An Analysis and Communication Review of a Flash Flooding Event in Leland, North Carolina

On 19 May 2023, a coastal low-pressure system located offshore of the Georgia-South Carolina coast moved east-northeast along the coast, impacting the coastal waters within the County Warning Area of NWS Wilmington, NC. This event was driven by positive vorticity advection, upper-level divergence, and an abundance of moisture, leading to prolonged flash flooding in parts of Brunswick County, North Carolina, specifically affecting the Leland area. The 12Z sounding from NWS Morehead City, NC (MHX), northeast of Leland, recorded a precipitable water value of 1.72 inches, exceeding the 90th percentile for sounding observations at MHX and the former Cape Hatteras, NC site (HAT), indicating abundant low-to-mid-level frontogenesis. Rainfall began before sunrise and lasted until the early afternoon, with totals reaching 10-13 inches over approximately 9 hours. The flat terrain exacerbated the flooding, as it hindered efficient drainage. Additionally, the flood prediction application CREST failed to account for the area’s rapid urban development over recent years and does not incorporate tidal data. The flooding coincided with high tide, and tidal creeks were the main drainage tributaries to the Cape Fear River. The tide peaked at 0.1 feet below minor coastal flooding levels, possibly contributing to the slow drainage. As a result, with rapidly rising floodwaters, NWS Wilmington, NC, issued a Flood Advisory early that morning, soon followed by a Flash Flood Warning that lasted through most of the day. Water intrusion resulted in seven buildings being inundated, including a local high school cafeteria with 3 inches of water, prompting a shelter in place for all Brunswick County Schools. More than 20 road closures were also reported. The event's significance led Leland Emergency Management to establish an Incident Command Post (ICP) to coordinate response actions. Throughout the event, NWS Wilmington, NC, maintained contact with the Leland Emergency Manager, which supported meteorologists in warning operations by providing ground truth and validation. Despite this collaboration, there were instances of confusion and delay in information exchange between meteorologists and emergency managers, highlighting the importance of maintaining close and effective communication with core partners. The timely issuance of warnings allowed for a swift emergency response, although improvements are possible for future incidents. As the event unfolded, there were internal discussions about upgrading the base Flash Flood Warning to include an Impact-Based Warning (IBW) Tag in order to activate the Wireless Emergency Alert (WEA) system. Although the warning was never upgraded at the time, subsequent analyses during the after-action review suggested that leveraging the “Considerable” IBW Tag could have better served the community and core partners, given the significant impacts and necessity for multiple water rescues. This presentation will examine the synoptic and mesoscale setup, including the extreme climatological values that contributed to the event’s significance. It will also analyze the limitations of current observations and forecasting tools in assessing the severity in a tidally influenced area. The critical role of consistent and clear communication between the NWS and its core partners during severe weather events will be highlighted, aided by the utility of IBW Tags as it relates to this May 2023 event. The Leland, NC event highlights the complexities of tidally influenced flooding, the lack of tools and observations needed to assess its severity, and how effective communication, impact-based warnings, and close relationships with local emergency management can help overcome these challenges and provide more advanced warning.