On the evening of March 9th, 2017, a strong cold front was pushing southeast from southern Missouri/Kentucky extending northeast into the Mid-Atlantic area of the U.S. Fairly strong low and mid-level winds (850 mb winds ~40 knots/500 mb winds of 60-70 knots) were in place ahead of the cold front. Ahead of the front (over the Huntsville Forecast area and further northwest), several other severe weather ingredients were in place. These ingredients came together ahead of the front to produce a Mesoscale Convective System (MCS) comprised of strong to severe thunderstorms. They included: surface based instability values of up to 500 J/KG, steep mid-level lapse rates between 700 mb and 500 mb of ~ 7 degrees Celsius/km, and 0-1 km helicity (low level turning of winds) values of 200-400 m2/s2. This MCS raced southeast through northern Alabama and Southern Middle Tennessee around midnight through ~ 3 AM. This activity weakened towards daybreak as the front pushed southeast into central Alabama.
As seen below on the left, a severe thunderstorm watch was issued at 11:20 PM for all of the counties in NWS Huntsville's forecast area. This was in effect until 4:00 am. On the right is a map of storm reports received after these storms pushed through the area. The most widespread effect of this MCS was damaging winds, but enough low-level helicity was in place ahead of the front to produce an isolated EF-1 tornado that affected Moore and Franklin counties in Tennessee. A few instances of large hail were observed as well given the strong forcing with the front and steep mid-level lapse rates.