Chart: Preliminary Maximum 72-hour Average Recurrence Interval, Ozarks
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.
SIMPLIFIED VERSION BELOW