Examination of Forecast and Warning Services for the Lower Mississippi River Valley Severe Weather and Flash Flood Event of 2–3 April 2017

An unusual mix of severe weather and excessive rainfall brought severe impacts to the NWS Jackson, MS forecast area during 2-3 April 2017. In all, there were 41 reports of severe weather, seven confirmed tornadoes, including two strong tornadoes in Northeast Louisiana, and 33 reports of flash flooding. The potent, spring storm system featured strong dynamical forcing, a very moist, highly unstable atmosphere, and strong low-level wind shear. Two mesoscale convective systems (MCS) were the primary features which drove the event. The first MCS came through during the afternoon and produced the bulk of severe weather over Northeast Louisiana and western Mississippi, while the second MCS produced additional severe weather and most of the flash flooding during the overnight.

Initial supercell thunderstorms produced several tornadoes during the afternoon, resulting in major damage to homes and widespread tree damage. Warning services were aided tremendously by the newly installed dual-polarization radar (KULM) located at the University of Louisiana-Monroe in Monroe, LA. This radar allowed tornado debris signatures (TDS) to be identified over Northeast Louisiana, thus allowing for critical tornado confirmation wording through our impact based warnings. In addition, a warning forecaster was dedicated to analyzing radar data for possible TDS, and this allowed for more efficient and effective warning operations. As the event evolved, flooding became a bigger concern as increasingly favorable shear and mesoscale influences combined to support training of convection. The High Resolution Rapid Refresh model showed signals for this flash flooding set-up well in advance, and it prompted forecasters to raise the threat level in the hazardous weather outlook to warn our partners and the public that significant flooding, including the flooding of homes and closure of roads, would be possible. Flash flood warning lead time was boosted by the utilization of Oklahoma University’s Flooded Locations and Simulated Hydrographs application. These indicated 50 to 100 year return intervals, with amounts as high as 100 to 200+ year return intervals for the most severely impacted areas.

By the end of the event, many observing stations had received up to 6-10 inches of rainfall, with most of that falling in less than 12 hours. The extreme rainfall rates resulted in major flash flooding of suburban neighborhoods in the Vicksburg to Jackson area that required evacuations and even prompted the sounding of sirens in Rankin County, MS. The most significant flash flooding occurred along the smaller gauged and ungauged streams around Vicksburg and Jackson, including a rise of 15 feet in less than six hour at Hanging Moss Creek. Larger rivers, such as the Pearl River at Jackson and the Big Black River at Bovina, rose nearly 22-24 feet to moderate flood stages during the overnight of April 2nd into April 3rd.

In this examination, we will detail the setup for this potent spring severe weather and flash flood event, and describe the benefits of new technology and operational procedures toward improving our forecast and warning services.