####018006497#### AGUS74 KWCO 171503 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT FRI MAY 17 2024 .Synopsis... Moderate to major river flooding in East Texas... Locally considerable flooding impacts possible for portions of the Lower Mississippi Valley and Southeast... Considerable flooding impacts remain possible in Hawaii... Localized flooding impacts possible in the Lower Ohio Valley and Central Appalachians... .Discussion... .East Texas... Major river flooding along the Lower Trinity and Lower Neches, as well as moderate river flooding along the Lower Sabine and San Jacinto, is ongoing and forecast. Most locations are still rising, with crests expected through the weekend and into next week as the flood waves route downstream. With no significant rainfall expected over the next week, recessions will begin and continue unimpeded. .Lower Mississippi Valley and Southeast... Locally considerable flash and urban flooding impacts are possible along the Central Gulf Coast through day 2 (Sat) as multiple rounds of heavy rainfall pass across the region. Flooding impacts will also be possible for inland portions of the region, with flash and urban flooding the primary threat, but some small stream and minor river flooding cannot be ruled out. While the top layer (0 - 10 cm) of soils along the coast are nearly saturated due to recent rainfall (85%+ RSM, NASA SPoRT), soil moisture decreases with depth in the column, with overall values between 60 - 70% (0 - 100 cm) throughout the region. These soil conditions should initially allow rainfall to be absorbed into the system, especially for the inland areas, however, each additional round of rainfall will continue to prime the environment for hydrologic responses and quickly diminish available infiltration capacity. Given the likelihood of enhanced rainfall rates and training convection, top soil conditions may quickly be overwhelmed, resulting in rapid runoff into already elevated streamflows and leading to subsequent flooding impacts. As the storm system moves into GA/SC tonight, antecedent soil conditions become less wet, with top soil conditions generally near normal (45 - 55% RSM), which should help to mitigate any widespread flooding responses. While overall rainfall totals are expected to be less (1 - 2" WPC), moderate to heavy rainfall rates are possible, thus localized flash and urban impacts cannot be ruled out if higher rates were to materialize or if training were to occur. The National Water Model (NWM) Short Range Forecast (SRF) has been showing a slow but steady increase of probabilities for rapid-onset flooding and high water flows (1.3 ARI flows) in the region between Jackson, MS, New Orleans, LA, and Montgomery, AL. Additionally, the NWM SRF Maximum High Flow Magnitude Forecast is showing areas of significant stream rises (2 - 4% AEPs) from just south of Jackson to Hattiesburg (MS) this morning, and a secondary area of potentially significant flows (2 - 4% AEPs) from Thomasville to Montgomery (AL) late this morning into the afternoon. These signals, combined with wet antecedent conditions, support the potential for locally significant flooding impacts, especially in urban areas and locations where repeated heavy rainfall rates occur over smaller and more responsive basins. Looking further, the NWM SRF is signaling rapid-onset flooding potential in the region between Pensacola, FL, to Dothan, AL, and east towards Tifton, GA beginning late this afternoon. These probabilities are mostly low-end (less than 25%) but will likely continue to increase throughout the day as the event unfolds and the QPF moves into the forecast window. Corresponding AEPs are mostly near 50%, however, isolated pockets of AEPs below 20% are seen throughout this region, suggesting locally out-of-bank small stream rises are possible. .Hawaii... Considerable flash, urban, and small stream flooding impacts remain possible for Kauai, Niihau, and Oahu today as a slow-moving band of heavy rainfall moves across the islands. Landslides are also possible in areas of steep terrain. .Lower Ohio Valley... Localized flash and urban flooding impacts remain possible through this evening for portions of northwest TN through central KY. Antecedent top soil conditions are wet, but not saturated (65 - 80%, 0 - 10 cm RSM, NASA SPoRT) throughout the region suggesting infiltration capacity is still available, however, should heavier rainfall amounts materialize, these conditions could be overcome and increase the potential for runoff into already elevated streams. Rainfall should be relatively progressive which will help to diminish the overall flood threat, but isolated flooding impacts cant be ruled out should heavier rainfall/training occur. The NWM SRF is signaling low-end probabilities for rapid-onset flooding (less than 25%) with some isolated probabilities approaching 50% between Clarksville, TN, and Bowling Green, KY. Corresponding AEPs are generally at or above 50%. This suggests most streamflow rises, should they occur, will remain in-channel, but some out-of-channel rises cant be ruled out in smaller, more responsive basins. Nonetheless, any impacts will be dependent on where the heaviest rainfall occurs given the overall progressiveness of the system. .Central Appalachians... Localized urban and small stream flooding impacts are possible along the foothills of the Applachians through day 2 (Sat) from periods of moderate to locally heavy rainfall (1 - 3") to the region. Soil moisture is generally near normal to somewhat wet across the entirety of the region, suggesting some remaining infiltration capacity. Streamflows follow a similar pattern with most flows normal to slightly above normal for this time of year indicating remaining channel capacity. These conditions will help to mitigate widespread hydrologic responses overall, but the complex terrain will increase the potential for out-of-bank rises and flash flooding impacts in small, quick responding basins, especially if there are repeated periods of convection. //Freeman $$