Flash Flood — Nance, Nebraska

2024-06-21 to 2024-06-22 · near Genoa, Nance, Nebraska

Event narrative

Based on radar estimation and various measured totals from NWS/NeRAIN/CoCoRaHS observers, widespread rainfall of 2.50-7.00 inches fell across most of the county mainly between 8 p.m. CDT on the 21st and 1 a.m. CDT on the 22nd. Per media reports, the county supervisor reported flood damage to numerous rural roads.

Wider weather episode

Although it impacted only a handful of west central and especially northern counties of South Central Nebraska, a persistent narrow band of heavy rain on the evening of Friday the 21st into the pre-dawn of Saturday the 22nd caused what was likely the overall most impactful flooding event of 2024 within the 24-county area. Over the course of 10 hours between 5 p.m. and 3 a.m. CDT, numerous and mostly sub-severe thunderstorms repeatedly tracked through counties along and especially north of Interstate 80, dumping several inches of moderate to heavy rain that flooded countless rural roads (damaging many), prompted at least minor flooding in several communities (most notably Dannebrog, which suffered moderate impacts) and sent several creeks and streams spilling out of their banks (including the Turkey/Oak Creeks in Howard County and Davis/Clear Creeks in Polk County). Fortunately, the vast majority of convection checked in on the weaker side in terms of damaging wind/large hail. However, there were a few instances of severe-criteria winds mainly early in the event, highlighted by a few pockets of tree damage and several flipped irrigation pivots from apparent microbursts in the Cozad vicinity around 7:30 p.m. CDT (speeds estimated 70-90 MPH).

There was little doubt though that heavy/rain flooding was the biggest story. Delving into various details, the broader west-east oriented mesoscale rain band was 25-35 miles wide, aligned through nearly all of Sherman/Howard/Nance counties, along with southern parts of Valley/Greeley counties and primarily northern sections of Dawson/Merrick/Polk counties. Within the outer northern and southern fringes of this band, 2-4 inch totals were common. However, the real trouble struck within the heart of the mesoscale band, as a roughly 10-15 mile wide swath of real estate centered roughly along a Rockville-St. Paul-Genoa line was drenched with widespread totals of 4-6, and localized higher amounts to 7+. Per measurements from NWS/NeRAIN/CoCoRaHS observers, a few of the highest official totals featured: 7.60 six miles southeast of Wolbach (this exceeded the 200-year average recurrence interval), 6.20 nine miles northeast of Palmer, 5.23 three miles north-northeast of St. Paul, 5.10 two miles northwest of Rockville and 4.70 in St. Paul itself. In addition to widespread flooding of innumerable rural roads and several small creeks/streams, the overall-biggest impact to any particular community targeted Dannebrog. Slightly outside of town, an evacuation was needed at a farmstead, and a river gage along the Turkey Creek north of Dannebrog registered a record crest of 21.63 around sunrise on the 22nd (this also eclipsed moderate flood stage of 21.0'). Incredibly, this gage was only three weeks removed from a notable crest of 20.40'...at the time its 2nd-highest on record. In Dannebrog itself, the Oak Creek overran its banks during the day on the 22nd, closing down Highway 58, flooding the city park along with several streets, and causing a few homes to need sandbagged. In all, seven homes reportedly suffered water damage and 14 city blocks flooded. Farther east, although not as impactful as in Dannebrog, a portion of Highway 81/92 was closed for several hours on the west edge of Osceola as Davis Creek spilled out of its banks. While the majority of flooding receded (or at least greatly improved) by late evening of the 22nd, a gage along the Clear Creek in northeastern Polk County did not slip below flood stage until late evening of the 24th. Putting this hydrologic event into some broader context, while this 3-6+ of evening-overnight rain was certainly enough to prompt flooding on its own accord, plentiful antecedent precipitation had primed the pump (so to speak) by saturating soils. Outlining specific examples from two long-time NWS Cooperative Observer stations, 60-day observed precipitation leading into this event (since April 23) was already a notable 5.53 above normal/167 percent of normal at St. Paul, and 7.11 above normal/180 percent of normal at Osceola. Then, in the wake of this event (which dumped 4.70 on St. Paul/2.96 Osceola), 60-day totals at these stations became downright remarkable. At St. Paul, the 60-day tally for April 24-June 22 ballooned to 18.50, not only marking the wettest stretch on record between those particular dates (out of 128 years), but also the overall-wettest 60 days in five years (since summer 2019). In Osceola, the 60-day total from April 24-June 22 soared to 18.96, marking the 2nd-most on record (out of 124 years) between those particular dates and also the overall-wettest 60 days since 1991.

Finishing up with a look at event evolution and timing, this was actually a fairly classic setup for training convection and resultant, narrow heavy rain banding. In the mid-upper levels, moderately-strong west-southwesterly flow prevailed, containing embedded, low amplitude disturbances. Meanwhile, at the surface a well-defined, quasi-stationary front loosely bisected Nebraska in southwest-northeast fashion at mid-afternoon, attendant to low pressure centered over the NE/KS/CO border. The local South Central Nebraska area resided entirely south of this front to start the late afternoon/early evening, with gusty southerly breezes maintaining a seasonably-moist airmass with dewpoints mid-upper 60s F. The first storms of the afternoon fired up near the surface low/frontal intersection in far southwest Nebraska and started tracking northeast, entering the local area into Dawson County between 5-6 p.m. CDT. During the next couple of hours, convection quickly evolved into a nearly solid, west-southwest to east-northeast oriented axis extending from Dawson County up into/across the aforementioned counties mainly north of I-80. Also during this time, a few storms assumed transient supercell characteristics within a mesoscale environment featuring 1000-1500 J/kg and 30-40 knots of effective deep-layer shear...including the storm that caused the Cozad area wind damage. However, as darkness set in, it was increasingly-clear that heavy rain/flooding would take center stage as a south-southwesterly low-level jet (evident at 850 millibars) ramped up to at least 40-45 knots, lifting copious amounts of moisture into a low-level frontal zone that was shifting slowly southward across South Central Nebraska...augmented by convective outflow. By 10 p.m. CDT, training convection stretched the width of the local area...from Dawson/Sherman/Buffalo counties in the west to Nance/Merrick/Polk counties in the east. Fortunately, this was also about the time that the western fringes of convection stopped continuously regenerating over the Dawson County area, allowing storms to begin gradually vacating South Central Nebraska. This was a slow process though, with the back edge of heavy rain not clearing the Highway 281 corridor until around midnight CDT, before finally exiting Polk County and the far northeast corner of the local domain between 2-3 a.m. CDT.

View location on OpenStreetMap → (41.3939, -97.7043)

Source: NOAA Storm Events Database, event_id 1182830. Narrative written by the NWS forecast office that issued the report.