
Analyses from observations as well as the Storm Prediction Center (SPC) at 1200 UTC revealed that at 250 mb, a 100+ knot jet and associated divergence was evident over the Ohio Valley. Furthermore, the atmosphere showed a sharp cold front from the mid-levels to the surface with plentiful moisture. Ahead of the cold front, surface observations indicated temperatures above normal for this time of year, reaching the upper 50s to low 60s °F by 1200 UTC, with above average dewpoints in the 50s °F pooling ahead of the boundary. Temperatures dropped sharply behind the front, with about a 20 °F drop across approximately 100 miles in central Illinois, according to 1200 UTC METARs. The 0-6 km shear values were at roughly 60 knots and 0-1 km, as well as 0-3 km, storm-relative helicity (SRH) held steady near 800 m2/s2. Area soundings and SPC's Mesoscale Analysis webpage revealed surface-based CAPE values that morning had not reached any higher than approximately 100 J/kg. While the CAPE values were being downplayed by the data, surface observations highlighted the abnormal area of warm, moist air ahead of the frontal boundary nosing into central Kentucky. The synergistic effect of these parameters coming together generated embedded mesovortices within the QLCS, focusing most of its intensity and damage in and around the Louisville metropolitan area.
While there was evidence days in advance that this event had the potential to generate severe weather, as indicated in local and national forecast discussions, both the long- and short-range models struggled with key atmospheric features and parameters. Short-term and near-term forecast updates were necessary throughout the morning hours to account for increasing instability. Critical atmospheric differences ultimately aided in creating the most prolific January tornado outbreak for the Louisville Weather Forecast Office's area of responsibility. This presentation will focus on how the changing environment ahead of the advancing cold front enhanced the generation of embedded mesovortices within this cool-season QLCS, and discuss the resultant impacts on the local community.