AGUS74 KWCO 041502 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1001 AM CDT SAT MAY 4 2024 .Synopsis... Catastrophic flooding impacts across East Texas... Ongoing and new flooding potential across the Southern and Central Plains into the Midwest/Middle Mississippi Valley... Continued flood threat this weekend in Puerto Rico and the USVI... Spring breakup flooding in Alaska... Snowmelt runoff induced flooding across the Great Basin and Intermountain West... Continued flood threat this weekend in American Samoa... .Discussion... .Texas... Catastrophic river flooding impacts are ongoing and will continue through this weekend across portions of the Lower Trinity, Upper San Jacinto, and Lower Brazos river basins in southeast TX. Major river flooding is expected to continue this weekend along the Navasota, East/West Fork San Jacinto/San Jacinto, Trinity, and Neches rivers. Many of these and other rivers in east TX are expected to crest in their top 5 highest crests on record, speaking to the significance of the flooding ongoing across the region. Additional heavy rainfall (1 - 2.5") is expected across the region days 1 - 2 (Sat - Sun), which will immediately convert to runoff given the completely saturated antecedent conditions, and will likely exacerbate and extend the duration of the areal and river flooding impacts across the region. Peak flows on many of the smaller ungauged creeks, rivers, and streams are expected within the next 24 - 48 hours according to the National Water Model (NWM). Moderate to heavy rainfall is possible day 1 (Sat) over portions of central and west Texas, bringing the potential of flash and urban flooding to the region. Soils within the region have available infiltration capacity with soil moisture generally 45 - 60% however, heavy rainfall within hilly terrain should be able to overwhelm infiltration capacity allowing for an increased threat of flash flooding. Rapid rises may be possible on area streams and rivers as well. The recent increase in potential QPF across some areas that have seen recent rainfall has introduced a locally considerable flooding threat to the region for late day 1 (Sat) into day 2 (Sun). .Southern and Central Plains into the Midwest/Middle Mississippi Valley... Minor to isolated moderate river flooding is ongoing or forecast across central MO into western IL, and across northwest IA. Many rivers have crested or are cresting over the next couple of days with only a few locations (Illinois River for example) experiencing broader crests from routed flows through this weekend. Deep layer soils remain wet (0 - 100 cm, NASA SPoRT) and streamflows are much above normal (USGS), creating favorable conditions for rapid runoff into elevated streams. Areas of heavy rainfall will return to portions of eastern OK, AR, southeast KS, and southern MO on day 2 (Sun) and shift north across eastern KS/NE, western IA, and northern MO on day 3 (Mon). This area remains wet, with soil moisture generally above 60% (0-10 cm RSM, NASA SPoRT). The combination of wet soils, complex terrain, and heavy rainfall will likely result in flash flooding and rapid runoff into already elevated streams. Rainfall shifts east across the Middle Mississippi Valley and lower Ohio River Valley on days 4 - 5 (Tue - Wed) where conditions are not quite as primed as the aforementioned regions, but nonetheless, isolated small stream and flash flooding will be possible. The NWM MRF continues to signal flows on many smaller rivers across the entire region with AEPs around 50% and some isolated AEPs as low as 20%, continuing the confidence that most small stream rises will not be significant with the additional rainfall over the next few days. Nonetheless, new river flooding and isolated rapid onset flooding potential still exists across many of the basins that have seen recent rainfall (NWM MRF). Peak flows on most rivers and streams will have crested by days 3 - 5 (Mon - Wed) across the entire region (NWM MRF). .Puerto Rico and the USVI... Persistent rainfall through this weekend will continue to provide a threat of flooding impacts, including mudslides and locally considerable flash flooding, across PR and the USVI. Soils remain saturated across PR and streamflows remain elevated, especially across the eastern half of the island. Considering the antecedent conditions, any additional heavy rainfall will likely lead to rises on rivers and flooding impacts. .Alaska... Flooding impacts from spring breakup and snowmelt runoff will continue for the next several days and beyond. Out-of-bank flows on Moose Creek and Trooper Creek near Glennallen could impact Glenn Highway and the surrounding floodplain. An ice jam on the Nushagak River between Dillingham and Portage Creek has caused localized flooding across the floodplain. An ice jam is in place on the Tanana River downstream of Manley, and has caused some minor flooding within the town. Finally, ice jams have been reported on the Yukon and Forty Mile rivers in Canada with the breakup front reaching Eagle Wednesday, flooding will be possible in the area if any jams form near the town. .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. .American Samoa... Heavy rainfall could result in flash and urban flooding impacts through this weekend. //JEC $$