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Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
529 AGUS74 KWCO 051530 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT FRI JUL 5 2024 .Synopsis... New and renewed flooding and flash flooding threats continue across the Upper and Middle Mississippi Valleys... Heavy tropical rainfall to begin late this weekend across South Texas... Isolated flash and urban flooding possible for portions of Central Plains... Increased snowmelt and glacier runoff, heavy rainfall to continue in Alaska... Flooding possible this weekend in American Samoa... .Discussion... .Upper and Middle Mississippi Valleys... Across MN, IA, and WI, scattered thunderstorms today may cause additional flash and urban flooding as storms continue to impact this already saturated region. Hydrologic conditions remain primed with wet soils and swollen streams. All forcings of the NWM indicate scattered to widespread small stream responses with peak flows ongoing or will occur this afternoon as runoff routes into the stream network. Corresponding annual exceedance probabilities (AEPs) from the National Water Model (NWM) Short Range Forecast High Flow Magnitude forecast continues to show that most rises will be modest, although some locally significant rises cannot be ruled out in areas of heavier rainfall. Additionally, moderate to major river flooding continues across portions of southern MN, and much of IA, with moderate to major flooding forecast along portions of the mainstem Missouri and Mississippi rivers through early next week. .South Texas... Periods of tropical rainfall associated with Hurricane Beryl will impact portions of the Hill Country, Deep South TX, and the Rio Grande Valley on days 3 - 6 (Sun - Wed). Rainfall totals of 4 - 8" (with locally higher amounts) is forecast for the aforementioned areas, and have been trending upward over the past couple of days. Antecedent conditions are generally dry, as relative soil moisture is low (0 - 30%, 0 - 10 cm RSM NASA SPoRT) and streamflows are generally below normal (USGS) for this time of year. These conditions would suggest that at least initially, there is ample infiltration capacity and in-channel storage in the streams and rivers. However, caution must be exercised as very dry soils can still be somewhat hydrophobic and elicit some runoff responses. Once the rainfall begins the antecedent conditions will transition to be more hydrologically favorable for enhanced runoff and out-of-bank rises. The NWM MRF is beginning to suggest some rises to high water and AEPs as low as 4% on the smaller streams and tributaries begin on day 4 (Mon) in southern and eastern TX. These signals would suggest that the main threat would likely be flash and urban flooding responses, particularly in low-lying areas and areas of complex terrain found further inland. River ensembles (HEFS and PQPF) are also beginning to suggest some isolated riverine responses may be possible. Nevertheless, confidence in the exact location and magnitude of potential flooding remains uncertain and is highly dependent on the track and intensity of Beryl. .Central Plains... Isolated flash and urban flooding, along with the new rises on small streams may be possible for portions of southern KS, western MO, and northern OK from additional rounds of rainfall (1 - 3") this weekend into early next week. Surface and shallow soils (0 -10 cm and 0 -100 cm) are very dry (0 - 30% RSM, NASA SPoRT) with variable climatological streamflow percentiles for this time of year (USGS). Given the dry soils, runoff may still be a factor if intense rainfall rates exceed infiltration rate/capacity. However, streams look to be in rather good shape overall and will likely have enough storage capacity available to mitigate more widespread flooding responses. Some pluvial flooding may still occur in typically flood prone areas and areas with poor drainage. Both the GFS and NBM-forced NWM are signaling magnitudes of 10 - 4% AEPs with peak flow arrival times by day 3 - 4 (Sun - Mon). While widespread flooding responses are unlikely, it is possible that locally significant flooding responses may occur where training thunderstorms develop, particularly in urban areas. .Alaska... Periods of moderate to heavy rainfall (1 - 2"+) will continue into the weekend across much of the Brooks Range, Interior, and into the south central portions of the state. This influx of rainfall may enhance runoff, generate rapid rises and rivers and streams, and potentially lead to ponding and isolated flooding. Elsewhere, elevated flows and isolated minor flooding from increased snowmelt and glacial melt continues along the Klutina River near Copper Center and the Nuyakuk River northeast of Dillingham. .American Samoa... Rainfall from a developing trough may cause instances of isolated flash flooding through day 2 (Sat). //Capp $$