####018005231#### AGUS74 KWCO 221522 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT FRI MAR 22 2024 .Synopsis... Potential urban flooding in South Florida...Flash, urban, and small stream flooding impacts this weekend for Coastal Mid-Atlantic and Northeast...Isolated river and small stream flooding impacts possible in East Texas into the Lower Mississippi Valley...Ongoing river flooding with potential renewed rises late this week into the weekend in the Southeast... .Discussion... .South Florida... Urban flooding impacts are possible through day 2 (Sat) due to repeated rounds of moderate to heavy rainfall, 2 - 3" with locally higher amounts possible (WPC QPF) off the coast of the western FL Peninsula, from a Gulf Coast low pressure system. Generally dry conditions and a network of wetlands for the FL Peninsula will help to mitigate a wider flood threat however, higher rainfall rates, particularly along and south of I-75, may overwhelm some of the poorly drained roadways or typically flood prone urban areas. .Coastal Mid-Atlantic and Northeast... Flash, urban, and small stream flooding impacts, along with riverine and coastal flooding, are possible day 1 (Fri) though day 2 (Sat) for portions of the coastal Mid-Atlantic region into the Northeast due to a frontal boundary moving across the Eastern Seaboard. Rainfall from the system is forecast to be 2 - 4" with locally higher amounts possible, particularly along the I-95 urban corridor from NJ up to CT. Antecedent conditions suggest dry soil conditions (0 - 100 cm RSM NASA SPoRT) and near normal streamflows (USGS) for coastal Mid-Atlantic and more wet further north. Scattered minor river flooding is forecast this weekend. Ensemble guidance (MMEFS) suggests that some basins, notably the Lower Hudson and Lower Delaware, may also have isolated rises to moderate flood stage if higher rainfall totals materialize. Both GFS and NBM-forced NWM MRF is signaling the possibility of rapid-onset flooding and AEPs of 10 - 20%, especially in the lower order streams, suggesting highwater thresholds may be met and exceeded, resulting in flooding impacts in typically flood prone and low-lying areas. Additionally, the threat of urban flooding impacts are possible due to the nature of immediate runoff and pooling of water in poorly drained areas. .East Texas into the Lower Mississippi Valley... Minor river flooding is ongoing and forecast to continue into next week in portions of eastern Texas and the western Lower Mississippi Valley due to multiple rounds of rainfall over the past 7 days. The region will see a couple of days of dry weather allowing for area streams and rivers to recede unimpeded. By day 4 (Mon), rainfall will return to the region again regenerating rivers causing new and renewed rises along with stalled recessions. Due to the extended timeframe of the event, confidence in any additional hydrologic impacts, such as small stream and rapid-onset flooding, remains low for now. .Southeast... Moderate rainfall is forecast today for much of the Southeast with the highest rainfall totals (2 - 4" WPC QPF) located in GA, the Carolina Piedmonts, and along the Carolina coasts. Antecedent conditions vary, with wet soils (0 - 10 cm RSM NASA SPoRT) and above normal stream flows (USGS) in the western portions of the region and progressively more dry as you get closer to the coastal ranges of the Carolinas. Additionally, the NWM MRF highwater arrival times and stream AEPs indicate that some of the smaller order streams and tributaries will meet their highwater thresholds over the next 24-hours for portions of coastal GA and the Caronlinas. Some areas may see some urban and small stream flooding over an area of the heaviest rainfall, but impacts should widely be mitigated. Renewed rises on area rivers are likely and will be the primary flooding concern for this event. Yet another round of rainfall (2 - 4", WPC QPF) is forecast later on in the forecast period beginning on day 4 (Mon) and continuing through day 6 (Wed). The NWM MRF rapid-onset flooding has multiple signals that align well with the two separate events. For the first round of storms today, rapid-onset flooding probabilities trend on the lower end and are isolated along the GA and Carolina coasts. The rest of the signals further east MS are for the second round of rainfall and suggest an enhanced urban and small stream flooding event. The NWM Highflow Magnitude Forecast indicates that the AEPs are generally at their highwater thresholds, adding some confidence to the diminished threat later in the week. Due to the extended nature of the forecast for this second event, confidence is initially low in the exact timing and placement of these signals however, confidence is increasing that portions of the area will receive moderate rainfall that may produce isolated hydrologic impacts next week. //Capp $$