####018005385#### AGUS74 KWCO 231522 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SAT MAR 23 2024 .Synopsis... Isolated flooding impacts this weekend and coastal flooding impacts early next week for the Coastal Mid-Atlantic and Northeast... Isolated river and small stream flooding impacts possible in East Texas into the Lower Mississippi Valley... Ongoing river flooding with a return of rainfall early and again late next week in the Southeast... Urban and small stream flooding impacts possible for Puerto Rico and USVI... .Discussion... .Coastal Mid-Atlantic and Northeast... Isolated flash, urban, small stream and isolated minor/moderate river flooding impacts are possible this weekend due to persistent rainfall (1- 3", WPC Day 1). Rainfall is currently impacting the Coastal Mid-Atlantic region and will continue to expand northward into the northeast through the day bringing the threat of isolated flash, urban, and small stream flooding impacts through at least early Sunday morning. Light rainfall this morning has been wetting soils however, soils have been on the drier side and may still take some rainfall at least initially. Streamflows are generally near their climatological norms suggesting that they should be able to handle most of the excess runoff. These factors suggest that flooding impacts will likely be more isolated in nature and limited to urban areas and typical flood prone areas. The National Water Model (NWM) Short Range Forecast (SRF) has consistently shown the potential for rapid-onset flooding (ROF) across the region with the highest ROF probabilities across the DelMarVa area (50 - 100%) and lower probabilities across southeast PA/NY, NJ, and southern New England (25 - 75%). The SRF Maximum High Flow Magnitude signals are fairly bullish with scattered lower Annual Exceedance Probabilities (AEPs) across those same areas suggesting potential for out-of-bank flows where rainfall remains persistent for several hours. As rainfall routes into the larger streams and rivers later today, those same streams and rivers will start to rise later this morning and afternoon with most rivers cresting on day 2 (Sun) before receding. RFC forecasts show scattered elevated rivers and isolated minor river flooding forecast through the weekend. River ensemble forecasts suggest that isolated moderate flooding will also be possible. As the region starts to dry out early next week, minor to moderate coastal flooding impacts begin due to a combination of high tides and onshore flow. The Coastal Mid-Atlantic will likely see the greatest impacts as suggested by the more widespread moderate coastal flooding forecast points. .East Texas into the Lower Mississippi Valley... Minor river flooding is ongoing due to multiple rounds of rainfall over the past 7 days and forecast to continue into next week in portions of eastern Texas and the western Lower Mississippi Valley. The region will see a couple of days of dry weather allowing for area streams and rivers to recede unimpeded. By day 3 (Mon), rainfall will return to the region again regenerating rivers, causing new and renewed rises along with stalled recessions. Both the GFS- and NBM-forced NWM Medium Range Forecast (MRF) show good agreement with modest stream responses generally across the southeast AR and northern MS however, the MRF High Flow Magnitude Forecast suggests only modest rises for now. River ensemble forecasts also show the potential for new and renewed river rises although widespread impacts are not expected. .Southeast... Rainfall returns to the Southeast early on day 4 (Tue) bringing the potential for isolated flash, urban, and small stream flooding impacts along with river rises across AL. Currently, both the GFS- and NBM-forced NWM MRF show the majority of the stream responses further west however, isolated signals are present across western AL. Another round of heavy rainfall develops mid next week across the Southeast Coastal Plain. Despite the extended range, global models do show some early agreement on placement of the rainfall. Both the GFS- and NBM-forced NWM MRF show stream responses from the Big Bend of FL and across the Southeast Coastal Plain into the Carolinas. The GFS-forced NWM however, has been considerably more bullish on some previous runs suggesting remaining uncertainty, particularly in magnitude. Antecedent conditions are generally above normal following recent rainfall. Rivers are running high with minor and moderate (Savannah River) river flooding ongoing and forecast across the Altamaha, Savannah, and Santee Rivers indicating that the region will be sensitive to additional heavy rainfall. Given these factors, the region will need to be closely monitored in the coming days. .Puerto Rico and USVI... Urban and small stream flooding impacts will be possible on Monday for Puerto Rico due to a passing cold front. As that front continues east, the U.S. Virgin Islands will see the same elevated concerns for flooding later in the evening. //Smith $$