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Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
697 AGUS74 KWCO 171527 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT MON JUN 17 2024 .Synopsis... Flood potential increasing across portions of the Central Plains and the Upper Midwest...Heavy rainfall and associated flooding impacts likely across the Western and Central Gulf Coast... .Discussion... .Central Plains and the Upper Midwest... Repeated rounds of heavy rainfall are likely across these regions throughout this week (7-day totals of 5 - 7"), and will likely generate flash, urban, small stream, and river flooding. This region has already received well above average rainfall over the past month (2 - 6"+ positive departures from normal precipitation) and rainfall over the last 48 hours (widespread 1 - 3", locally higher, MRMS) have wetted soils (60 - 90% RSM, 0 - 10 cm NASA SPoRT) and brought area streams to above average annual median flows (USGS). Each additional round of rainfall will serve to further compound the flood potential. Day-to-day flooding impacts will heavily depend on the intensity of rainfall and whether storms train and/or persist over the same area for prolonged periods, potentially causing locally considerable flood impacts. River flooding is increasingly becoming a concern as well, especially across MN and western WI, where stream flows are the most elevated and where river flooding is ongoing. Current river ensemble guidance (HEFS) suggests the potential of new and renewed minor to major river flooding towards the end of this week after subsequent rounds of heavy rainfall. For today, the short range HRRR-forced National Water Model (NWM) indicates a chance of rapid-onset flooding (ROF). The past few runs have been consistent, showing high ROF probabilities (50 - 75%), mostly situated in southeastern MN. Corresponding annual exceedance probabilities (AEPs), per the High Flow Magnitude forecast, have continually indicated significant stream rises (AEPs as low as 2%) southwest of Minneapolis, increasing confidence that this area may experience hydrologic impacts. In the medium range, the GFS and NBM are indicating the potential for ROF, with the GFS showing more widespread signals. Furthermore, both models are indicating widespread small stream responses across much of the region with lower AEPs concentrated in eastern MN and into northwestern WI. Peak flows are not expected until day 5 (Fri) for smaller streams, and day 8 (Mon) and beyond for larger mainstem streams. .Texas and Coastal Louisiana... Tropical moisture is bringing heavy rainfall and the threat of considerable flash, urban, small stream, and riverine flooding impacts to TX and coastal LA. Rainfall is ongoing along the LA coast, and is expected to shift into TX on days 2 - 4 (Tue - Thu). Widespread QPF totals for this event are in the 7 - 10" range along coastal TX, and decrease further inland, but even as far inland as the Austin/San Antonio area is still forecast to receive up to 5". Considerable flash, urban, and small stream flooding impacts are expected in response to this rainfall, along with new and renewed river flooding. Isolated rainfall amounts of 15 - 20 inches may be possible in southern TX by the end of this event. In LA, antecedent conditions are becoming more conducive to efficient runoff due to the ongoing rainfall. NASA SPoRT is beginning to respond to the rainfall that fell this morning with current topsoil RSM in the 35 - 65% range. Streamflow responses are lagging, as would be expected, and are still near or below normal annual mean flow (USGS). Recent forecast trends have shifted most of the day 2 (Tue) and beyond QPF out of LA into TX, thus the hydrologic threat is greatly reduced. Nonetheless, heavy rainfall today could still lead to locally significant flash and urban flooding impacts anywhere heavy rainfall persists. In TX, antecedent conditions vary, with east TX being on the wetter side after an abnormally wet April and May (5 - 8"+ positive departure from normal precipitation). The area has been spared significant rainfall over the last week, allowing soils to dry to near-normal conditions (40 - 50% RSM, 0 - 10 cm, NASA SPoRT), 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. With the current hydrologic sensitivity of eastern TX, persistent heavy rainfall is likely to lead to at least locally considerable flash, urban, and small stream flooding impacts. Thankfully, recent forecasts have shifted the highest QPF totals to the southwest, and current 7 day totals for the area are at worst 3 - 4" (WPC). 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 NWM is showing muted small stream responses despite the high QPF totals, but is not surprising due to the high annual recurrence interval (ARI) set for TX and small stream flooding impacts are still likely. On the other hand, south and west of Houston, soil conditions are dry and streamflows are running at normal to much below normal flows, allowing some infiltration and in-channel storage capacity for the initial rounds of expected rainfall. This will help buffer any flooding, but with high forecasted rainfall totals and likely high rainfall rates, considerable flooding impacts are likely. Texas 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 (Isolated rainfall amounts of 15 - 20 inches may be possible in southern TX by the end of this event.). As for river flooding, ensemble guidance has been slow to pick up on the southwest shift in QPF, with only the HEFS 10% exceedance showing any 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 annual recurrence interval (ARI) value from the NWM in TX. The GFS-forced version of the NWM is much more aggressive, with widespread AEPs less than 20% from Victoria, through San Antonio and into Texas Hill Country. These signals are likely overdone in their wide areal coverage, but isolated occurrences of these extreme signals is possible should the rainfall exceed current forecasts. These signals support the possibility of locally considerable flooding impacts. Flash and urban flooding, which are not as dependent on ripe antecedent conditions, are still likely anywhere repeated rounds of heavy rainfall occurs. //Bliss/Wood $$