####018005134#### AGUS74 KWCO 151505 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT WED MAY 15 2024 .Synopsis... Considerable flooding impacts likely across East Texas through the Central Gulf Coast late this week... Considerable flooding impacts likely across Hawaii through Friday... Isolated flooding impacts possible across the Southern Plains through the Ohio Valley late this week... .Discussion... .East Texas through the Central Gulf Coast... Considerable flash, urban, and river flooding is expected once again across the region beginning on day 2 (Thu) across East TX into central LA as additional rainfall (2 - 4"+) impacts this already sensitive region. The Neches/Angelina, Lower Trinity, Sabine, Brazos/Navasota, and Calcasieu river basins are most at risk for moderate-to-major river flooding, as the heaviest rainfall is expected to fall on these already swollen streams and rivers. There is very little-to-no available soil infiltration capacity, given the saturated soils from recent rainfall and moderate to major river flooding that is ongoing, and thus the new additional rainfall will immediately runoff into already flooded streams. At this time, both the GFS and NBM-forced National Water Model (NWM) continue to indicate isolated to scattered small stream responses across East TX. Given the higher ARI (3.2 ARI) threshold for high water flows across eastern TX, signals are a bit muted across the region in comparison to what expectations are for riverine response late this week. Both medium-range forcings of the NWM continue to depict isolated annual-exceedance probabilities (AEPs) below 20% (NWM High Flow Magnitude Forecast (HFM)), suggesting significant stream rises are once again expected. In regards to river flooding, the ongoing moderate and major river flooding will likely be prolonged and worsened, and new minor-to-moderate and isolated major river flooding is expected in areas of prolonged heavy rainfall (HEFS, PQPF). Late day 2 (Thu) through day 3 (Fri), the heavy rainfall (2 - 5"+) progresses east across central LA and southern MS/AL, which will likely bring new and renewed flooding impacts to the already wet region. Soils and streamflows will not recover from recent rainfall events over the past several weeks, and this additional rainfall will likely eliminate any remaining soil capacity, resulting in immediate runoff into already swollen streams and urban areas, especially across portions of northern LA, southwestern MS, and coastal MS/AL where 4 - 6"+ of rain has occurred over the past 72 hours (MRMS QPE). Additionally, the heaviest rainfall is expected to occur along and south of the I-20 corridor. Both the GFS and NBM-forced NWM continue to indicate widespread small stream responses beginning late on day 2 (Thu) and continuing through day 5 (Sun), with peak flows primarily on day 4 (Sat) across much of LA, MS, and central/south AL. There are isolated AEPs 20% (NWM HFM) across the region that suggests some of the responses may be significant in areas of prolonged heavy rainfall. The overall placement of the rainfall has been relatively consistent, giving increased confidence in the location and the magnitude of impacts, as suggested by the increase in rapid onset flooding probabilities above 25% across central LA and southern MS for day 3 (Fri). In regards to river flooding, ongoing minor flooding will likely worsen/be prolonged and additional areas of minor-to-moderate river flooding are possible (MMEFS, HEFS), especially given the very moist deep layer soils (40 - 100 cm relative soil moisture, NASA SPoRT) already in place. .Hawaii... A Kona Low will bring periods of prolonged heavy rainfall to the region, with flash and urban impacts likely, some of which may be significant. Antecedent conditions are wet following recent rain (3 - 6"+, 72-HR MRMS QPE), and it is likely that additional heavy precipitation will quickly overwhelm current capacity. Given these wet conditions, landslides are also possible in areas of steep terrain. .Southern Plains through the Ohio Valley... Periods of rainfall are expected days 2 - 3 (Thu - Fri), with eastern OK through southern MO and into western KY having the best chance for flooding impacts, given that this is expected to be the region with the best overlap of heavier QPF (1 - 3"), wet top layer soils (NASA SPoRT), and elevated streamflows (USGS). Ensemble guidance (HEFS, MMEFS) suggests potential for isolated river flooding impacts beginning on late day 2 into day 3 (Thu - Fri). This is in addition to the potential for isolated flash flooding as well. Scattered small stream flooding impacts are likely, as suggested by the consistent NWM signals of isolated flows on small headwaters with AEPs 50% for the day 2 - 3 (Thu - Fri) period. //JEC $$