####018006166#### AGUS74 KWCO 281539 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 915 AM CST WED FEB 28 2024 .Synopsis... Localized flash and urban flooding remains possible for portions of the Ohio River Valley... Rainfall and snowmelt-induced flooding impacts are possible in the Northeast... Rounds of rainfall forecast in California and the Pacific Northwest... .Discussion... .Ohio River Valley... Localized flash, urban, and small stream flooding impacts remain possible through early this afternoon across portions of eastern KY, southern OH, WV and southwest PA as a line of showers and thunderstorms, with locally enhanced rainfall rates, moves across the region. Ahead of this activity, antecedent conditions are conducive for rapid runoff, as recent rainfall has left soils wet (65 - 90% RSM, 0 - 10 cm NASA SPoRT) and streamflows above climatological normals for this time of year (USGS). While storm motions are expected to be relatively fast/progressive, training convection within the line, as well as enhanced rainfall rates (up to 2"/hr) on top of already wet soils will at least locally overwhelm remaining infiltration capacity, generating immediate runoff into nearby small streams and urban areas, leading to subsequent flooding. The latest HRRR-forced National Water Model (NWM) Short Range Forecast indicates scattered small stream rises across this region, with peak flows expected by early this afternoon. Corresponding annual exceedance probabilities (AEPs), per the HRRR-forced High Flow Magnitude forecast, show rises generally near the high water threshold (1.0 - 1.6 ARI). However, locally significant AEPs (2 - 4%) are noted near/north of Charleston, WV, suggesting at least locally the potential for rapid stream rises. Flooding impacts are most likely along areas of steep terrain and where more robust rainfall rates materialize. River flooding is not likely, but rises to action stage are possible along the Monongahela and Youghiogheny rivers in northern WV/far southwestern PA. .Northeast... The same system bringing showers and storms across portions of the Ohio Valley will traverse across the region, generating up to 1.5" of rainfall through early day 2 (Thu), as well as high elevation snow melt, may elicit localized small stream and river flooding impacts across the region. The snowmelt will be the biggest contributor to total runoff and flooding potential. Ahead of this rainfall, 2 - 6" of snow water equivalent (SWE) is available in the snowpack across portions of the Adirondacks (NY), Green (VT), and White (NH) mountains above 500 and into the western/northern ME mountains, with locally higher amounts across portions of the White Mountains. In regard to antecedent conditions, soils are generally wet across these regions (65 - 85% RSM, 0 - 10 cm NASA SPoRT) and streamflows are generally near or above climatological normals for this time of year due to recent snowmelt. This, in conjunction with additional snowmelt (up to 2"), and at least partially frozen soils in some areas, will lead to immediate runoff into nearby streams and rivers, leading to subsequent flooding. Scattered rises to action stage are possible region wide, with isolated minor flooding now forecast across portions of upstate NY and northern VT. The latest guidance from both the GFS and NBM-forced NWM continues to indicate scattered stream rises across portions of the Adirondacks and Downeast ME through day 3 (Fri). Of note, behind this frontal system there will be snow and below-freezing temperatures filtering into the region, which will likely attenuate/slow some of the hydrologic responses across the region. .California... A series of storm systems bringing low elevation rain and high elevation snow is forecast across the state this week, increasing the potential for localized flooding impacts. The first round of precipitation is expected during the late day 1/early day 2 (Wed - Thu) period across northern and central CA, with additional rounds on days 3 - 4 (Fri - Sat) impacting the same region, but also extending further south into SoCal. SNODAS and the NWM continue to indicate that there is very little, if any, SWE left to melt in the lower elevations of northern and central CA, suggesting that snowmelt will likely not be a significant contributor to runoff. However, antecedent conditions ahead of this rainfall are favorable for efficient runoff, with wet soils (NASA SPoRT) and above climatological normal streamflows (USGS) across the region. These periods of rainfall will likely diminish any remaining infiltration capacity and lead to small stream rises and flooding of poorly drained/low-lying areas. The latest guidance from both the GFS and NBM-forced NWM Medium Range Forecast (MRF) continues to suggest isolated small stream responses along the Coastal Range and across the Central Valley on days 3 - 5 (Fri - Sun). Corresponding AEPs, per the MRF High Flow Magnitude forecast, continue to indicate that most streams should remain within the channel, although some out-of-channel responses are possible across the Central Valley (AEPs down to 2 - 4%). This suggests flooding impacts, if any, will remain highly localized and be dependent on where the heaviest rainfall occurs. .Pacific Northwest... Both small stream rises and isolated minor to moderate river flooding impacts are expected through early next week for areas west of the Cascade Mountains in response to long-duration light to moderate rainfall, both recent and forecast, in combination with high elevation snowmelt (up to 0.5" of SWE loss, SNODAS). The NWM MRF continues to signal scattered small stream responses along the foothills of the Cascades, with high water arrival times mostly in the 2 - 6 (Thu - Mon) day range. //JDP $$