####018004049#### AGUS74 KWCO 141502 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT TUE MAY 14 2024 .Synopsis... Considerable flooding impacts expected to continue in East Texas...Flooding impacts once again possible across the Lower Mississippi Valley and Southeast...Flooding likely across Hawaii... .Discussion... .East Texas... Considerable flash, urban, and river flooding is expected once again across the region on day 3 (Thu) as additional rainfall (2 - 4"+) impacts this already sensitive region. At this time, the Neches and Lower Trinity river basins are most at risk, as the heaviest rainfall is expected to fall on these already swollen streams and rivers. Despite the brief reprieve in rainfall, there is likely no available infiltration capacity, given thoroughly saturated soils and widespread moderate to major river flooding that is ongoing, and this new 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. This is unrepresentative of the overall flood threat given the higher ARIs (3.2 ARI) across the region, which is muting the response. Both medium-range forcings of the NWM continue to depict isolated annual-exceedance probabilities (AEPs) below 20% (High Flow Magnitude Forecast), suggesting at least locally significant stream rises are once again possible. In regards to river flooding, the ongoing moderate and major river flooding will likely be aggravated, and new river flooding may be possible in areas of prolonged heavy rainfall (HEFS, PQPF). .Lower Mississippi Valley and Southeast... After a brief reprieve through day 2 (Wed), another round of heavy showers and thunderstorms will likely bring new and renewed flooding impacts across northern LA, central/southern MS, and central/southern AL on days 3 - 4 (Thu - Fri). It is likely that soils and streamflows will not recover from recent rainfall events over the past several weeks, and this additional rainfall (2 - 4"+) will likely eliminate any remaining soil capacity, resulting in immediate runoff into already swollen streams and urban areas, especially across portions of northern LA and southwestern MS, 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. The forecast intense rainfall rates will likely quickly overwhelm urban drainage causing areas of standing water on roadways, overflowing of ditches and culverts, and pluvial ponding in low-lying areas. Both the GFS and NBM-forced NWM continue to indicate scattered small stream responses on days 3 - 4 (Wed - Thu) with peak flows primarily on day 4 (Fri) across much of LA, MS, and central/south AL. There are isolated AEPs (per the High Flow Magnitude Forecast) across the region that suggest 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. In regards to river flooding, ongoing minor to isolated moderate flooding will likely worsen/be prolonged and additional areas of river flooding are possible (MMEFS, HEFS). .Hawaii... A Kona Low will bring periods of prolonged heavy rainfall to the regions, with flash and urban impacts likely, some of which may be significant. Ahead of this rainfall, antecedent conditions are wet following recent rainfall (3 - 6"+, 72-HR MRMS QPE), and it is likely that additional heavy rainfall will quickly overwhelm current capacity. Given these wet conditions, landslides are also possible in areas of steep terrain. //Pritchard $$