For the third consecutive night, this time from the evening of Thursday the 4th into the early morning of Friday the 5th, several rounds of thunderstorms rumbled across much of the 24-county South Central Nebraska area. And for the second straight night, the majority of storms concentrated within the southeastern half of the area. Although the first few hours of evening convection yielded several hail reports up to mainly quarter size within Hall, Hamilton, Merrick and York counties, by far the biggest story of the night, and for some places for the next several days thereafter, was heavy rainfall and resultant flooding. During the 24-hour period ending around sunrise on the 5th, widespread rainfall of 2-4 drenched several counties, primarily including: Adams, Clay, Fillmore, York, Nuckolls, Thayer, southern Hall and southern Hamilton. Combined with as much as 1.50-3.00 that fell across some of these same areas during the preceding night, the net result was 36-hour rain totals of generally 3-7, with the overall-highest concentration of 5-7 impacting a fairly narrow, roughly 10-mile wide corridor extending through southeast Hall, northeast Adams, southern Hamilton, northern Clay and western Fillmore counties. As just a few specific examples of 36-hour measured totals ending around sunrise on the 5th: 6.44 at Harvard and 5.28 four miles west of Geneva (NeRAIN observers). For the calendar days of June 3rd-5th, Hastings airport totaled 5.39, of which 4.74 fell on the 4th. This total set a new Hastings daily record for June rainfall, and was the third-highest daily total for any day of the year. Needless to say, widespread flooding of creeks, streams and primarily rural roadways was well-underway in the aforementioned counties by daybreak on the 5th. Adams county Emergency Management issued a press release alerting citizens that some roads are impassable and dangerous due to standing water and possible unknown damage in the northeast part of the county. Earlier on, during the initial stages of heavy rain on the evening of the 4th, significant street flooding impacted many communities including Grand Island, where several city streets were closed for a few hours. Also near Grand Island on the evening of the 4th, the Platte River (which had already been slightly above flood stage for a few weeks) briefly rose far enough out of its banks to overflow Locust Street near the Interstate 80 exit 314 interchange. Getting back to the daytime hours on the 5th, although flooding of roads and small streams began to very slowly subside, issues began ramping up on larger creeks and rivers. Most notably, this included the West Fork Big Blue River, which was centered almost directly under the narrow axis of heaviest rain and experienced historic flooding over the next several days (please refer to the separate paragraph below for more details regarding West Fork Big Blue River flooding).
Taking a closer look at the timing and evolution of local thunderstorm activity from the afternoon of the 4th into the early morning of the 5th...there were several different rounds. Following a brief lull in the wake of widespread activity during the early morning of the 4th, the next round of mainly non-severe storms affected counties primarily along/north of Interstate 80 through most of the afternoon hours. These storms yielded only one severe weather report: quarter size hail near Pleasanton. Following another brief lull during the late afternoon and early evening, a narrow swath of hail and torrential rain-producing severe storms blew up between 7-9 p.m. CDT along an axis from near Grand Island and Central City, eastward to near Stromsburg and York. Between 9-11 p.m. CDT, this vigorous line of convection gradually weakened as it sank southward toward the Kansas border, but continued dumping heavy rain. Beyond 11 p.m., the next several hours featured widespread, persistent development of non-severe-but-heavy-rain producing convection over most counties along and south of Interstate 80, largely driven by convergence/lift along the nose of a nocturnal, 30-40 knot low level jet. Finally, between 3-6 a.m. CDT on the 5th, storms steadily diminished in intensity and ended from north-to-south, leaving only lingering light rain over the Nebraska-Kansas border area by sunrise. Turning to the synoptic and mesoscale setup: this was a fairly similar situation to the night before, as a quasi-stationary composite surface front/outflow boundary remained in place near the Nebraska-Kansas border. Although the primary large scale mid-upper trough remained well to the west along the California coast, a series of downstream low-amplitude disturbances passed through the Central Plains in quasi-zonal flow, interacting with an unstable and very moist airmass to trigger vigorous convection. Around the time of the initial evening hail-producing storms, the mesoscale environment featured around 2000 J/kg mixed-layer CAPE and 30-40 knots of 0-6 kilometer deep-layer wind shear.
June 5th-12th Summary of Historic Flooding Along the West Fork Big Blue River from near the Adams/Clay County line downstream through southeastern York County...deemed to be the worst flooding in 50-80 years: As detailed in the first paragraph above, 36-hour rainfall totals averaged 5-7 along much of the local stretch of the West Fork Big Blue River between the evening of June 3rd and the morning of June 5th. As a result, significant flooding commenced along the river and its smaller tributaries on the 5th, with the primary flood crest initially working its way across the northern Clay and southern Hamilton County area. By the daylight hours on Saturday the 6th, the seriousness of the flooding situation became more apparent for downstream communities such as McCool Junction. Residents in southern Hamilton County were already calling it the worst flooding they had witnessed in 50-80 years, as the crest reached the Lushton area of York County by late morning. Around this time, officials in McCool Junction began evacuations of several homes on the east side of town along the rising river, along with sandbagging operations. Around 12:30 p.m. CDT, water entered a church camp just south of town, cutting off access to it. The West Fork Big Blue crested in the immediate McCool Junction area that evening, but only a few homes suffered significant flooding. It was reported that at least five homes were saved due to advance notice of the approaching crest and resultant sandbagging. By daylight on Sunday morning June 7th, water levels had receded 1-2 feet at McCool Junction, as the crest worked downstream to near the York/Seward County line. Over the next few days, a welcomed-stretch of warm and dry weather allowed the river to slowly recede back within its banks over southern York County, and the initial multi-day Flood Warning was allowed to expire on Wednesday, June 10th. However, Mother Nature soon threw one last curveball at the McCool Junction area, as yet another round of heavy rain on the order of 1.50-3.00 fell into much of the West Fork Big Blue basin on the evening of the 10th into the morning of Thursday the 11th. As a result, a secondary, less-severe round of flooding was reported at McCool Junction by 10 a.m. CDT on the 11th, with the river reported to be one foot out of its banks. This second round of primarily minor flooding persisted into the daytime hours on Friday, June 12th, but largely abated by sunset that evening, finally ending over one week of flooding issues along the West Fork Big Blue River in eastern portions of South Central Nebraska.