####018009309#### AGUS74 KWCO 051521 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1021 AM CDT SUN MAY 5 2024 .Synopsis... Catastrophic flooding impacts continue in portions of East Texas... Isolated flooding impacts through Monday across the Southern Plains, Ozarks, and Central Plains... Rain and snowmelt across Central and Eastern Montanna... Isolated flooding impacts possible across the Middle Mississippi, Lower Ohio, and Tennessee Valleys, Puerto Rico and the USVI, and Alaska through mid week... Isolated snowmelt induced flooding continues in portions of the Great Basin and Intermountain West... Heavy rainfall continues for American Samoa... .Discussion... .East Texas... Catastrophic river flooding impacts are ongoing and will continue into early this week across portions of the Lower Trinity, Upper San Jacinto, and Lower Brazos river basins in southeast TX. Major river flooding will continue this week 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 considerable flash, urban, and small stream flooding impacts are possible across southeast TX day 1 (Sun) as very heavy rainfall is expected across the extremely vulnerable region this morning, with potential redevelopment of intense rainfall late this morning into the afternoon. Soils are completely saturated (NASA SPoRT/NWM) and rivers are already flooding (NWPS) in response to 4 - 12"+ (RFC QPE) of rain from the previous flooding event within the last 72 hours. Rain will immediately convert to runoff and result in potentially significant new and renewed river flooding and areal flooding in urban areas. New and renewed areas of inundation may drastically inhibit ongoing recovery efforts this morning into this afternoon. The National Water Model (NWM) Short Range Forecast (SRF) continues to highlight potential for new rapid-onset flooding (ROF) impacts this morning from just north of Houston to the Huntsville area as the heavy rain progresses through the region. Depending on if heavy rainfall redevelops along a potential lingering boundary generally from Austin to Houston late this morning into the afternoon, additional areas of ROF can be expected across the aforementioned region. The area of greatest confidence for new rapid small stream and river rises is from just east of Austin, TX through Beaumont, TX and north towards Bryan, TX, where probabilities for ROF are in the 25 - 50% range. Additionally, significant out of bank flows on many smaller tributaries and headwaters is consistently being signaled across this same region, with widespread 2 - 4% AEP flows being forecast by the NWM SRF. In terms of the NWM performance, this region already has stringent flow thresholds in place for subsequent NWM service visualization, so to see forecast flows of this magnitude colocated with fairly decent probabilities for ROF in a completely saturated area with ongoing moderate to major river flooding is quite concerning, and considerable flooding impacts will be possible during the morning hours into the afternoon/evening. Peak flows on many smaller streams are not expected until the late morning and afternoon hours (NWM SRF). Mostly dry weather conditions are expected for the next week after todays rainfall, allowing water to begin to route unimpeded, rivers to crest, and recessions to eventually begin early this week. .Southern Plains, Ozarks, and Central Plains... A complex of thunderstorms will move across eastern OK into central AR this morning into this afternoon (day 1), bringing heavy rain (2 - 4" (HRRR, WPC)) and the potential for flash, urban, and small stream flooding impacts. Soils across eastern Oklahoma are very wet (NASA SPoRT), with relative soil moisture (RSM) generally 60% or greater, increasing to over 75% RSM across central Arkansas, suggesting that there is at least some available infiltration capacity initially. However, with the initial heavy rainfall and prolonged moderate rainfall, in combination with the hilly terrain of the Ozarks, rain will quickly convert to overland flow, potentially resulting in flash flooding and runoff into already elevated streams. The NWM SRF continues to indicate the potential for ROF this morning into the afternoon with scattered ROF probabilities of 50 - 75% and more isolated values greater than 75% across eastern OK into central AR. However, with the HRRR somewhat underperforming in regards to the location and the magnitude of precipitation, the potential for ROF is most likely more widespread than indicated by the NWM ROF probability forecast. Additionally, NWM SRF High Flow Magnitude Forecast AEPs are generally 20-50%, with isolated streams as low as 2%. However, with the HRRR underperforming, flows of 20% AEP or lower are more likely than indicated by the NWM visualization. Ensemble guidance (SR QPF River Ensembles) suggests low end potential for isolated minor to moderate river flooding as well. On day 2 (Mon), 1 - 2" of rain is expected across eastern NE/KS and western MO/IA, which could elicit isolated flooding impacts across the region. River flooding is still ongoing across northwest IA and central MO in response to the previous flooding event, so basins across these areas will see the best potential for new and renewed small stream and river flooding impacts on day 2 (Mon) given the expectation of efficient runoff. Flash and urban flooding will be the primary flooding threats across eastern NE/KS as conditions are not quite as wet as areas further east, but small stream flooding impacts cannot be ruled out (NWM NBM-forced Medium Range Forecast). 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 (Tue - Thu) across the entire region (NWM MRF). .Central and Eastern Montanna... Periods of rain days 2 - 4 (Mon - Wed) are expected across the region, which in combination with snow melt (SNODAS), will likely produce small stream rises and potentially isolated minor river flooding. Isolated flash flooding cannot be ruled out either. .Middle Mississippi, Lower Ohio, and Tennessee Valleys... Rainfall shifts east across the Middle Mississippi, Lower Ohio River, and TN Valleys on days 4 - 5 (Wed - Thu) where isolated small stream and flash flooding will be possible. There is ample soil infiltration capacity and rivers are generally flowing at near normal levels (USGS) with exception to portions of the lower Ohio Valley, so most basins across the region should be able to handle the large majority of the expected rainfall (1 - 2"), and flooding impacts should be isolated. One thing that will need to be considered/monitored is the accumulated rainfall (1 - 2") days 1 - 3 (Sun - Tue) ahead of the heavier rainfall days 4 - 5 (Wed - Thu) which could alter the antecedent conditions/vulnerability and increase the flood threat locally across some areas, so trends will have to be monitored. .Puerto Rico and the USVI... Persistent rainfall through mid week 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. Considering the antecedent conditions, any additional heavy rainfall will likely lead to rises on rivers and flooding impacts. .Alaska... Significant rises on Moose creek near Glennallen due to snowmelt runoff are expected and minor flooding is already occurring. The ice breakup front has already passed Eagle and is 50 miles upstream from Circle as of Saturday afternoon. There is potential for minor to moderate flooding in Circle as a heavy run of ice moves through. An ice jam that caused flooding in Kalskag has released and water levels are subsiding, however, there is potential for a rejam to impact Kwethluk. .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 early this week. //JEC $$