This past weekend, the Ozark Mountains of southern Missouri and northern Arkansas experienced two days of near-continuous heavy thunderstorms. The sustained heavy rain over the steep hills of this region led to rapid runoff and a dramatic rise in local creeks and streams. Numerous towns and cities from central Arkansas to southern Indiana have been inundated over the past few days by floodwaters that, in some cases, have reached record heights. While floodwaters have begun to gradually recede in many locations, another ongoing (but fortunately less extreme) rain event will keep rivers elevated for several more days.
As is the case with any extreme flooding event, numerous factors contributed to the disaster, but meteorological conditions are foremost. Late last week (on and around April 28, 2017), a sprawling low pressure system spun into life along the southern Rocky Mountains and drifted northeastward. This powerful system inspired a huge variety of spring weather across the middle portion of the United States, with severe and tornadic storms across the southern Plains, a record-strong late-season blizzard in the High Plains of Kansas and Colorado, and heavy rain across much of the lower Missouri and Mississippi River valleys (Figure 3). Below, a spectacular GIF of infrared satellite images from the new GOES-16 shows thunderstorms blossoming across the central part of the country before the main upper level low pushes through. (See also this YouTube video of the lightning from these storms, as captured by GOES-16.)
While the tornadoes proved deadly and the blizzard may have damaged some sensitive vegetation, by far the most devastation from this storm was associated with the flooding. Figure 4 shows a snapshot of radar across the region from the evening of April 29, along with a selection of storm reports from the April 28-May 1 period. All kinds of severe weather occurred over this period, but note that flooding was widespread from MO/AR through the Ohio River Valley.
The most intense flooding was concentrated across the Ozarks of southern Missouri, northern Arkansas, and far northeastern Oklahoma, where 14 river gauges hit all-time record crests this week. Of course, factors such as floodwall and levee development along rivers can certainly affect the flood depths achieved — the more a river’s floodplains are restricted, the higher the water must go. However, we at MetStat like to examine the precipitation factor from a frequency perspective to determine just how extreme rainfall events are (Figure 5).
As the Average Recurrence Interval (ARI) analysis above indicates, some areas experienced rain on par with a 1-in-100 year event or more, particularly across the Ozarks and on into southern Illinois. This product compares the observed rainfall to rainfall frequency distributions in order to illustrate the relatively rarity of an event. On average, a 100-year ARI event would be expected to occur just once every 100 years for a given point. This event brought an impressively large area of 100- to 1000-year rains (reds to purples in Figure 5), which certainly played a leading role in producing such catastrophic flooding.
To some, this storm may seem like a case of déjà vu. A remarkably similar storm system brought rains of remarkably similar magnitude to this same region in late December 2015. As the comparisons below show, the rains in 2015 were slightly heavier, more widespread, and further northwest than the ones this time around:
Clearly, while the 2015 heavy rain was more widespread, the 2017 event had more intensely concentrated pockets of heavy rain over hillier terrain, and this likely contributed to the extreme nature of recent floods. While these two very rare events occurred within a short time of each other, it must be kept in mind that recurrence intervals just represent a probability, and that large events can happen at any time. This past weekend’s event is the first major event of the season, and more are sure to follow. We are constantly monitoring extreme precipitation events all around the country, so monitor this space for updates from MetStat.