####018006370#### AGUS74 KWCO 121508 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT SUN MAY 12 2024 .Synopsis... Considerable flash, urban, and major flooding in East Texas... Locally significant flash and urban flooding in the Lower Mississippi Valley and Southeast... Urban and small stream flooding in Puerto Rico and American Samoa... Ice jam-induced flooding possible in Alaska... Snowmelt in Central Montana... Rainfall returns to Missouri... .Discussion... .East Texas... Considerable flash/urban flooding and major river flooding is expected ahead of multiple rounds for rainfall through day 2 (Mon) and again late this week. Two day rainfall totals of 2 - 4" (isolated amounts of 6"+) is forecast across the region where widespread minor and isolated moderator river flooding continues. Record or near record major flooding is ongoing on the Lower Trinity River highlighting the vulnerability of the region. USGS streamgages are reporting widespread above normal flows both historically and for this time of year. Soil moisture continues to recover but still remains on the wet side just above 50% relative soil moisture (NASA SPoRT). Despite the heavy rainfall totals, any infiltration capacity could be quickly overwhelmed producing runoff into already swollen rivers and streams. The National Water Model (NWM) Short Range Forecast (SRF) Rapid-onset Flooding forecast is starting to capture most of the QPF for the heavy rainfall with isolated signals for rapid-onset flooding meandering across East TX. The SRF Maximum High Flow Magnitude forecast is doing the same with isolated areas of streamflows with lower Annual Exceedance Probabilities (AEPs) ranging from 20 - 2%, suggesting at least isolated areas of significant out-of-bank streamflows are possible. The NWM Medium Range Forecast (MRF) is fully capturing this first rainfall event that expands into day 2 (Mon). Both the NBM- and GFS-forced NWM continue to suggest isolated significant stream responses including isolated rapid-onset flooding over the next 48 hours centered on the Lower Brazos and Lower Trinity Rivers with the GFS-forced NWM more aggressive overall and more widespread with the significant streamflows. River ensemble forecasts suggest many of the area's rivers could see new and renewed river rises which will exacerbate ongoing flooding. The region remains hydrologically active and vulnerable as rainfall returns again mid to late week. The NBM- and GFS-forced NWM MRF responses have tended to back off in magnitude since yesterday but do show some additional isolated responses later this week. While there is some uncertainty with this next event, this area should be closely monitored in the coming days. .Lower Mississippi Valley and Southeast... Rounds of rainfall (3 - 5"+) will again impact the region beginning this afternoon and continue through day 3 (Tue), bringing the potential for significant rapid-onset flooding particularly across the central Gulf Coast, some of which may be considerable. The latest runs NBM- and GFS-forced NWM have fallen into better agreement, both showing widespread stream responses with isolated areas of lower AEPs suggesting isolated significant out-of-bank streamflows. But there is a catch, the QPF over the next 48 hours varies significantly from model to model with the HRRR being much more aggressive overall and further south (central Gulf Coast) with the placement of QPF. The HRRR-forced SRF only ingests 18 hours of QPF so it will not fully capture the rainfall until later today. With that said, the SRF will likely show expanding signals for highwater and rapid-onset flooding across the region as we move closer to the event and thus locally considerable flash and urban flooding impacts cannot be ruled out. Additionally, river ensemble forecasts show the potential for scattered minor river flooding at the 30% chance of exceedance. Isolated moderate river flooding cannot be ruled out either. .Puerto Rico Urban and small stream flooding impacts, along with the chance for isolated flash flooding, will remain possible through today. Wet soil conditions and locally elevated streamflows will increase the potential for hydrologic impacts, including mudslides in areas of steep terrain. .American Samoa... Isolated flash and urban flooding impacts will be possible through today, with landslides/mudslides/rockslides also possible in areas of steep terrain. .Alaska... Flooding is possible due to various small ice jams in the lower Kuskokwim River near Eek, Tuntutuliak and other riverside communities through Sunday afternoon. Looking north to the Yukon River, an ice jam front is also expected to impact the community of Circle through Monday. Ice jam breakups and formations are expected to continue along this breakup front as it makes its way down the Yukon River. Ice jams and breakup fronts can evolve rapidly, so refer to the local forecast offices for up-to-date information. .Central Montana... Warming temperatures will continue to bring snowmelt to the mountain ranges of central Montana and may lead to small stream flooding and rises on area rivers through Monday. .Missouri... Rainfall (1 - 2") returns to MO on day 2 (Mon) which could bring some renewed river rises and isolated flash and urban flooding impacts to the State. Soils are still on the wet side from recent rainfall of 2 - 3" across central MO last week. Streamflows are elevated on a few streams but are generally normal for this time of year (USGS). These factors suggest that the soils and streams can handle much of the water however, isolated flash and urban flooding cannot be ruled out. The NWM is currently signaling isolated responses from both the NBM- and GFS-forced NWM with good agreement in placement and magnitude. New river flooding may also be possible as HEFS suggests the potential for new and renewed river rises including minor river flooding at the 30% chance exceedance. //Smith $$