Flash Flood — Thayer, Nebraska

2015-05-06 to 2015-05-07 · near Davenport, Thayer, Nebraska

Event narrative

Central portions of Thayer County was the hardest hit in the NWS Hastings coverage area. A swath of 5 to 9 inches of rain, with locally higher amounts of 10 to 11 inches, was reported across the the county. Portions of Highways 81, 136 and 53 were closed for a period of time during the evening. In Hebron, a bridge was washed out, reducing the access to the local hospital and nursing/assisted living facilities down to just one road. In Deshler, water at least 6 inches deep resulted in the evacuation of 6 homes as well as a nursing home. A total of 43 residents were taken to a shelter at the local high school. At least 4 bridges and many miles of county roads were damaged by the flooding.

Wider weather episode

Thunderstorms developed early this Wednesday afternoon and evening, and predominantly produced tornadoes and flash flooding. With no cap in place, scattered thunderstorms began developing as early as the noon hour over north central Kansas. Most storms rapidly became supercellular. These supercells rapidly lifted north into south central Nebraska and began producing severe weather by 330 p.m. CDT. Most of the tornadoes occurred over Nuckolls and Thayer counties. One tornado did occur over Adams county and a brief spin-up also occurred over Hall county. Eventually, an MCS developed. The leading edge of the cold pool becoming stationary. New storms repeatedly developed over north central Kansas and lifted northeast along the boundary, through Nuckolls and Thayer counties and further downstream into southeastern Nebraska. Cell training produced a swath of 5-11 inches of rain over portions of Nuckolls and Thayer counties. The highest amount reported was 10.91 inches just west of Hebron. This amount of rainfall in 24 hours typically occurs once in 1,000 years at any given location, or stated another way, has a .1% chance of occurrence in a given 24 hour period. Cooperative observers in Hebron and Superior reported 8.59 and 7.13 inches, respectively. These were the highest 24-hour rainfall amounts ever recorded at these locations, dating back to 1893. Flash flooding damaged roads and some bridges were washed out. U.S. Highways 81 and 136 were closed for a time due to the flooding.

A complex low pressure system extended from the Canadian Prairies into the western United States. These storms developed in the warm sector, far away from any discernible surface boundaries. However, an axis of confluence was present over north central Kansas. A deep upper-level trough was over the western U.S., with a ridge over the East. In the southwest flow between these features, a shortwave trough was lifting through the Central Plains. These storms formed in the southeast quadrant of the associated vorticity maximum. Around the time thunderstorms developed, temperatures were in the lower 70s with dewpoints in the lower 60s, resulting in low cloud bases. Even at mid-afternoon, cloud bases were only 2000-2500 feet. The surface conditions and mid-level lapse rates of 7 degrees C/km resulted in SBCAPE around 1500 J/kg. Deep layer shear was only 25-30 knots. However, low-level wind fields were strong with 40-45 knots at 850 mb. This resulted in large hodographs with 0-1 km SRH values of 200-250 m2/s2, and 0-3 km SRH of 250-300 m2/s2. Combined with low cloud bases, this created a favorable environment for supercells and tornadoes.

View location on OpenStreetMap → (40.2781, -97.8207)

Source: NOAA Storm Events Database, event_id 576494. Narrative written by the NWS forecast office that issued the report.