Flash Flood in Mahoning County, Ohio

Widespread flash flooding across the Boardman area. Local dispatch, local media, and the local stormwater district reported over 250 residences that experienced basement flooding, with a few reports of water filling entire basements. Widespread street flooding with reports of dozens of car stranded in high water as high as three feet deep. Radar estimated rainfall totaled up to 4.5 in just under a three hour period, which has a 0.2% chance or less probability of annual occurrence per NOAA Atlas 14. Instantaneous rates of up to 6 per hour were observed, with a peak 15 minute rainfall accumulation around 1.25.

A surface cold front moved south-southeastward across Lake Erie and northern Ohio during the late morning through early evening of the 4th. Isolated to scattered showers and thunderstorms occurred along and ahead of the front, within a humid warm sector comprised of weak to primarily moderate MUCAPE and moderate effective bulk shear. Radar data suggest the weak and brief tornado was spawned by a supercell after the northeastward-moving supercell experienced a constructive merger with a separate multicell thunderstorm moving north-northeastward during the early evening. Mixed layer LCL's were estimated to be near 500 to 750 meters AGL within the supercell's ambient environment, which were favorable for mesocyclonic tornadogenesis. Perhaps chaotic surface airflow associated with the merging convective cold pools enhanced low-level vertical wind shear and surface-based effective storm-relative helicity prior to tornado occurrence. The thunderstorms produced prolific rainfall rates of up to 6 per hour due to precipitable water values near 1.8 and efficient rainfall processes.