This late Monday evening the 25th into early Tuesday morning the 26th featured one of the most noteworthy, albeit very-localized heavy rain and flooding events of summer 2014 within South Central Nebraska, with the greatest concentration of 3+ inch amounts centered directly over and near the city of York. The vast majority of this rain fell during a three-hour period between 1130 pm-230 am CDT, with a few of the highest rain totals in the immediate York area featuring 6.40 on the southwest side of town (NeRAIN), 4.93 on the southeast side (NeRAIN) and 4.71 inches a few miles north of the city (NWS coop observer). As a result of this deluge, significant street flooding temporarily stranded several motorists during the night and Beaver Creek remained out of its banks well into the Tuesday daytime hours within the city. Although flooding in central York County was the main story on this night, hail up to quarter size was also reported in York, and farther west, penny to nickel size hail fell in Hastings and Phillips.
This event was triggered by the rapid 11 pm-12 am CDT initiation of a west-southwest to east-northeast corridor of elevated convection that started out no more than 10 to 25 miles wide, and extended from the Minden and Hastings areas in the west and then eastward through the Aurora and York areas. While this primary area of convection steadily ended from west-to-east during the next few hours as it expanded in area somewhat, the York area bore the brunt of heavy rainfall as it not only received heavy rain from the initial development, but also had to endure training convection moving in from the west. Although small pockets of weak thunderstorms lingered over South Central Nebraska throughout the night, the main heavy rain event wrapped up between 130-230 am CDT.
This was a fairly classic case of a narrow corridor of elevated nocturnal thunderstorms blossoming within the exit region of a modestly-strengthening low level jet centered near/slightly above 850 millibars. In the mid-upper levels, the local area resided with a zone of slightly enhanced southwesterly flow focused well-downstream from a positively tilted western United States trough centered over Nevada. At the surface, South Central Nebraska resided north of a generally west-east quasi-stationary front draped across Kansas. Around the time of late-evening convective initiation, mesoscale parameters featured around 1000 J/kg most-unstable CAPE and 0-6 kilometer deep layer wind shear around 30 knots.