####018005930#### AGUS74 KWCO 061512 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 915 AM CST WED MAR 6 2024 .Synopsis... Flash, urban,small stream, and river flooding possible this week across the Central Gulf Coast and the Southeast and Mid-Atlantic and Northeast...Urban flooding impacts in Southern California... .Discussion... .Central Gulf Coast and the Southeast... The system that brought heavy rainfall to AL and GA yesterday and this morning has moved into the SC and will continue to work its way northward inland from the coast. Rainfall totals will be modest, and coupled with normal to slightly below normal soil moistures (35 - 50% RSM, 0 - 10 cm, NASA SPoRT), flooding impacts are not likely to materialize beyond some nuisance ponding. Minor river flooding is ongoing and forecast across northern GA with most rivers cresting today and tomorrow. A second system will sweep across the Southeast beginning day 3 - 4 (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. Yesterdays rainfall dropped widespread totals of 1 - 2", and notably the Atlanta metropolitan area received a bullseye of up to 4". This rainfall has primed the region for efficient runoff processes with soils in the 55 - 85% RSM (0 - 10 cm, NASA SPoRT) and streams running above normal to much above normal compared to climatological normals (USGS) however, current NASA SPoRT and USGS products do not reflect the increase in the overall soil saturation and streamflows. With this second round of rainfall, flash, urban, and small stream flooding impacts are expected, and there is potential for additional 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/GFS-forced Medium Range Forecasts (MRFs) are generally in good agreement as far as timing and placement of flooding impacts, but the GFS continues to show stronger signals with respect to magnitude that may be overdone. Signals from the NBM-forced NWM Maximum High Flow Magnitude forecast have trended downward, with widespread Annual Exceedance Probabilities (AEPs) in the 50 - 20% range, but the isolated pockets of lower AEPs are no longer found. Nevertheless these signals still support the occurrence of impactful flash, urban, and small stream flooding. .Mid-Atlantic and Northeast... Flash, urban, small stream, and minor to isolated moderate river flooding impacts remain on the table for the region through early day 2 (Thu) as moderate to heavy rainfall moves northward across eastern portions of the Mid-Atlantic and into the Northeast. Rain is already falling across eastern NC/VA and is expected to progress northward throughout the day. As stated in previous discussions, the antecedent conditions are primed for runoff. Near surface (0 - 10 cm) relative soil moisture is greater than 65% (NASA SPoRT) suggesting that the soils can not take very much rainfall if any, leading to rapid runoff. Streamflows are also elevated with many USGS sites reporting flows greater than the 90th percentile for historic observations. Rainfall is ongoing across the Mid-Atlantic coast and the HRRR-forced NWM Short Range Forecast (SRF) is starting to capture most of the rainfall with rapid-onset flooding (ROF) signals expanding across the region from northeast NC and up the Eastern Seaboard to southern New England. ROF probabilities are increasing from NC up to New York City ranging from 25 - 75%. Probabilities will likely continue to increase across southern New England with successive runs of the NWM SRF. Highwater arrival times range from within the next several hours in portions of eastern NC/VA to late this evening and overnight across southwest NY and southern New England. Additionally, the Maximum High Flow Magnitude forecast is suggesting scattered areas of significant stream rises. Overall, models generally show good agreement with the placement and magnitude of heavy rainfall across the region, suggesting confidence in the NWM responses. Additionally, river ensemble forecasts are still suggesting the potential for minor to isolated moderate river flooding. Lastly, there is potential for significant snowmelt (1 - 2" SNODAS) across a ripened snowpack in the higher elevations of VT/NH and into northern portions of ME through day 2 (Thu) which could exacerbate flooding across the region. Given the saturated conditions and frozen soils, much of the melt will be routed quickly into streams and rivers. Despite the expected snowmelt, much of the heavier rainfall will remain closer to coastal New England. Based on these factors, the potential for flooding is on the table for much of New England. Rainfall is expected to return late on day 3 (Fri) into day 4 (Sat) however, the magnitude of the NWM MRF responses have continued to back off on recent runs for the weekend rainfall. Nevertheless, given the already saturated conditions and extensive urban area, the region will have to be monitored as we get closer to the weekend. .Southern California... Rainfall from a fast-moving system today may produce urban flooding impacts. With soils still being wet and streamflows running high from recent rainfall, this area is susceptible to additional rainfall. However, any flooding impacts are expected to be localized. //Bliss/Smith/Stackhouse $$