####018006664#### AGUS74 KWCO 191513 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1013 AM CDT SUN MAY 19 2024 .Synopsis... River flooding impacts continue across Southeast Texas and Southwest Louisiana... Flooding impacts remain possible across portions of Hawaii today... Increasing flood threat across portions of the Lower Missouri River Basin and Upper Mississippi River Basin and ArkLaTex through the Lower Ohio River Basin this week... Flood potential increasing this week for Montanna and Puerto RIco and USVI... Ice jam flooding impacts for portions of Alaska... .Discussion... .Southeast Texas and Southwest Louisiana... Moderate-to-major river flooding is ongoing in response to all of the recent heavy rainfall. Many rivers have crested, with flooding impacts lingering through this week. There is potential for additional rainfall mid-to-late next week, but confidence is very low on if this will cause any hydrologic impacts to the region. .Hawaii... Flash, urban, and small stream flooding impacts remain possible for Niihau and Kauai today as a slow-moving band of heavy rainfall moves across the island. Landslides are also possible in areas of steep terrain. .Lower Missouri River Basin and Upper Mississippi River Basin... Multiple days of rain and thunderstorms will introduce a threat for flooding impacts beginning today and continuing through day 3 (Tue), then returning day 6 (Fri). Rainfall totals of 1 - 2" are expected for days 1 - 2 (Sun - Mon) across much of NE, northern IA, southern MN, and WI. This rainfall will likely not result in much of a hydrologic response across the region, but will prime soils across the region ahead of a heavier rainfall event (1 - 4") on day 3 (Tue) across central/southern MN, northern IA, and western WI. Top layer 0 - 10 cm soils leading into this week are generally near normal (50% RSM, NASA SPoRT), while deeper layer soil columns (0 - 100 cm) contain a bit more moisture (55 - 70 % RSM, NASA SPoRT), suggesting streams and rivers, which are already running above normal (USGS) across southern MN, IA, and portions of WI, may begin to respond fairly quickly after the top layer soils become wet-to-saturated and runoff becomes efficient on day 3 (Tue). Ensemble guidance (HEFS, CR ensemble QPF hydrographs) shows potential for widespread riverine response across MN, IA, and western WI, with 10% exceedance (HEFS) and 5% exceedance (CR ensemble QPF hydrographs) forecast flows showing potential for widespread moderate to isolated major river flooding, and while this extent or magnitude of river flooding is not expected, it does suggest that there is potential for impactful riverine flooding if some of the higher event total QPF forecasts (4 - 5"+) come to fruition across the region. The National Water Model (NWM) Medium Range Forecast (MRF) continues to signal widespread small stream response across southern MN, northern IA, and WI beginning on day 3 (Tue). Peak flows are expected on days 4 - 7 (Wed - Sat) in response to rainfall through day 3 (Tue), however additional rises may be possible late this week into the weekend as additional precipitation moves into the region. Probabilities for high water flows (1.1 - 1.6 ARI flows, NWM MRF) have increased substantially from yesterday, with numerous basins now seeing probabilities in the 50 - 80% range across southern MN, northern IA, and central/southern WI. Probabilities for rapid onset flooding have also increased 25% (NWM MRF) across the same region, however corresponding AEPs are generally around the 50% threshold, suggesting that widespread high impact river flooding is not likely at this time. Both the GFS and NBM QPF represent reasonable scenarios for this week across the region, thus the NWM MRF response from both forcings is fairly reasonable, but perhaps a bit underdone in terms of magnitude of response. .ArkLaTex through the Lower Ohio River Basin... Rain (1 - 2"+) moves into the region on days 4 - 5 (Wed - Thu), bringing an isolated flooding threat along with it, as most of the region still has wet soils (NASA SPoRT) and elevated streamflows (USGS) in place from recent rainfall events. The heaviest rainfall totals and greatest threat for flooding impacts will be across the ArkLaTex region where the NWM MRF, HEFS, SR PQPF River Ensembles, and CR ensemble QPF hydrographs are all showing potential for riverine responses across the region. The magnitude and areal coverage of potential flooding impacts is still quite uncertain. .Montanna... Rainfall is expected across much of the state this week, with very light precipitation on days 1 - 2 (Sun - Mon) keeping conditions wet ahead of the heavier rainfall (1 - 3") on days 4 - 6 (Wed - Fri). Streams remain elevated (USGS) from recent precipitation events, and trends are suggesting small stream flooding impacts, and potentially isolated rapid onset flooding, will be possible later this week, specifically across basins draining the Kings Hill area southeast of Great Falls. QPF is in general agreement (NBM, WPC, GFS) in terms of placement and magnitude in this region, and thus confidence in the subsequent responses being forecast by the NWM (NBM and GFS forced) are fairly high in this area. Probabilities for high water flows (flows with an ARI of 1.6) are starting to creep above 75% based on the last 7 runs of the NWM, and both the GFS and NBM forced NWM are showing potential for flows with AEPs 20%, suggesting impactful flows in days 4 - 7 (Wed - Sat). Some snow is expected across the region, especially above 7500 feet, so that will help mitigate total runoff amounts. .Puerto Rico and USVI... Periods of heavy rainfall are expected through all of this week across the islands, increasing the potential for flooding impacts. Soils across the islands are already very wet given the recent rainfall, increasing runoff efficiency, the potential for rapid rises on creeks and streams, and landslides in areas of steep terrain. Urban and flash flooding is expected. .Alaska... Ice jam flooding impacts are possible through Monday on the Yukon River near Pilot Station. Water levels downstream at St. Marys Pitkas Point and Mountain Village will rise significantly once this ice jam releases or moves downstream. //JEC $$