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Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
269 AGUS74 KWCO 061751 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SAT JUL 6 2024 .Synopsis... Heavy tropical rainfall to begin Sunday afternoon across Texas into the Lower Mississippi Valley...Isolated flash and small stream flooding remains possible across the Upper and Middle Mississippi Valleys... Isolated flooding possible for portions of Kansas and Oklahoma... .Discussion... .Texas into the Lower Mississippi Valley... Flash and urban flooding impacts, some of which may be locally considerable, are likely day 2 - 4 (Sun - Tue) for portions of the TX Gulf Coast and East TX due to tropical rainfall associated with Tropical Cyclone Beryl. Heavy rainfall (5 - 10", locally 15") is forecast and has been trending up as the track continues to shift. Antecedent conditions are variable and their influence on potential impacts is highly dependent on where TC Beryl makes landfall. Antecedent conditions in South TX 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. The further north and east you go, conditions become more favorable for hydrologic responses in East TX and LA. While this area has had their soils dry out somewhat (35 - 60% RSM) the streams are still generally normal to above normal. Furthermore, the complex terrain of the southern extent of the Ozarks in AR may further enhance the flooding characteristics by increasing flows downslope causing areas of flash flooding and rapid rises in streams. The National Water Model (NWM) Medium Range Forecast (MRF) is signaling some rises to high water and AEPs as low as 4% on the smaller streams and tributaries begin on day 3 (Mon) in southern and eastern TX. The GFS-forced rapid-onset flooding service is beginning to suggest some streams may see some rapid rises. The basins around the Mississippi Delta that are being highlighted do seem to still be bouncing around somewhat so confidence is lower on how those may materialize. River ensembles (HEFS and PQPF) are suggesting some isolated riverine responses of minor to isolated moderate flooding may be possible in coastal TX and eastern TX. Initially, the primary hydrologic impacts associated with this tropical cyclone will be flash and urban flooding. However, small stream and river flooding could become particularly problematic as this event unfolds due to the impacts the spring flood season has had on the basins in East TX. The timing and magnitude of any flooding impacts will be highly dependent on the track and intensity as the models are continuing to resolve their solutions. .Upper and Middle Mississippi Valleys... Lingering rainfall is forecast to continue through the weekend for portions of MN, IA, WI, northern MO, and western IL which may continue to cause isolated flooding, along with delayed recessions on elevated streams and rivers. Hydrologic conditions remain primed with wet soils and swollen streams (0 - 10 cm RSM NASA SPoRT, USGS). The NWM suggests scattered to widespread small stream responses in the northern extent of this region and decreases further to the south in MO. Generally, peak flows are ongoing and expected to continue as runoff routes into the stream network. Corresponding annual exceedance probabilities (AEPs) from the NWM MRF High Flow Magnitude forecast continues to show that most rises will be modest and the lingering rainfall will likely prolong elevated flows. Furthermore, these tributaries are resolving the high flows into rivers that are in flood. 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. The broad flood wave on the Mississippi River is currently just south of La Crosse, WI. By day 4 (Tue) dry conditions settle across the region helping to improve the hydrologic conditions, and allowing for unimpeded recessions to resume. .Kansas and Oklahoma... Forecast heavy rainfall (1 - 3"+) may cause isolated flash and urban flooding, along with new rises on small streams, beginning on day 2 (Sun) for south-central KS into much of central OK. 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. The NWM MRF forcings continue to signal magnitudes of 10 - 4% AEPs on the smaller streams and creeks with peak flow arrival times by day 2 - 3 (Sun - Mon). These signals would suggest that smaller order streams may experience some isolated higher flows if these totals were to materialize. However, small stream flooding is the secondary threat for this event with primary concern being pluvial flooding in typically flood prone areas and urban areas with poor drainage. Some lingering rainfall will be possible by day 4 (Tue) before dry conditions settle across the region. //Capp $$