####018005877#### AGUS74 KWCO 011514 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT WED MAY 1 2024 .Synopsis... Considerable flooding impacts possible today 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 and the USVI... Snowmelt runoff and ice jams are causing isolated river and small stream impacts in Alaska... Ongoing small stream and river flooding across the Great Basin and Intermountain West... .Discussion... .East Texas and the Lower Mississippi Valley... Considerable flooding impacts, including new and renewed moderate to isolated major river flooding and significant small stream, flash and urban flooding, are possible in portions of central and eastern TX today as a widespread 2 - 4" of rainfall is expected. NASA SPoRT data indicate that soils top and mid-layer soils remain wet to near saturated from recent rainfall and streamflows remain much above normal (USGS) in the region. As a result, rainfall is expected to quickly become runoff, and intense rainfall rates will only reduce time to runoff if they occur. Portions of the Trinity and Neches rivers remain in flood stage this morning, and renewed rises on these rivers and new rises on other rivers in the region are expected. Considering the antecedent soil conditions, significant flash, urban, and small stream flooding is also possible in these areas. The focus shifts toward the ArkLaTex and along the TX/LA border and into the Lower MS Valley on day 2 (Thu), where 1 - 2" of rainfall is expected. Top and mid-layer soils are also wet to near saturated in these areas, but streamflows are a mix of above to much above normal. Isolated flash, urban, and small stream flooding impacts and minor river flooding are possible. .Central and Southern Plains into the Midwest... 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 through day 2 (Thu) with additional flooding impacts. Soils remain wet (0 - 100 cm, NASA SPoRT) and streamflows are generally above normal (USGS) creating favorable conditions for rapid-runoff into elevated streams. Additionally, isolated flash and urban flooding impacts can not be ruled out across the entire region, particularly where training occurs. Isolated minor river flooding continues 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 2 days. This may result in isolated new and renewed rises on the smaller streams. Drier conditions settle across the area for a few days before another wet pattern is forecast to return by day 6 (Mon) but, the confidence in exact placement and magnitude of forecast rainfall is low due to the extended timeframe. The NWM MRF continues to signal rapid-onset flooding probabilities between 25 - 50% across the region. Corresponding AEPs are around 50% with some isolated smaller order streams signaling AEPs as low 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. .Puerto Rico and the USVI... Persistent rainfall resulting in totals of 4 - 8" through day 5 (Sun) will bring with it all modes of flooding through that period, with the focus on flash and urban flooding impacts, including mudslides. The highest rainfall amounts are generally expected to occur over the eastern and central portions of the island through day 3 (Fri), and transition more towards the central and westward side of the island by day 5 (Sun). Additionally, rapid 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 this evening. Out-of-bank flows on Moose Creek near Glennallen could impact Glenn Highway and the surrounding floodplain. Furthermore, snowmelt driven rises are ongoing for the Yukon River at Eagle and Dawson. The ice jam that was located approximately 23 miles upstream of Eagle has broken up and reformed further downstream. However, it is expected to break up again in the near future, diminishing the overall flood threat for Eagle. Any further ice jam development can still pose a localized threat immediately around its formation. Across the western and central portions of AK, GloFAS flood summary continues 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 the flood threat on streams and rivers across the region. .Great Basin and Intermountain West... Isolated minor to moderate river flooding and 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. //Capp/Watson $$