####018003363#### AGUS74 KWCO 311512 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SUN MAR 31 2024 .Synopsis... Urban and small stream flooding impacts possible in Southern California... Flooding potential next week across the Mid-Mississippi and Ohio Valleys into the Mid-Atlantic...Ongoing highwater and flooding from Northeast Texas through the Southeast... .Discussion... .Southern California... Moderate to heavy rainfall is ongoing and will continue through this afternoon along with the potential for localized flooding impacts primarily in urban areas and along small streams draining from steep terrain. Rainfall over the past 48 hours (2 - 5" QPE, MRMS, gage) has wetted top-layer soils, and combined with non-impervious urban areas and the steep/complex terrain, rapid runoff is possible, especially with the potential rates of 1 - 1.5"/hr (WPC). The National Water Model (NWM) Short Range Forecast (SRF) continues to signal potential for rapid-onset flooding (ROF) across the region with many ROF probabilities greater than 50% south of Los Angeles. The SRF Maximum High Flow Magnitude Annual Exceedance Probabilities (AEPs) suggest potential for out-of-bank flows, with AEPs ranging from 10 - 50% around San Diego. Given these signals from the NWM and the antecedent conditions, there is potential for lingering urban and small stream responses. .Mid-Mississippi and Ohio Valleys into the Mid-Atlantic... Light rainfall will continue prior to the arrival of moderate to locally heavy rainfall tomorrow, bringing potential for riverine flooding and isolated flash flooding. Current soil moisture indicates availability for initial infiltration (50 - 70% RSM, 0 - 10 cm NASA SPoRT), and streamflows are generally below normal to much below normal for this time of year (USGS), with the exception of small portions of MO, OH, and IL, where streamflows are elevated from recent rainfall. The flooding threat will likely be mostly flash and urban, but river flooding is also expected. While guidance from both the NBM and GFS-forced NWM indicate scattered small stream responses across these regions, the NWM does not handle the regions reservoirs well. The exact location and magnitude of impacts still remain uncertain given the variability in model guidance. At this time, the GFS and NBM-forced National Water Model (NWM) Medium Range Forecast (MRF) is signaling widespread ROF probabilities, NBM and GFS QPF have come into somewhat better alignment with the WPC QPF, which has modestly increased the confidence in the placement of the NWM signals. MMEFS is signaling potential for widespread minor to isolated moderate river flooding. Isolated flash flooding also remains a concern especially with potential heavy rainfall and training. .Northeast Texas through the Southeast... Minor river flooding across the region is expected to persist into early next week due to runoff routing downstream. Most locations are anticipated to crest this weekend before gradually receding. Light rainfall returns to the region day 2 and 3 (Mon and Tue), but specific hydrological impacts are limited. //Ayala $$