####018006506#### AGUS74 KWCO 301535 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT TUE APR 30 2024 .Synopsis... Flash, urban, and small stream flooding, along with moderate to major river flooding is ongoing or forecast for East Texas and the Lower Mississippi Valley...Isolated flooding impacts remain possible across portions of the Central and Southern Plains into the Midwest...Isolated flash and urban flooding impacts for Puerto Rico...Snowmelt runoff and ice jams are causing isolated river and small stream impact in Alaska...Ongoing flooding across the Great Basin and Intermountain West... .Discussion... .East Texas and the Lower Mississippi Valley... Looking ahead to days 2 - 3 (Wed - Thu), another storm system moving across the region is forecast to bring potentially heavy rainfall (2 - 4", locally higher possible) to the region. The heaviest precipitation is focused in headwaters of Neches and Lower Trinity river basins which are still experiencing moderate and major flooding further downstream. Antecedent soils conditions (NASA SPoRT) and streamflows (USGS) are still primed for enhanced runoff and highflow responses from previous heavy rainfall. Additionally, isolated flash and urban flooding responses are possible where training occurs, particularly along and south of the I-20 corridor. The placement of the heaviest precipitation in the headwaters of the basins does allow for some mitigation of more severe river flooding impacts. However, if the rainfall maxima were to materialize locally considerable impacts may not be ruled out. By the end of the week, conditions improve slightly providing some time for rivers crests and continue routing downstream. The National Water Model (NWM) Medium Range Forecast (MRF) is continuing to suggest that some rivers and streams are experiencing highwater conditions. The High Flow Magnitude forecast is also signaling annual exceedance probabilities (AEPs) as low as 2% which are in good agreement with the river forecast. NWM signals can be somewhat muted in this region of TX due to the annual recurrence interval thresholds. With this in mind, the NWM may not be capturing the event in totality, and that small stream responses may be more widespread than is suggested. .Central and Southern Plains into the Midwest... Widespread minor to isolated major river flooding is ongoing or forecast over southeastern KS, through MO, and into western IL. Many locations have crested or are cresting over the next couple of days with only a few locations experiencing broader crests from routed flows into this weekend. Steady rainfall totalling 1 - 3", locally higher possible, will continue to threaten the region with additional flooding impacts, particularly in eastern KS and MO where the aforementioned river flooding is ongoing. Soils remain wet (0 - 100 cm, NASA SPoRT) and streamflows are above their climatological norms (USGS) creating conditions that are favorable for rapid-runoff into streams with little storage available. This runoff in combination with elevated flows may generate rapid rises and out-of-bank responses on smaller streams. Flash and urban flooding impacts can not be ruled out across the entire region where the higher rainfall rates materialize or where training occurs. The threat of flooding impacts diminishes further north into the Midwest. However, there remains some isolated minor flooding in portions of IA and MN from previous rainfall. Moderate rainfall totals of 1 - 3" are forecast through portions of the Midwest with the bulk of the forecast rainfall occurring over the next 3 days. This may result in isolated new and renewed rises on the smaller streams. The NWM MRF is generally signaling rapid-onset flooding probabilities between 0 - 50% across the region. Corresponding AEPs are around 50% with some isolated smaller order streams signaling AEPs as 10%. Peak Flow Arrival Time Forecast is suggesting most streams and rivers peaking by this weekend with some of the larger mainstem rivers cresting later in the period, correlating well with the river forecasts. Model guidance is in fair agreement and the placement and magnitude of the upcoming rainfall providing some added confidence in the NWM output in these sensitive basins. .Puerto Rico... Persistent heavy rainfall (6 - 8", 120 hr rainfall forecast totals) from a developing wet pattern will bring with it all modes of flooding through early next week with the focus being on flash and urban flooding impacts. The elevated risk of excessive rainfall will generally be in the eastern and central portions of the island through day 3 (Thu), and transition more towards the central and westward side of the island by day 6 (Sun). Additionally, river rises are also possible throughout this event due to routed flow from runoff. .Alaska... Snowmelt runoff in the Copper River Basin may cause isolated flooding impacts through the evening of day 2 (Wed). Out-of-bank flows on Moose Creek near Glennallen could impact Glenn Highway as these high flows route downstream. Furthermore, snowmelt driven rises are ongoing for the Yukon River at Eagle and Dawson. An ice jam is located about 23 miles upstream of Eagle, and once released may cause additional ice jams and flooding impacts to low-lying areas. Across the western and central portions of AK, GloFAS flood summary is continuing to signal the potential for flows to reach their respective 5-year return periods, with some rivers cresting through the weekend. ECMWF hydrographs are suggesting an increase in base flow indicating that the runoff from snowmelt is ongoing and may enhance flooding conditions on streams and rivers. Additional crests are possible later in the forecast period, some of which are beyond the valid period of this NHD. .Great Basin and Intermountain West... Elevated flows will continue along area rivers, creeks, and streams across portions of southern ID and northern NV in response to runoff from snowmelt and recent rainfall. Isolated minor to moderate river flooding is ongoing through this week. //JAC $$