Flooding in Nebraska, October 1 to 2, 2013

A highly unusual, albeit minor and fairly low-impact episode of Platte River flooding that began in South Central Nebraska in late September continued into the first few days of October before river levels commenced a steadier drop. More specifically, minor flooding during the first few days of October primarily concentrated along the river through Buffalo and Hall counties (and the adjacent extreme northern borders of Phelps and Kearney counties), with the gauge near Kearney slipping below minor flood stage of 6.0 feet on Wednesday morning the 2nd. This ended roughly 10 days of minor flooding along the Platte within South Central Nebraska that mainly impacted lowlands immediately adjacent to the river as minor flooding first reached the Gothenburg area around Sep. 22 and gradually spread as far downstream as the Merrick/Hamilton County border northeast of Grand Island. However, by the time flood waters got very far past Grand Island, the multiple channels of the river helped disperse enough water to diminish any flooding threat farther downstream.

The unusual aspect of this river flooding event was that it had very little, if anything to do with rain that fell directly over South Central Nebraska or even nearby upstream areas over western Nebraska. Instead, this event resulted from historic rainfall that inundated parts of the Colorado Front Range and northeast Colorado between Sep. 9-15. During this second week of September, an upper-level low pressure system formed and became cut off from the mean flow over the Intermountain West. This low slowly migrated from near Las Vegas north to Idaho. Subtropical high pressure remained stationary over the Southern Plains. This synoptic configuration channeled a stream of anomalously high moisture into the Rockies. With surface high pressure essentially stationary over the Northern Plains, several days of easterly upslope flow into the Colorado Rockies resulted in tremendous rainfall and catastrophic flash flooding in and around Boulder, Colorado. The heaviest rains of all occurred on Sep. 11-13. Widespread multiple-day rainfall ended up totaling from around 7 to as high as 17 inches in some areas. In the city of Boulder, all rainfall records were broken for the month of September, for any calendar month and for any year. Given that northeastern Colorado is the headwaters of the South Platte River, this tremendous volume of water began a slow but steady journey down the South Platte, reaching the confluence with the North Platte River near North Platte, Nebraska, on Friday, September 20th.

By Sunday Sep. 22nd, the leading edge of flooding reached western Dawson County within the local South Central Nebraska area. River levels rose rapidly and dramatically at Gothenburg and points downstream. Prior to the arrival of the flood waters, Platte River levels were quite low due to prevailing below normal long term precipitation within and near the local Platte basin. In fact, river flow in some areas prior to the flooding was down to a narrow trickle up to a couple feet wide. As a result, the ensuing flood water filled the breadth of the riverbed and its multiple channels, with standing water occurring in some low-lying areas adjacent to the riverbanks. As the water reached the gauge near Cozad, the river level increased from one foot to five feet. The peak crest at Cozad was around 7 ft. on Sep. 24. It was a similar story as the water continued to move downstream, with river levels peaking in the Overton and Kearney areas between Sep. 25-29 and peaking near Grand Island in the Sep. 29-Oct. 1 time frame. At all locations along the river, once water levels reached the crest they were slow to recede and took around three days to drop one foot from the crest height. This stood as a testament to the significant volume of water generated from the flooding rains far upstream, and it was especially impressive given that it took little more than a single day for river levels to initially rise several feet.