####018006573#### AGUS74 KWCO 111502 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1001 AM CDT THU APR 11 2024 .Synopsis... River flooding and lingering flood impacts continue across East Texas and the Lower Mississippi Valley and the Southeast... Rain and snowmelt-induced river and small stream responses in the Northeast... Localized flooding impacts and renewed river rises possible across the Ohio Valley and Southern Appalachians into the Mid-Atlantic... Flash flooding possible in Hawaii... .Discussion... .East Texas and Lower Mississippi Valley... Widespread minor to moderate river flooding, and isolated major river flooding in east TX, is ongoing and forecast throughout the region in response to recent heavy rainfall. Most locations are still rising, with crests expected through the weekend into early next week as the elevated flows route downstream. With no significant rainfall expected over the next week, recessions will continue unimpeded. Ongoing high water flows along smaller creeks and streams, and any associated impacts, will also continue over the next few days as flows route unimpeded into the mainstem rivers. .Southeast ... Significant heavy rainfall (4 - 6", locally higher) and flash flooding occurred overnight in and surrounding Tallahassee, FL, north into portions of GA, leading to swollen rivers and streams and numerous flooding impacts. Although rainfall will have ended for the affected regions, associated impacts may linger beyond today. Rainfall will continue through the afternoon across eastern portions of GA/FL into the Carolina Coastal Plain potentially causing localized flooding impacts should higher amounts occur over urban and typical flood-prone locations. However, dry antecedent conditions and the prevalence of sandy soils, coupled with the overall progressive nature of the storm and its expected low QPF totals should help to mitigate widespread flooding impacts. Minor to moderate river flooding is ongoing and forecast across western AL and the FL Panhandle into central/southern GA following the recent heavy rain event. Most locations are still rising, with crests expected through the weekend into early next week as the elevated flows route downstream. With no significant rainfall expected over the next week, recessions will continue unimpeded. .Northeast... There is potential for urban and small stream flooding, as well as new and renewed river flooding, from eastern NY into ME through day 2 (Fri) as a storm system brings periods of moderate rainfall to a ripened snowpack. The combination of gusty winds (30 - 40 kts at high elevations) and high dew points (mid 50s) on an isothermal snowpack is expected to bring 3 - 4" of melt in 24 hours. Early rainfall today will likely compress the snowpack and with the arrival of a warmer rain on day 2 (Fri), there is potential for rapid melt. Full melt-out below 2,000 ft is likely, particularly on S/SE facing slopes of the Green (VT) and White (NH) Mountains. This melt will route downstream, causing potential flooding impacts on low elevation rivers. Melt combined with saturated antecedent conditions sets up ideal conditions for rapid rises and potential moderate river flooding in the Upper Merrimack (NH), Saco (ME), Androscoggin (ME), Kennebec (ME), and Penobscot (ME) river basins. Outside of the snow covered areas, locally heavy rainfall may elicit poor drainage and nuisance flooding in urban areas, especially if heavier rainfall totals materialize. Both the GFS and NBM-forced National Water Model (NWM) Medium Range Forecast guidance continue to indicate small stream responses across the region. Corresponding annual exceedance probabilities (AEPs) below 50% are noted on small streams draining the steep terrain of the White (NH) and Adirondack (NY) mountains, indicating confidence in the likelihood of small stream rises. RFC ensemble forecasts continue to suggest that renewed rises are possible, however, at this time rises are generally expected up to minor flood stage. .Ohio Valley... Localized urban and small stream flooding impacts, as well as new and renewed river rises, are possible through this weekend due to widespread rainfall (1 - 2", locally up to 3"). MMEFS guidance from the NAEFS and GEFS is signaling minor to moderate river flooding across IN and OH with rivers forecast to peak days 2 - 3 (Fri - Sat). Soils are primed from previous rainfall, and combined with elevated streamflows and ongoing river flooding, will limit infiltration and storage capacity leading to increased susceptibility to rapid runoff. However, rainfall should be relatively progressive which will help to diminish the overall flood threat, but flooding impacts in urban, low-lying, and poorly drained areas, and in areas with complex terrain, remain possible where the heaviest rainfall amounts occur. .Southern Appalachians into the Mid-Atlantic... Localized urban and small stream flooding impacts are possible today along the foothills of the Applachians into central PA from periods of moderate to locally heavy rainfall (1 - 2") to the region. Soil moisture and streamflows are elevated from rainfall last week, making the region prone to flooding impacts from any additional heavy rainfall, especially given the complex terrain. The NWM Short Range Forecast is signaling scattered rapid-onset flooding probabilities less than 25% and AEPs are generally at or above 50%, suggesting that most rises will remain in-bank, although some out-of-bank responses are possible on small streams draining steep terrain, as well as creeks draining into urban areas. Any impacts will be highly dependent on where the heaviest rainfall occurs. Moderate to major coastal flooding impacts remain possible today along the Chesapeake Bay to Long Island Sound. Check with your local NWS office for more information related to coastal flooding impacts. .Hawaii... Flash and urban flooding impacts are possible across Kauai and Niihau through day 2 (Fri) as slow-moving rainfall sweeps across the islands. Flood prone roads and other low lying areas could be closed due to elevated runoff and overflowing streams. //Freeman $$