AGUS74 KWCO 111528 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SAT MAY 11 2024 .Synopsis... Major river flooding impacts ongoing in East Texas... Potential for flooding impacts across the Lower Mississippi Valley and Southeast, Puerto Rico, Hawaii, and American Samoa... Ice jam-induced flooding possible in Alaska... Snowmelt in Central Montana... Ongoing river flooding in the Middle Mississippi Valley... Coastal flooding possible on the Mid-Atlantic coast... .Discussion... .East Texas... Prolonged major river flooding continues along the Lower Trinity and Lower Neches, as well as isolated moderate river flooding ongoing elsewhere in the region. Rainfall (1 - 3") returns on day 2 (Sun) bringing the threat of flash and urban flooding. The new rainfall will likely exacerbate ongoing river flooding, producing new and renewed rises. Despite the rapidly expanding green vegetation, the region still remains vulnerable to flooding impacts. River flooding continues and USGS streamgages are reporting widespread above normal flows both historically and for this time of year. Soil moisture remains on the wet side as well, suggesting limited infiltration capacity. River ensemble forecasts suggest many of the area's rivers could see new river rises and prolonged river flooding. The National Water Model (NWM) Medium Range Forecast (MRF) is showing very little response, some of which may be in part to the higher thresholds for highwater relative to other parts of the US. Despite the minimal overall responses from the river ensembles and NWM, localized flash and urban impacts cannot be ruled out if higher rainfall rates were to materialize or if training of showers and thunderstorms were to occur. The region remains hydrologically active and vulnerable as rainfall returns again mid to late next week. The placement and magnitude of the expected QPF is much less certain. The NBM- and GFS-forced NWM responses show some overall disagreement with the GFS-forced NWM showing more of an aggressive response later next week. The NBM-forced NWM responses are far more subdued. While there is some uncertainty with this next event, this area should be closely monitored in the coming days. .Lower Mississippi Valley and Southeast... Rounds of rainfall will again impact the region on days 2 - 4 (Sun - Tue), bringing the potential for new and renewed flooding impacts. The heaviest rainfall totals are currently forecast on days 3 - 4 (Mon - Tue) along the central and eastern Gulf Coast, and across the FL Panhandle. Both the NBM- and GFS-forced NWM are responding accordingly with widespread responses and the potential for isolated rapid-onset flooding across southern MS/AL and the FL Panhandle. Both models show good agreement with the placement of the rainfall however, the GFS is much more aggressive with the overall magnitude of the responses. River ensemble forecasts also favor the potential for new and renewed river rises including minor river flooding. .Puerto Rico Urban and small stream flooding impacts, along with the chance for isolated flash flooding, will remain possible through today. Wet soil conditions and locally elevated streamflows will increase the potential for hydrologic impacts, including mudslides in areas of steep terrain. .Hawaii... Flash and urban flooding impacts are possible through today as locally heavy rainfall falls on the islands. Landslides will also be possible in areas of steep terrain. .American Samoa... Isolated flash and urban flooding impacts will be possible through Sunday, with landslides also possible in areas of steep terrain. .Alaska... Various small ice jams in the Kuskokwim Delta between Kwethluk and Napakiak are causing high water In Tuluksak. Looking north to the Yukon River, an ice jam front is also expected to impact the community of Circle. Ice jam breakups and formations are expected to continue along this breakup front as it makes its way down the Yukon River. Ice jams and breakup fronts can evolve rapidly, so refer to the local forecast offices for up-to-date information. .Central Montana... Warming temperatures will bring snowmelt to the mountain ranges of central Montana and may lead to small stream flooding and rises on area rivers through Monday. .Middle Mississippi Valley... Minor to isolated moderate river flooding will continue across the region with rises expected on the Iowa River (IA), Tennessee River (TN), and much of the Middle Mississippi River from Keithsburg, IL to St. Louis, MO. Rainfall (1 - 2") returns to central Missouri on day 3 (Mon) which could bring some renewed river rises and isolated flash and urban flooding impacts to the State. The NWM is currently signaling isolated responses with both the NBM- and GFS-forced NWM varying slightly. The GFS-forced version has more focused but higher magnitude responses and the NBM-forced version more widespread with lower magnitude responses. These factors suggest the isolated flash and urban flooding will be possible along with some renewed river rises. .Mid-Atlantic... Increasing onshore flow will bring coastal flooding impacts to the Mid-Atlantic coast through early on day 2 (Sun). Please refer to the local offices for additional information. //Smith $$