####018005669#### AGUS74 KWCO 271523 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT WED MAR 27 2024 .Synopsis... Isolated flash and small stream flooding impacts, along with riverine flooding, is ongoing and forecast in the Southeast and Coastal Mid-Atlantic... Flooding impacts possible due to a combination of snowmelt and rainfall in the Northeast... Flash and urban flooding impacts are possible late in the week for Southern California... Unimpeded river recessions continue for Northeast Texas and Lower Mississippi Valley... .Discussion... .Southeast and Coastal Mid-Atlantic... Isolated flash and small stream flooding impacts are possible for days 1 - 2 (Wed - Thu) as potentially heavy rainfall of 1 - 3" (WPC QPF), with locally higher amounts, from a low-pressure system impacts the Big Bend and northeastern FL peninsula through the coastal Carolinas, and into coastal VA. Minor to isolated moderate coastal flooding is ongoing and forecast through day 2 (Thu) along the Eastern Seaboard due to onshore flows correlating with hightide. While some nuisance street flooding or flooding of poorly drained areas is possible in FL today due to the higher rainfall rates, the flood threat is generally confined to more of a small stream and river flooding threat further north along the coasts. Soils are still dry (0 - 10 cm RSM NASA SPoRT) along the coastal ranges of GA and the Carolinas providing confidence that there is infiltration capacity into the soils before rapid-runoff becomes an issue. Additionally, streamflows in NC and VA are near normal to slightly below normal (USGS) suggesting that some in-channel storage is available limiting initial flood responses. The National Water Model (NWM) Short Range Forecast (SRF) continues to signal scattered rapid-onset flooding probabilities of up to 50% from the Big Bend (FL) into portions of central VA. The NWM Medium Range Forecast (MRF) continues to signal peak flow arrival times on the lower order streams over the next 24 hours. However, some of the slower draining mainstem rivers along the Eastern Seaboard are forecast into the weekend. These signals are well-aligned with the river forecasts adding confidence in the NWM output. Stalled recessions and new and renewed rises are expected to continue through day 2 (Thu) before dry conditions settle in for the remainder of the week allowing unimpeded recessions to begin. .Northeast... Moderate rainfall, combined with warmer temperatures and higher dew points to produce snowmelt, is forecast on day 2 (Thu) as a low-pressure system continues north bringing with it the chance for isolated flooding impacts. The primary flood impacts are likely rapid rises on streams and rivers, particularly where snowmelt compounds runoff. Isolated flooding impacts on some low-water crossing or poorly drained urban areas may occur where the higher precipitation amounts materialize. Additionally, an increase in water output may be enough to cause movement of ice along the Saint John River (ME). Ensemble guidance (MMEFS) indicates that new and renewed rises are possible beginning day 2 (Thu) as runoff from rain and snowmelt begins to route into the tributaries and mainstems. Furthermore, SNODAS indicates that 0.5 - 1.5" of snow water equivalent loss may occur throughout the region. The NWM MRF continues to show some consistency in highwater arrival times beginning later this week, along lower annual exceedance probabilities (AEPs) thresholds being met, increasing confidence in the magnitude and timing of potential hydrologic impacts. .Southern California... Rainfall beginning on day 3 (Fri) and persisting through the weekend will bring the chance for isolated flooding impacts. Rainfall totals of 1 - 3" (WPC QPF) are expected, with higher amounts up to 6" possible in the higher elevations. Isolated flash and urban flooding impacts may be possible, particularly along the foothills of the Transverse and Peninsular Ranges, and in low-lying and flood prone areas. Furthermore, rapid rises on low order streams and creeks is expected due to the excessive runoff routing down from the higher terrains combined with urban runoff. The NWM MRF continues to signal scattered high water arrival times over the next 3 - 4 days (Fri - Sat) correlating well with the expected rainfall. Rapid-onset flooding probabilities are increasing for the Ventura, San Gabriel Coastal, Santa Ana, Laguna, and San Diego Coastal river basins. The NWM Maximum Highflow Magnitude Forecast suggests AEPs are broadly in the 50th percentile, providing further confidence to the overall increased flood threat for the area. Additionally, due to the heavier rainfall earlier in the season, it is possible that rock and mudslides may also impact residents in areas of steeper terrain. .Northeast Texas and Lower Mississippi Valley... Scattered minor flooding is ongoing and forecast through next week from repeated rounds of rainfall over the past week (1 - 3" RFC QPE). Although many faster responding streams and rivers have crested, rises along portions of slower-responding rivers are expected to continue through much of this week as flood waves route downstream. Unimpeded recessions are expected throughout the remainder of the week as dry conditions settle across the region. //Capp $$