####018004987#### AGUS74 KWCO 141522 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SUN APR 14 2024 .Synopsis... Flash and urban flooding impacts possible in Hawaii... New flooding impacts possible across the Ohio Valley into the Northeast... River flooding continues in East Texas, the Lower Mississippi Valley, and Southeast... Flooding impacts possible in Southern California, Puerto Rico and USVI, and Upper Midwest... .Discussion... .Hawaii... Heavy rainfall and thunderstorms may produce additional flash and urban flooding impacts across Kauai and Niihau through tonight. Flood prone roads and other low-lying areas may close due to elevated runoff and overflowing streams, as this rain falls on saturated soils. .Ohio Valley into the Northeast... Localized flash and urban flooding impacts are possible later today across portions of eastern OH, western/central PA, and southern NY as showers and thunderstorms bring enhanced rainfall rates to the region. Despite this system being quick moving and modest rainfall amounts (1" mean areal average), primed soils and elevated streamflows across this region may elicit at least localized hydrologic responses, especially in urban and poor drainage areas, and along complex terrain. The latest guidance from the HRRR-forced National Water Model (NWM) is indicating scattered small stream responses across western/central PA and southern/western NY this evening, with peak flows expected tonight. Corresponding annual exceedance probabilities (AEPs) are only showing isolated significant stream rises, which increases confidence in the hydrologic impacts being rather localized. Minor to isolated moderate river flooding impacts continue across portions of IN and OH. Most points have crested, and should continue to slowly recede mostly unimpeded after today. Additional rainfall on days 3 - 5 (Tue - Thu) may elicit renewed rises and/or stalled recessions along portions of the White (IN) and Wabash (IN) rivers, however, confidence in the exact location and magnitude of impacts remains somewhat uncertain, and will be heavily dependent on where convection trains or persists. .East Texas, Lower Mississippi Valley, and Southeast... Widespread minor to moderate river flooding, and isolated major river flooding (TX/LA), will continue throughout these regions through at least mid-week as runoff from recent heavy rainfall routes through the systems. With no significant rainfall expected over the next several days, crests and subsequent recessions will continue unimpeded, minimizing the risk of any additional river flooding impacts. .Southern California... Isolated urban and poor drainage flooding impacts remain possible across portions of southern CA through today as additional rainfall (up to 1") impacts the region. Fast storm motions and modest rainfall rates should largely mitigate flood impacts, however, isolated rapid stream rises and subsequent flooding is possible along the western slopes of the Transverse Ranges and in urban areas, especially if heavier rainfall rates are to materialize. The HRRR-forced NWM is indicating isolated small stream responses across this region, with corresponding AEPs generally at or above 50%, suggesting most of the small stream rises should remain within bank. .Puerto Rico and USVI... Periods of heavy rainfall will increase the potential for urban and small stream flooding impacts on days 2 - 4 (Mon - Wed) across the islands. Isolated flash flooding and landslides may also be possible, especially in areas of steep terrain. .Upper Midwest... Localized flooding impacts are possible on day 3 (Tue) from portions of northeastern NE and northern IA to southern MN and southwestern WI in response to locally heavy rainfall (1 - 2", locally higher) expected. Ahead of this event, antecedent soil conditions are relatively dry (less than 55%, 0 - 100 cm RSM, NASA SPoRT), with areas of moderate to extreme drought. While this does suggest ample soil infiltration capacity at least initially, the lack of available evapotranspiration (dormant vegetation), coupled with the potential for enhanced rainfall rates, may locally overwhelm infiltration capacity and lead to runoff into nearby streams and urban areas. Both the GFS and NBM-forced NWM continue to indicate isolated to scattered small stream responses across this region, with the most robust responses across northern IA and southern MN. Corresponding AEPs are generally at or above 50%, which suggests that small stream rises should generally remain within bank. Impacts across this region will be driven by enhanced rainfall rates and training convection. //Pritchard $$