Episode narrative: On March 3rd, 2023, a historically strong low pressure system moved across the lower Ohio Valley, bringing heavy rainfall, severe weather, and most notably, very strong gradient winds as it moved across the region. On the synoptic scale, a shortwave upper-level trough located over the mid-Mississippi Valley became negatively tilted during the morning hours of March 3rd. A 120 knot 500 mb jet developed across central Kentucky and middle Tennessee in the presence of a very strong height gradient. The footprint of this jet core was transferred into the lower levels of the atmosphere, with a 70 knot jet at 850 mb present across the Ohio and Tennessee Valleys. The orientation of the shortwave trough aloft with respect to the low level cyclone was nearly ideal for mutual amplification, with both the surface low and the upper trough undergoing deepening during the morning and early afternoon hours on March 3rd. Ultimately, the surface low's minimum central pressure dropped to approximately 976 millibars near Vincennes, Indiana by mid-afternoon on the 3rd, creating an anomalously strong low-level pressure and height gradient across central Kentucky and southern Indiana.

Before the severe weather and strong gradient winds began, the first impact from this strong mid-latitude cyclone would come in the form of heavy rainfall, primarily across southern Indiana. With the strong low-level jet helping to advect warm and moist air northward, precipitable water values were anomalously high, ranging from 1.3 to 1.6 inches across southern Indiana and central Kentucky during the morning hours on March 3rd. Coupled with an intensifying low level warm frontal boundary which provided a large-scale lifting mechanism, this axis of high moisture content contributed to a broad swath of precipitation, which lasted through much of the morning and early afternoon hours across southern Indiana. Ultimately, many locations would receive between 3 and 6 inches of rain over the span of about 12 hours, causing isolated to scattered reports of flooding.

After the initial wave of precipitation associated with the warm front lifted into central Indiana, southern Indiana and central Kentucky would briefly be located in the warm sector of the cyclone during the late morning and early afternoon hours, providing an opportunity for severe weather. Due to the proximity of the surface low and the relatively limited period for destabilization, instability was very limited for this event, with between 250-750 J/kg MLCAPE present across central Kentucky and southern Indiana. However, with the extremely strong dynamics in place, wind shear parameters were incredibly high. Effective bulk shear generally ranged from 70-100 kt, while effective Storm Relative Helicity ranged from 300-600 m2/s2. In addition, low-level Storm Relative Helicity was maximized (300-400 m2/s2) just ahead of a broken line of low-topped supercells which went on to produce three EF-1 tornadoes across southern Indiana. Across central Kentucky, the main severe impacts would come from damaging straight-line winds within a broken quasi-linear convective system. While limited instability clearly reduced the overall strength of the QLCS, some convective cores were deep enough to pull higher momentum air from aloft down to the surface, causing scattered damage and wind gusts above 58 mph. A few additional severe wind gusts would also be produced by the aforementioned line of low-topped supercells as they moved across southern Indiana and northern Kentucky.

While the heavy rainfall and severe weather were important aspects of this system, the most widespread impacts from this historic system would come from extremely strong gradient winds immediately following the passage of showers and storms along the surface cold front. As was previously mentioned, the rapid cyclogenesis as the surface low moved into western Kentucky and southwestern Indiana created an unusually strong pressure gradient across the region, even for cool season mid-latitude cyclones. This strong pressure gradient, combined with a brief period of steeper low-level lapse rates across central Kentucky thanks to temporary clearing in the wake of the cold front, helped to bring extremely strong winds and wind gusts to all of central Kentucky and most of southern Indiana. Widespread sustained winds of 30-40 mph and wind gusts of 60-80 mph were observed across the area, causing widespread tree and power line damage, as well as more scattered occurrences of structural damage. At one point, around half a million customers were without power across the state of Kentucky, with several counties seeing a temporary near-total loss in electrical service. As the mid-level cyclone moved overhead during the early evening hours, cloud cover would return, helping to reduce wind speeds and gusts somewhat, with conditions continuing to calm through the overnight hours into the morning of March 4th.

Tornado event in DUBOIS County, INDIANA
Start: 03-MAR-23 14:12:00
End: 03-MAR-23 14:15:00
Direct deaths: 0
Event narrative: An EF-1 tornado began just east of Dale, IN in northern Spencer County, near the intersection of Spencer County Roads 2100 North and 500 East. It continued to the north-northeast damaging property before crossing into Dubois County, where it snapped or uprooted several large trees along the county line. A couple of homes received minor roof and fascia damage. The tornado passed just northeast of Johnsburg, downing several large tree limbs and damaging a metal farm building before dissipating.