####018004244#### AGUS74 KWCO 031517 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 915 AM CST SUN MAR 3 2024 .Synopsis... Flood potential this week across the Gulf Coast States into the Appalachian Foothills...Localized flooding possible mid-week across the Mid-Atlantic and Northeast... .Discussion... .Gulf Coast States into the Appalachian Foothills... Localized flash, urban, and small stream flooding impacts are possible beginning on day 2 (Mon) across portions of LA/southern MS and will expand into much of AL, GA, the FL Panhandle, and the Appalachian Foothills on day 3 (Tue) as several rounds of showers and thunderstorms move through the region. Current antecedent conditions indicate some degree of susceptibility to flooding impacts as soil moisture (60 - 75% RSM, NASA SPoRT) and streamflows (USGS) are elevated due to recent rainfall (2 - 5"+, 72 HR MRMS QPE), suggesting an already limited infiltration capacity for rainfall. The repeated nature of this rainfall on saturated soils will be the driving force for impacts as this will likely overwhelm remaining infiltration capacity, leading to immediate runoff into nearby streams and urban areas. The latest guidance from both the GFS-forced and NBM-forced National Water Model (NWM) are in decent agreement the placement of these small stream responses, as they are indicating scattered small stream responses across southeast LA/southern MS beginning on day 2 (Mon) and across much of AL, GA, and the Appalachian Foothills on days 3 - 4 (Tue - Wed). Corresponding annual exceedance probabilities (AEPs), between both forcings, differ in magnitude. The NBM is currently the more modest solution, suggesting only isolated significant stream rises (AEPs less than 10%) across AL and GA while the GFS indicates widespread AEPs as low as 2% across the same region. Nevertheless, given the current antecedent conditions, as well as the long-duration rainfall, confidence is increasing in the likelihood of small stream flooding impacts. Localized areas of river flooding are also possible, as river ensemble guidance suggests some rises to minor flood stage across portions of LA and MS. The wet pattern will continue over the region as potentially heavy rainfall returns later this week (days 5 - 7). Both the GFS and NBM-forced NWM continue to indicate scattered to widespread small stream responses across this same region, primarily on days 6 and 7 (Fri - Sat). As soils will likely remain saturated from the rain expected early this week, this round of rainfall may elicit new and renewed (in some areas) river rises/flooding, as river ensemble guidance from both MMEFS and HEFS suggests minor to isolated moderate flooding later this week. .Mid-Atlantic and Northeast... A storm system tracking along the East Coast will provide a focus for moderate to locally heavy rainfall from northern VA to southern New England (MA/CT/RI) on day 4 (Wed), potentially generating urban and small stream flooding impacts. 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 capacity for runoff. Both the GFS and NBM-forced NWM are indicating scattered small stream responses across the region on days 4 - 5 (Wed - Thu). The exact location of the impacts remain uncertain, as the GFS remains the heaviest and furthest west solution, while the NBM remains closer to the coast. Regardless, given saturated antecedent conditions and the large urban footprint of the region, this area is vulnerable to urban and small stream flooding impacts. River flooding may be a concern as well, as MMEFS and HEFS guidance suggests rises to minor flood across the region. Further north into ME, prolonged above freezing temperatures will increase the risk of the remaining river ice to break up, potentially leading to ice jams. //Pritchard $$