####018004768#### AGUS74 KWCO 051528 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 915 AM CST TUE MAR 5 2024 .Synopsis... Flash, urban, and river flooding possible this week across the Central Gulf Coast and the Southeast and Mid-Atlantic and Northeast... .Discussion... .Central Gulf Coast and the Southeast... Rainfall is forecast for parts of AL, GA, and the FL Panhandle through today. Totals of 1 - 1.5" are expected (WPC), with a bullseye of up to 2" QPF over the Atlanta metropolitan area. Localized flash, urban, and small stream flooding impacts are possible, as well as rises to area rivers. The system is then expected to expand into the Appalachian foothills and across the Carolinas/southern VA on day 2 (Wed), bringing light QPF (1", WPC). Flooding impacts are not expected on day 2, but the rainfall will serve to prime soils for additional rainfall later in the week. NASA SPoRT has current soil conditions across the region generally between 40 - 60% RSM, 0 - 10 cm, and streams are running at above normal to much above climatological normals (USGS), which indicate some infiltration capacity but diminished in-channel storage for runoff. The HRRR-forced SRF National Water Model (NWM) has scattered, low-end, rapid onset flooding signals of 25% across central MS and into AL, and the high-flow magnitude service is indicating most affected streams only reaching the highwater threshold, but has some isolated stream reaches in the 50 - 10% annual exceedance probability (AEP) range. These signals add confidence that this early rainfall will be more of a primer event, with widespread within-bank rises and only localized flash, urban, and small stream flooding impacts possible. A second system will sweep across the Southeast beginning day 4 - 5 (Fri - Sat), bringing heavy rainfall and additional flooding impacts to the area. Widespread QPF totals of 1 - 3+" are expected, with the highest totals expected across central AL. This system will bring the expected 5-day QPF total for the Southeast to 2 - 5"+. Flash, urban, and small stream flooding impacts are expected, as well as the potential for riverine flooding. MMEFS is signaling widespread 30% probability or greater for minor riverine flooding across the region, with some isolated moderate flooding possible, mostly across central GA. Most rivers are expected to peak over the weekend. The NWM NBM and GFS-forced MRFs are generally in good agreement as far as timing and placement of flooding impacts, but the GFS may be over-signaling with respect to magnitude. The more-representative NBM-forced NWM high-flow magnitude forecast is showing widespread AEPs in the 50 - 20% range, with isolated pockets of 10 - 2% AEPs across central AL/GA. These more robust signals support the occurrence of impactful flash, urban, and small stream flooding. .Mid-Atlantic and Northeast... Isolated flash, urban, small stream, and minor to isolated moderate river flooding impacts are possible late on day 2 (Wed) into day 3 (Thu) and again late on day 5 (Sat) into day 6 (Sun). Forecast rainfall totals over the next 7 days are heaviest (3 - 5") across NJ, southeast NY and southern and coastal areas of New England. Soils are wet across the region (65 - 90% RSM, 0 - 10 cm NASA SPoRT) and streamflows are generally above climatological normals for this time of year (USGS) in the wake of recent rainfall, suggesting limited infiltration and channel capacity. Additionally, there is potential for modest snowmelt (1" SNODAS) across the higher elevations of VT/NH and into northern portions of ME through day 2 (Wed) which could play a part in expected river rises however, much of the heavier rainfall is expected closer to the New England coast. Recent runs of both the GFS- and NBM-forced NWM have backed off on overall signal magnitude with a few exceptions. The more robust responses have been relegated to RI and eastern MA with highwater arrival times late on day 2 (Wed) into day 3 (Thu). The magnitude of the NWM responses have backed off on recent runs most notably for the weekend rainfall however, there remains some uncertainty with this second round. Additionally, river ensemble forecasts are still suggesting the potential for minor to isolated moderate river flooding. Given the already saturated conditions, potential priming rainfall earlier in the week, and extensive urban area, the region will have to be monitored as we get closer to the weekend. //Bliss/Smith $$