####018004779#### AGUS74 KWCO 201507 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT MON MAY 20 2024 .Synopsis... Flooding impacts, including river flooding, possible across portions of the Lower Missouri River Basin and Upper Mississippi River Basins and the ArkLaTex through the Lower/Middle Mississippi Valley this week... Rainfall and snowmelt bringing the potential for flooding impacts later this week in Montana... Flooding impacts possible this week for Puerto Rico and USVI... Significant river flooding continues in Southeast Texas and Southwest Louisiana... Ice jam flooding impacts continue for portions of Alaska... .Discussion... .Lower Missouri River and Upper Mississippi River Basins... Continued and forecast rainfall in NE, IA, WI, and MN will bring a threat for flooding impacts through day 2 (Tue) as rainfall totals of 1 - 3" are expected across those areas. Top-layer soils have wetted across IA where rainfall has already fallen, and are otherwise wet but not saturated across MN, eastern NE, and WI (NASA SPoRT). Portions of eastern NE, southern MN, and northern IA look to see rainfall on both days, and those areas along with the portions of central and northern MN that are forecast to see heavy rainfall on day 2 (Tue) are most likely to see isolated flash and urban flooding impacts. The Minnesota River (MN) is currently in action stage on its recession limb, and many streamflows in the area remain well above normal (USGS). CR Ensemble QPF guidance still indicates that widespread moderate to major river flooding is possible at 5% chance of exceedance, but that is a worst case scenario-type forecast that is unlikely to occur. Meanwhile, the National Water Model (NWM) Medium Range Forecast (MRF), both the GFS and NBM versions, are indicating annual exceedance probabilities (AEPs) of 50% or greater with only localized instances of AEPs lower than 50%. The most likely scenario is somewhere between those two possibilities, since the QPF forcing from the GFS and NBM seem reasonable, and the Minnesota River, already in action stage, may be the most vulnerable to minor or worse river flooding. .ArkLaTex through the Lower/Middle Mississippi Valley... Periods of moderate to heavy rainfall totaling 2 - 4" on days 3 and 4 (Wed and Thu) will bring the potential for isolated flooding impacts to the ArkLaTex and southern MO. Top and mid-layer soils are currently wet but not saturated (NASA SPoRT) and streamflows are near to above normal (USGS), indicative of a near-primed environment that will still have some time to recover ahead of the rainfall. Isolated flash and urban flooding impacts are possible, especially in areas of steep terrain, and river ensemble forecasts indicate that rises on rivers and minor river flooding are also possible. .Montana... Moderate to heavy rainfall (1 - 2") on days 3 and 4 (Wed and Thu), combined with snowmelt in some areas, could produce isolated small stream flooding impacts in portions of central and southern MT. The most vulnerable areas are likely the Little Belt and Highwood Mountains of central MT, where Belt Creek remains elevated from recent rainfall. Snowfall is expected above 7000 feet there, but with saturated soils in lower elevations and snowmelt likely a contributor to runoff, out of bank rises and associated flooding impacts are possible. Elsewhere in the region, flooding impacts are possible, but drier antecedent conditions should mitigate more widespread and significant small stream responses. .Puerto Rico and USVI... Periods of heavy rainfall are expected through all of this week across the islands, bringing potential for flooding impacts. Soils across the islands are already very wet given recent rainfall, which will increase runoff efficiency, the potential for rapid rises on creeks and streams and isolated flash and urban flooding, and landslides in areas of steep terrain. .Southeast Texas and Southwest Louisiana... Moderate to major river flooding is ongoing in response to recent heavy rainfall in these regions. Many rivers have crested, and flooding impacts will linger through this week. With little rainfall expected this week, recessions should continue unimpeded. .Alaska... The ice jam on the Yukon River near Pilot Station has released however, elevated water levels are still possible today in and near Pilot Station, and significant rises are possible downstream in Mountain Village and St. Marys today. //Watson $$