####018008307#### AGUS74 KWCO 091518 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT TUE APR 9 2024 .Synopsis... Considerable flooding impacts expected across portions of Northeast Texas, Lower Mississippi River Valley and the Southeast... Rain and snow melt potentially leading to river rises late this week in the Northeast... Localized flooding and potential renewed rises across the Ohio Valley and Central Appalachians into the Mid-Atlantic...Heavy rainfall and potential flooding in Puerto Rico and USVI... .Discussion... .Northeast Texas and Lower Mississippi River Valley... Considerable flooding impacts are likely from northeast TX through the MS Delta through day 3 (Thu) as multiple days of heavy rainfall (3 - 7+") are expected. Heavy rainfall over the past 24 hours (2 - 4", isolated totals up to 6") has impacted the region, already saturating soils and greatly diminishing soil infiltration capacity (70 - 90% RSM, 0 - 10 cm). The initial mitigation of runoff by vegetation has likely been reached in many areas and streams are elevated, leaving little room for the additional heavy rainfall that is ongoing and forecast. Guidance from the NBM and GFS forced NWM indicate widespread small stream responses in southern AR, northern LA, and northern MS, with corresponding annual exceedance probabilities (AEPs) as low as 2%. Widespread minor river flooding is now forecast across these regions, with moderate flooding currently forecast along the Sabine (TX), Big Black (MS) and Pearl (MS) rivers. Major river flooding is possible in areas of heavier rainfall. Shower and thunderstorms with efficient rates and training are expected through the morning with an additional 2 - 4+" forecast. Depending on where exactly this rainfall trains, there is potential for considerable flooding impacts, especially along the I-20 corridor between Dallas and Shreveport, LA, and across southern AR, where the heaviest rainfall fell overnight. Most likely impacts will be overwhelming of ditches and culverts, large areas of standing water along roadways, and poor drainage flooding in urban and low-lying areas. Specifics in terms of placement and magnitude still remain uncertain and will depend on convective and training aspects. .Southeast ... The potential for considerable flooding impacts associated with multiple rounds of heavy rainfall and flash flooding is possible day 2 into day 3 (Wed - Thu). Top soils are wet (60 - 80% RSM, 0 - 10 cm), suggesting elimination of infiltration capacity and rapid conversion of rainfall to runoff. Multiple axes of heavy rainfall are possible and high rainfall rates are expected where showers and thunderstorms develop. The GFS-forced NWM places the more robust stream responses over east MS and portions of west and central AL, while the NBM-forced NWM places the more robust stream responses over northern LA and southern AR. Nevertheless, considerable flooding impacts are possible in areas of heavier rainfall, especially urban centers along the I-20 corridor. River ensemble guidance (MMEFS/HEFS) as well as PQPF guidance suggests areas of minor to moderate river flooding across portions of the Pearl (MS), Big Black (MS), and Tombigbee (MS/AL) basins. Confidence in the exact placement of the greatest flooding impacts continues to be uncertain and will be dependent on where heaviest rates and training occur. .Northeast... There is potential for urban and small stream flooding, as well as new and renewed river flooding impacts from northern PA through western ME on days 3 - 4 (Wed - Thu) as a storm system brings periods of moderate rainfall, warm temperatures, and moist dew points to the region. On a regional scale, rainfall totals of 1 - 1.75" are expected, with up to 2.5" expected in the higher terrain of the Catskills, Adirondacks, Green, Whites, and western ME mountains, and this rainfall will occur on a ripened snowpack. Gusty winds are likely with this system as well, which will aid in accelerating the degree of melt ahead of the event. SNODAS appears to be too high with SWE totals across the region but is handling the areal extent of snow well. The NWM, on the other hand, appears to be slightly too low with SWE and too low with areal extent. A good compromise between these two models yields the initial state of the snowpack with 1 - 3" of SWE available to melt, with the highest SWE in higher elevations. In general, expect between 0.5 - 1" of SWE melt a day due to warm temps and winds prior to the event. During the event, high dewpoints are likely to cause rapid snowmelt of the remaining snowpack with melt totals up to 1.5" during the event. Regardless, given the already saturated antecedent conditions and atmospheric set up, it is likely that whatever is left of the snowpack, combined with the rainfall, will further enhance the river flooding potential. Both the GFS and NBM-forced NWM guidance continue to indicate widespread small stream responses across the region. AEPs as low as 2% are noted on small streams draining the steep terrain, indicating confidence in the likelihood of small stream flooding impacts. MMEFS guidance continues to show the potential for minor river flooding across NY, CT, RI, VT, NH, and MA later this week. Outside of the snow covered areas, locally heavy rainfall may elicit poor drainage and nuisance flooding in urban areas, especially if heavier rainfall totals were to materialize. .Ohio Valley... Localized urban and small stream flooding impacts, as well as new and renewed river rises, are possible as showers and thunderstorms impact the region beginning late today and continuing through day 3 (Thu), with the greatest potential for impacts across portions of western KY and southern IN. Rainfall totals of 1 - 2" are expected. While top soils have mostly recovered from last week's rainfall, streamflows remain elevated, especially across the Lower Wabash (IL/IN) and Green (KY) basins, as these areas remain in action stage and minor flood. Renewed river rises and prolonged river flooding remains the biggest concern, as this activity should be relatively progressive, though small stream and poor drainage flooding impacts in urban areas are possible dependent on training. Ensemble guidance (MMEFS/HEFS), as well as guidance from both the GFS and NBM-forced NWM continue to support the idea of river and small stream responses later this week. .Central Appalachians into the Mid-Atlantic... Localized urban and small stream flooding impacts are possible from portions of the Blue Ridge Mountains (WV/VA) to central PA as the same storm system bringing impacts to the Southern U.S. moves northeast, bringing periods of moderate to locally heavy rainfall (1 - 1.5+") to the region. Soil moisture and streamflows are elevated from rainfall last week, which suggests lowered infiltration capacity for runoff. Guidance from both the GFS and NBM-forced NWM are indicating scattered small stream responses across southern WV through central PA on days 3 - 4 (Thu - Fri), with corresponding AEPs near the high water threshold (50% AEP), 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. River rises are possible as well, as MMEFS and HEFS guidance suggests some action stage rises across VA and PA later this week. Coastal flooding impacts are possible through day 3 (Thu) from Chesapeake Bay to Long Island Sound. Check with your local NWS office for more information related to coastal flooding impacts. .Puerto Rico and the USVI... Periods of heavy rainfall through day 2 (Wed) may elicit urban and small stream flooding impacts, particularly across interior and eastern PR, and the USVI. //Ayala/Pritchard $$