A fairly significant-but-localized rural flooding event unfolded within north central and northeast portions of Polk County, ramping up during the early morning hours of Tuesday Aug. 18th and persisting into the afternoon of Wednesday the 19th before markedly improving (severe weather in the form of large hail/damaging winds were not an issue). The worst flooding impacts were confined to a rather narrow, roughly 5-8 mile wide corridor extending west-to-east several miles north of Osceola and Shelby, where generally 6-10 of rain accumulated over the course of roughly 36 hours (see below for timing details). Within this zone, county officials reported that 25-30 gravel roads were under water or closed, with a few completely washed out. In addition, Clear Creek flooded within northeastern portions of the county, as the gauge 10 miles north of Shelby crested just above the minor flood stage of 8 ft. on the morning of the 19th. Although ground-truth, event-total reports were limited within the narrow corridor of maximum rainfall, a NeRAIN observer seven miles north of Osceola measured 6.69, and an unofficial public gauge six miles south of Duncan captured 8.50. Another nearby NeRAIN observer commented that this was the worst road flooding they had observed within the local area in at least 63 years.
This was a somewhat unique flooding event considering that it did not occur solely as a result of one significant rainfall. Instead, it stemmed from the cumulative effect of primarily three rounds of heavy rain over the course of roughly 36 hours, with the third and final round serving as the proverbial final straw, resulting in the impactful flooding. In the mid-upper levels, broad quasi-zonal flow was in place over Nebraska during most of this time, before a larger scale shortwave trough finally passed through on the 18th, sweeping rain chances out of the local area. The first concentrated round of heavy rain moved through the county on the evening of Sunday the 16th, part of a larger complex of showers and thunderstorms focused near a slow-moving cold front. Following a rather short lull, the second primary round of rain flared up during the morning hours of Monday the 17th. Combined, these first two rounds dropped generally 2-4, but not enough to instigate flooding. Although a few spotty showers affected the area during the afternoon and early evening of the 17th, this was largely a dry period before a strengthening nocturnal low level jet sparked a narrow axis of persistent heavy rain over northern Polk County mainly between 9:30 p.m. and 12:30 a.m. CDT. This third round, which soaked the area with another 2-5, was clearly the one that prompted significant flooding issues, although one final burst of rain also passed through a few hours later around sunrise on the 18th. Fortunately, a solid stretch of dry weather returned to Polk County thereafter.

Flash Flood in POLK, NEBRASKA
From 17-AUG-15 23:00:00 to 18-AUG-15 05:00:00 (CST-6)
Location: POLK
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Many roads were flooded. Around 25 to 30 county roads (many were gravel roads) were closed in north central and northeastern portions of Polk County. At least one paved road had water running over it a few miles west of the Highway 81 and 64 junction. Some gravel roads were washed out in spots. Gauge readings and radar estimation indicated a narrow swath of six to 10 inches of rain fell in separate rounds over the course of roughly 30 hours. Minor flooding occurred along Clear Creek within northeast portions of the county. An NeRAIN observer seven miles north of Osceola reported 6.69 inches of rain between the evening of August 16th and the early morning of August 18th. The majority of the rain fell between 930 p.m. CDT August 17th through 1230 a.m. August 18th. Also, an unofficial public report of 8.50 of storm total rain occurred six miles south of Duncan.

Flood in POLK, NEBRASKA
From 18-AUG-15 05:00:00 to 19-AUG-15 12:00:00 (CST-6)
Location: POLK
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Around 25 to 30 county roads (many were gravel roads) were closed due to standing water in north central and northeastern portions of Polk County. At least one paved road had water running over it a few miles west of the Highway 81 and 64 junction. Some gravel roads were washed out in spots. Gauge readings and radar estimation indicated a narrow swath of six to 10 inches of rain fell in separate rounds over the course of roughly 30 hours. Minor flooding occurred along Clear Creek within northeast portions of the county. An NeRAIN observer seven miles north of Osceola reported 6.69 inches of rain between the evening of August 16th and the early morning of August 18th. The majority of the rain fell between 930 p.m. CDT August 17th through 1230 a.m. August 18th. Also, an unofficial public report of 8.50 of storm total rain occurred six miles south of Duncan.