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Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
709 AGUS74 KWCO 181510 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT TUE JUN 18 2024 .Synopsis... Considerable flooding expected in Texas... Locally considerable flooding possible in the Northern Plains and Upper Midwest... Isolated flooding possible in the Central Plains... .Discussion... .Texas... Considerable flash, urban, small stream, and riverine flooding is anticipated for parts of TX tonight through day 3 (Thu) as heavy rainfall from Potential Tropical Cyclone One impacts the region. Widespread QPF totals for this event range from 5 - 10" along coastal TX, decreasing further inland, with areas as far inland as San Antonio forecasted to receive up to 3" (WPC). Isolated rainfall amounts up to 15" may be possible in southern TX by the end of this event, potentially leading to moderate and isolated major river flooding. Antecedent conditions vary, with east TX being on the wetter side after an abnormally wet April and May (5 - 8"+ above normal precipitation). The area has been spared significant rainfall over the last week, allowing soils to dry to near normal conditions throughout the soil column (40 - 50% RSM, 0 - 100 cm), but streamflows remain well above average climatological norms (USGS). Reservoirs across east TX are at or near capacity, and river flooding is ongoing in the lower reaches of the Trinity, Sabine, and Neches basins. Recent forecasts have shifted the highest QPF totals to the southwest, with current 7-day totals for the area near 3 - 4". Given the current hydrologic sensitivity of eastern TX, persistent heavy rainfall will likely generate flash, urban, and small stream flooding impacts, some locally considerable. Ensemble guidance (HEFS 30%) continues to indicate the potential for minor to moderate flooding on rivers north and east of Houston. The NBM-forced version of the National Water Model (NWM) shows muted small stream responses despite the high QPF totals, which is not surprising due to the high annual recurrence interval (ARI) set for TX, however, small stream flooding impacts are still likely. South and west of Houston, soil conditions are dry and streamflows are running at normal to much below normal mean annual flows, allowing for some infiltration and in-channel storage capacity for the initial rounds of expected rainfall. This will help buffer initial hydrologic responses, but with high forecasted rainfall totals and high rainfall rates, considerable flooding impacts are still likely. The TX Hill Country will be an area to watch, as basins are flashier due to the high relief and the area is currently forecast to get 3 - 5"+ over the duration of the event, with isolated rainfall amounts up to 15" possible in southern TX by the end of this event. Considering the current reservoir capacities are below 30% across the region (except near Victoria where reservoirs are near capacity), this rainfall will be highly beneficial. As for river flooding, ensemble guidance has started to pick up on the southwest shift in QPF, with HEFS 30% exceedance showing minor to moderate river flooding in south TX. Regardless, rises on rivers in south TX to at least minor flood status are likely. The NBM-forced version of the NWM is showing scant signals considering the amount of rainfall forecast, which is due to the high ARI value from the NWM in TX. The GFS-forced version of the NWM is much more aggressive, with widespread annual exceedance probabilities (AEPs) less than 20% being signaled across south TX. These signals are likely overdone in their wide areal coverage, but isolated occurrences of these extreme signals are possible should the rainfall exceed current forecasts, supporting the potential for locally considerable flooding impacts. Flash and urban flooding, regardless of antecedent conditions, are still likely wherever repeated rounds of heavy rainfall occur. .Northern Plains and Upper Midwest... Flash, urban, small stream, and river flooding, some locally considerable, as a result of persistent rounds of moderate rainfall this week, threatens the Northern Plains and Upper Midwest. This region has already received well above average rainfall over the past month (2 - 5"+ positive departures from normal precipitation) and rainfall over the last 72 hours (widespread 2 - 4", locally higher, MRMS) have wetted soils, particularly across southern MN into WI where soils down to the 100 cm layer are nearing saturation (65 - 95% RSM, NASA SPoRT). Additionally, area streams are above average annual median flows (USGS), with ongoing and forecast minor to isolated major river flooding in southern MN and northern IA as a result from preceding rainfall. Each additional round of rainfall will serve to further compound the flood potential. In the short range, the heaviest rainfall is forecast to affect northern MN this evening, where the aforementioned antecedent conditions will facilitate efficient runoff, leading to the potential for all modes of flooding. For days 3 - 5 (Thu - Sat), another system will move through this primed region. As this consecutive round of precipitation occurs later in the forecast period, uncertainty arises regarding the placement of higher rainfall totals. However, if storms persist or linger over the same vulnerable area for extended periods, locally considerable flood impacts are possible. The HRRR-forced NWM suggests a chance of rapid-onset flooding (ROF) this evening. The past few runs have been consistent, showing high ROF probabilities (50 - 75%), mostly situated in northeastern MN. Regarding the overall magnitude of the streamflow responses, AEPs, per the High Flow Magnitude forecast, have continually indicated significant stream rises (AEPs as low as 2%) southwest of Minneapolis as well as in northeast MN. The consistency in the placement and magnitude of the signals increases confidence in hydrologic responses expected later today. In the medium range, the GFS and NBM are indicating the potential for ROF. Both models are indicating widespread small stream responses across much of the region with lower AEPs concentrated in southeastern MN and into northwestern WI. This is the greatest area of concern for potential considerable flooding impacts later this week, with peak flows not expected until day 4 (Fri) for smaller streams, and day 8 (Tue) and beyond for larger mainstem streams. .Central Plains... Heavy rainfall is forecasted for today and tomorrow across portions of the Central Plains, with the highest totals (2 - 5", WPC) expected in southwestern KS, posing a threat of isolated urban and flash flooding. Antecedent conditions are not conducive to flooding, except in northeastern NE, where widespread rainfall totals of 1 - 2 inches, with locally higher amounts, in the past 48 hours have primed the region for hydrologic responses. Elsewhere, near surface soils are dry (15 - 35% RSM, 0 - 10 cm NASA SPoRT) and streamflows are normal to much below normal due to ongoing drought conditions. Additionally, sandy soils characterized by high infiltration rates will further mitigate any widespread responses, allowing rainfall to be absorbed into the system. However, where storms train and/or persist in high intensities for prolonged periods, the threat of flash and urban flooding cannot be ruled out. The NWM has been consistently signaling the potential for ROF in eastern NE and across central KS, with a cluster of ROF probabilities (25 - 75%) situated in northeastern NE. Corresponding AEPs, per the High Flow Magnitude Forecast, indicate isolated small stream rises with AEPs less than 20% scattered across the region, suggesting small stream rises could be possible in flashier, more responsive basins. //Freeman/Wood $$