####018005656#### AGUS74 KWCO 131518 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT MON MAY 13 2024 .Synopsis... Considerable flash and urban flooding impacts, and major river flooding in East Texas...Locally significant flash and urban flooding in the Lower Mississippi Valley and Southeast...Isolated flooding impacts are possible in the Mid-Mississippi Valley... .Discussion... .East Texas... Considerable flash/urban flooding and major river flooding is expected ahead of multiple rounds of rainfall today and again late this week. The Neches and Lower Trinity river basins are once again bearing the brunt of abnormal rainfall (600% or greater, last 30-days percent of normal) and coincidentally the significant flooding impacts. Record or near record major flooding is ongoing on the Lower Trinity River further highlighting the vulnerability of the region. Antecedent soil (70% or greater, 0 - 100 cm RSM NASA SPoRT) and streamflow (USGS) conditions have not yet rebound from previous rainfall events experienced over the past few weeks, meaning that any rainfall will immediately transition to runoff and route into bloated streams and rivers. The National Water Model (NWM) Short Range Forecast (SRF) Rapid-onset Flooding forecast continues to signal isolated rapid-onset flooding across East TX. These signals may be somewhat under representative of the overall flood threat due to high highwater thresholds muting some of the response. The SRF Maximum High Flow Magnitude forecast is indicating isolated areas of streamflows with lower Annual Exceedance Probabilities (AEPs) ranging from 20 - 2%, indicating isolated areas of significant out-of-bank streamflows are possible today. River ensemble forecasts suggest many of the area's rivers will likely see new and renewed river rises which will exacerbate ongoing flooding. The region remains hydrologically active and vulnerable as an additional 2 - 3" of rainfall is forecast to begin on day 4 (Thu) for portions of East Texas, further adding to an increasingly significant and abnormal flooding scenario. The NWM Medium Range Forecast (MRF) 10-Day Peak Flow Arrival Time Forecast is in good agreement with the river forecasts, and is hinting at long duration routed flows (10 days and beyond). .Lower Mississippi Valley and Southeast... Multiple rounds of heavy rainfall (3 - 5", locally higher possible) forecast this week and into the weekend may cause significant flash and urban flooding impacts, along with rapid rises on streams and river flooding, from the LA parishes through the FL panhandle and inland into central MS/AL. Today, the most intense rainfall of the forecast period (2 - 4") may result in considerable flash and urban flooding impacts, and prime the area's streams and rivers for additional responses beginning on day 4 (Thu). The I-10 urban corridor through the central Gulf Coast is the primary concern for today as those high rainfall rates can quickly overwhelm urban drainage causing standing water on roadways, and pluvial flooding development where prolonged rainfall occurs. The NWM SRF is suggesting rapid-onset flooding may occur across the region with probabilities generally at 50% or greater. The placement and magnitude of responses have been stable leading to an increase in confidence in the forecast. Following a brief reprieve from wet weather, by day 4 (Thu) yet another slug of moisture pushes through the region, and with it brings a renewed threat of flooding impacts. The NWM MRF is indicating both rapid-onset flooding and some out-of-bank flows are possible, some of which may be significant with the inclusion of stream AEPs at 10% or less. River ensemble forecasts show the potential for scattered minor river flooding at the 30% chance of exceedance. Isolated flash and urban flooding impacts should not be ruled out, particularly where the heaviest axis of QPF occurs today. .Mid-Mississippi Valley... Isolated flash and urban flooding, along with new and renewed rises on streams and rivers, are possible through early day 2 (Tue) from forecast rainfall totals of 1 - 2", locally higher possible. Antecedent soil conditions and streamflows are both generally near normal which would help to initially mitigate any significant flooding concern today. However, training heavy rainfall can still pose a threat to flood prone urban and low-lying areas, particularly across Kansas City, MO into Chicago, IL. Receding trends are ongoing for the minor and isolated moderate river flooding in the region. However, HEFS suggests the potential for new and renewed river rises including isolated moderate river flooding at the 30% chance exceedance. The NWM SRF continues to suggest the chance for scattered rapid-onset flooding with probabilities generally within 50%. The Maximum High Flow Magnitude Forecast is signaling most streams in the area will be over the 50% annual exceedance probability (AEP) with only a few isolated reaches as low as 10% in central and southern MO. These AEPs indicate that while some stream responses are possible, they are unlikely to be significant. Some streams in northern MO into IA and IL are peaking later in the week, hinting at responses from additional rounds of rainfall that are forecast throughout the week. //Capp $$