Flash Flooding and Hail in Nebraska, April 26 to 27, 2016

Tuesday the 26th featured the first fairly widespread severe weather event of the year within South Central Nebraska, along with a few narrow-but-intense corridors of heavy rainfall resulting in flooding and flash flooding. The majority of severe storms concentrated within the southeast quadrant of the area (especially Nuckolls, Thayer and Clay counties), although a secondary zone of severe weather and heavy rainfall also flared up in western counties such as Harlan, Phelps, eastern Dawson and western Buffalo. Most severe weather reports consisted of hail, the largest of which was up to tennis ball size in Deshler. Fortunately, a possible tornado threat did not materialize (more on that below). There were actually two separate episodes of severe storms on this day, one during the pre-dawn hours (in far southeast counties) and then the main show during the late afternoon/evening.

In terms of highest impacts, the story of the day ended up being flash flooding in eastern Nuckolls and western Thayer counties, especially in and near Deshler. Mainly between 5-8 p.m. CDT, a nearly continuous axis of strong to severe storms streamed within roughly five miles either side of the Nuckolls/Thayer County line, dumping at least 4-6 of rain. Around 7:45 p.m. CDT, a storm chaser three miles west of Deshler posted fairly dramatic video showing a roughly 200-yard stretch of Highway 136 inundated by up to 4 feet of rushing water. Given that this deluge fell directly over the Spring and Snake Creek basins just upstream of Deshler, it did not take long for flash flooding to reach the community. By 9 p.m. CDT, evacuations were underway at nursing home and assisted living facilities, and one man had to be rescued when he became surrounded by rising water while boarding up his home. In addition to Highway 136, a number of rural roads in the Ruskin and Deshler areas remained water-covered for several hours. Fortunately, actual infrastructure damage was limited, and most roads re-opened the next day. Officially, the highest measured 24-hour total was 5.10 in Ruskin (NeRAIN observer), but radar estimation suggested that areas between Ruskin and Deshler likely received 6+ inches. This was the second-consecutive year that Deshler dealt with notable flooding (it also occurred May 6, 2015). Although not as serious overall, a secondary area of flooding materialized in western portions of South Central Nebraska, mainly within a narrow corridor extending from western Buffalo County southward through Phelps and northern Harlan counties, where 24-hour rain totals averaged 2-4, including 3.62 at the Holdrege airport. Most of this rain fell between 7-10 p.m. CDT. Within this zone, some roads were also impacted by flooding, including in the Elm Creek, Atlanta and Holdrege areas. While most water issues here also improved by the following day, flooding persisted through Thursday the 28th along the Turkey Creek in southern Buffalo County.

Focusing on non-flooding aspects of this severe weather event, the initial round of early morning storms were classic, elevated hail producers that affected mainly Thayer, Nuckolls and Fillmore counties between 3:00-6:30 a.m. CDT, with stones to at least quarter size reported in Hebron, Hardy and near Carleton. After this early morning activity exited to the east around sunrise, most of the day featured a solid lull before afternoon/evening redevelopment. Round 1 of the main event occurred primarily between 4:30-7:30 p.m. CDT, as a number of multicell to transient supercell storm clusters roamed Nuckolls, Thayer and Clay counties, and brushing as far north as Hamilton County. Within this zone, hail reports featured up to tennis ball size in Deshler, golf ball size near Superior and Hardy, and half dollar size in Fairfield. One storm southwest of Deshler briefly exhibited tornadic potential but no touchdowns were reported. As round 1 started to weaken in eastern counties, an entirely separate round 2 of severe storms rapidly fired up to the west within Harlan, Phelps, eastern Dawson and western Buffalo counties between 7-9 p.m. CDT. Although the aforementioned heavy rain situation was the main impact of these western storms, quarter size hail also fell in or near Atlanta, Ragan and Elm Creek. By mid-to-late evening, all storms within South Central Nebraska subsided to below severe limits, but areas of weaker convection persisted through the night.

This was a well-anticipated, synoptically evident severe weather event, as a large, deep trough ejected out of the Central/Southern Rockies into the Central/Southern Plains in a neutral-to-slightly negatively-tilted orientation. During the initial round of early morning hailers, the local area was positioned slightly north of a well-defined surface stationary front stretched west-to-east across northern Kansas, with convection driven by classic warm air/moisture advection at the nose of a 40 knot low level jet and in the presence of healthy elevated most-unstable CAPE up to 2000 J/kg. By the time of afternoon redevelopment, a 996 millibar surface low was centered over central Kansas. Except for the far southeastern edges around Thayer County, nearly all of South Central Nebraska remained slightly north of the quasi-stationary front/warm front throughout the late afternoon/evening hours. As a result, late afternoon surface temperatures only averaged in the upper 60s-lower 70s F, dewpoints only in the 50s and breezes were steady from the east-northeast. Despite residing north of the main frontal zone, mesoscale parameters were still supportive of severe storms as far north as the Interstate 80 corridor, featuring as much as 1000-2000 J/kg mixed-layer CAPE and deep-layer wind shear of 50-60 knots. However, the low-mid level wind profile proved rather inhospitable to longer-lived, discrete supercell storms, instead favoring clusters/line segments. For days leading up to the 26th, there was considerable speculation in some weather circles about this being a major severe weather/tornado outbreak through much of the Central Plains. This perception was fueled when the Storm Prediction Center placed much of South Central Nebraska and points southward under a Moderate Risk on the Day 2 outlooks issued on the 25th and maintained this level 4 category through most of the Day 1 period. As it turned out, although there were numerous reports of primarily large hail within the local area, fortunately a number of factors (including predominant storm mode) held a significant tornado outbreak at bay.