####018005216#### AGUS74 KWCO 251531 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1031 AM CDT THU APR 25 2024 .Synopsis... Flooding impacts excepted late this week across the Central and Southern Plains into the Midwest and across East Texas and the Lower MIssissippi River Valley... Daily rainfall continues in Puerto Rico... Snowmelt continues in the Great Basin and Intermountain West... .Discussion... .Central and Southern Plains into the Midwest... Confidence has increased for flash flooding impacts across much of central and eastern OK on day 3 (Sat), however the magnitude and areal coverage of river responses across OK and the surrounding region are still uncertain at this time given the variable antecedent conditions, but areas of minor river flooding can be expected. The three areas of interest at this time are east-central OK (primarily flash flooding), northeast OK/southeast KS/southwest MO (flash and river flooding), and northeast MO/southeast IA/northwest IL (primarily river flooding, isolated flash flooding). Relatively dry conditions are in place across eastern OK/KS and western MO/IA, with somewhat wet conditions in place across northeast TX, AR, eastern MO, and IL where soil moisture and streamflows remain elevated (NASA SPoRT, USGS) from recent rainfall. Periods of rainfall are expected across most of the region through day 4 (Sun), and rainfall totals and location on days 1 - 2 (Thu - Fri) will likely play an important role in setting the antecedent conditions leading into the heaviest rainfall on day 3 (Sat), so the greatest threat region could change between now and day 3 (Sat). Both the GFS and NBM-forced National Water Model (NWM) Medium Range Forecast (MRF) continue to indicate widespread small stream responses across these regions, mostly beginning on days 3 - 4 (Sat - Sun), with peak flow arrival times on days 4 - 7 (Sun - Tue). The most consistent signals from the NWM MRF have been across southeast KS through northern MO and into southeast IA/northwest IL, where probabilities for rapid onset flooding (primarily on day 3 (Sat) into day 4 (Sun)) are between 25 - 50%. Corresponding flow annual exceedance probabilities (AEPs) (NWM MRF) are mostly near 50% with isolated higher magnitude flows on smaller tributaries in the lower Middle Arkansas and upper Neosho Basins (southeast KS), and ensemble guidance suggests potential for isolated minor-to-moderate river flooding across southeast KS into northeast OK (PQPF Ensembles, HEFS). Across northeast MO/southeast IA/northwest IL there is a more cohesive signal for flows with 20 - 2 % AEPs. In this specific region, the GFS QPF aligns more with WPCs QPF guidance (higher local maxima) than the NBM, and the GFS forced NWM MRF is the version of the NWM that is showing potential for those more robust signals. HEFS (30% exceedance) is suggesting potential for isolated minor-to-moderate river flooding on tributary rivers across this region as well, with rises beginning late this weekend. .East Texas and the Lower MIssissippi River Valley... Rain will progress across the region on day 2 (Fri), however any flooding impacts will be isolated and most likely across northeast TX into AR. The primary concern for flooding impacts will be on days 4 - 5 (Sun - Mon) across the ArkLaTex through the ArkLaMiss where some places could see 2 - 4" of rain (WPC). Soils are wet (NASA SPoRT) and streamflows are above normal (USGS) as basins continue to recover from previous rainfall events. Rivers are still in recession across this region as well, so new and renewed river flooding will be a concern. Flash flooding, widespread small stream responses (NWM MRF), and isolated minor-to-moderate river flooding is possible (HEFS/PQPF Ensembles). Of note, the NWM MRF continues to signal potential for more significant flows on smaller tributaries in the upper Ouachita basin south of Little Rock, where conditions are hydrologically vulnerable, especially in the areas draining the terrain of the Ozarks. .Puerto Rico... Daily showers and thunderstorms will continue to generate urban and small stream flooding impacts through day 3 (Sat). As this rainfall continues on saturated soils and elevated streamflows, rapid river rises and landslides in areas of steep terrain are also possible. .Great Basin and Intermountain West... Mid-to-high elevation snowmelt (SNODAS) will continue to generate elevated flows on area rivers, streams, and creeks over the next several days. Rainfall late this week into this weekend is not expected to cause widespread impacts, although sharp stream rises are possible in areas of heavier rainfall. The Portneuf River at Pocatello, ID is expected to crest in moderate flood stage late this weekend and minor flooding along the Owyhee River (NV) will persist through next week. Cooler weather this weekend should slow the rate of snowmelt. //JEC $$